VEGETABLE PRODUCTION USING GREEN ROOF TECHNOLOGY AND THE
POTENTIAL IMPACTS ON THE BENEFITS PROVIDED BY CONVENTIONAL GREEN
ROOFS
By
Leigh Jane Whittinghill
A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
HORTICULTURE
2012
ABSTRACT
VEGETABLE PRODUCTION USING GREEN ROOF TECHNOLOGY AND THE
POTENTIAL IMPACTs ON THE BENEFITS PROVIDED BY CONVENTIONAL GREEN
ROOFS
By
Leigh Jane Whittinghill
Modern green roofs, which originated in Germany in the late 1800’s, reduce energy use,
mitigate urban heat island effects and reduce stormwater release. In recent years more attention
has been turned to two additional potential benefits of green roofs; carbon sequestration and
vegetable production. Although some work has been done on extensive Sedum spp. roofs, little
has been done on carbon sequestration of other green roof types and other ornamental
landscapes. Little literature also exists on the use of green roofs in urban agriculture.
Urban agriculture has many benefits including increased economic and food security, job
creation and community building, but is also faced with many challenges. The largest of which
is land availability and competition with other forms of land development. The use of green
roofs could eliminate this form of competition at some locations and help to alleviate other
concerns surrounding urban agriculture such as health hazards associated with heavy metal
contamination of food. It does however raise two other major concerns; how the added weight
of the green roof and the weight load restrictions of most flat roofs will affect the scale at which
it can be implemented and how higher nutrient requirements of vegetable and herb plants
compared to most other green roof plants will affect runoff water quality. Four studies were
designed to examine food production on extensive green roofs and the ability of extensive green
roofs and other ground level ornamental landscapes to sequester carbon.
Vegetable species selected for study were tomatoes (Lycopersicon esculentum), green
beans (Phaseolus vulgaris), cucumbers (Cucumis sativus), peppers (Capsicum annuum), basil
(Ocimum basilicum) and chives (Allium schoenoprasum). These were tested for their
productivity on an extensive green roof, extensive green roof platforms and in ground. All plants
survived and produced biomass at all three locations for three growing seasons. Use of three
mulching strategies (no mulch, pine bark mulch, and a living Sedum mulch) and three
2
fertilization regimens (25, 50, and 100 g/m of a 14-14-14 N-P-K slow release fertilizer applied
twice during each growing season) were examined and the use of pine bark mulch and higher
fertilizer application rates improved crop performance. Further research on other mulches and
fertilizers is recommended to optimize vegetable and herb production. To examine effects of
vegetable and herb production on the stormwater benefits provided by green roofs, stormwater
runoff quantity and water quality were compared with more traditional extensive green roofs
planted with a sedum mix and a native prairie mix. The prairie mix retained the most runoff and
the vegetable and herb mix the least because of differences in plant density and morphology.
Examination of runoff nitrate and phosphorus concentrations had mixed results with no
differences among treatments for nitrate and higher phosphorus concentrations in the first 125
mL of runoff from the vegetable and herb green roofs. Further research into nutrient runoff is
recommended. Finally the ability of nine in ground and four extensive green roof landscapes,
including vegetable and herb gardens, to sequester carbon were compared. The in ground
landscape systems sequestered more carbon than the corresponding green roof landscape
systems. Most carbon was sequestered by landscape systems containing plants with large
amounts of woody structures or high plant volumes, but more research on the effect of
management practices is recommended.
The following is dedicated to Ruth, who first helped me to discover my passion for green roofs
and my family for their continued support throughout the course of my education.
iv
ACKNOWLEDGEMNTS
There are several people to whom I would like to give special thanks. To Dr. Bradley
Rowe, thank you for your support and encouragement in pursuing this new aspect of green roof
research. To my advisory committee, Dr. Bert Cregg, Dr. Mathieu Ngouajio, Dr. Jeff Andresen,
and Dr. Mike Hamm, thank you for your help and insight throughout the research and writing
process. Thank you to Dan, for your support and encouragement in all its forms from help
weeding to hot meals and reminders to take some time to rest.
v
TABLE OF CONTENTS
LIST OF TABLES……………………………………………………………………………...viii
LIST OF FIGURES……………………………………………………………………………...xii
LITERATURE REVIEW:
CARBON SEQUESTRATION: A POTENTIAL BENEFIT OF LANSCAPE SYSTEMS....…...1
Introduction………………………………………………………………………………..2
The History of Green Roofs……………………………………………………………….3
Plant Selection for Green Roofs…………………………………………………………..4
Traditional Uses of Green Roofs………………………………………………………….5
Emerging Uses of Green Roofs…………………………………………………………...8
References………………………………………………………………………………..20
CHAPTER ONE:
THE ROLE OF GREEN ROOF TECHNOLOGY IN URBAN AGRICULTURE……………..28
Abstract…………………………………………………………………………………..29
Introduction………………………………………………………………………………29
Economic Improvement and Food Security……………………………………………. 35
Economic Barriers……………………………………………………………………….39
Access to Resources and Policy…………………………………………………………40
Human Health Concerns…………………………………………………………………43
Environmental Health Concerns…………………………………………………………46
Conclusions………………………………………………………………………………48
References………………………………………………………………………………..50
CHAPTER TWO:
EVALUATION OF VEGETABLE PRODUCTION ON EXTENSIVE GREEN ROOFS……..57
Abstract…………………………………………………………………………………..58
Introduction………………………………………………………………………………59
Methods……………………...…………………………………………………………...61
Results……………………………………………………………………………………67
Discussion………………………………………………………………………………..75
Conclusions………………………………………………………………………………78
References………………………………………………………………………………..80
vi
CHAPTER THREE:
EVALUATION OF NUTRIENT MANAGEMENT AND MULCHING STRATEGIES FOR
VEGETABLE PRODUCTION ON AN EXTENSIVE GREEN ROOF…..…………………….84
Abstract…………………………………………………………………………………..85
Introduction………………………………………………………………………………96
Methods…………………………………………………………………………………..90
Results……………………………………………………………………………………97
Discussion………………………………………………………………………………106
Conclusions……………………………………………………………………………..112
References………………………………………………………………………………114
CHAPTER FOUR:
STORMWATER RUNOFF QUANTITY AND QUALITY FROM TWO TRADITIONAL AND
ONE VEGETABLE EXTENSIVE GREEN ROOF……………………………………………118
Abstract…………………………………………………………………………………119
Introduction……………………………………………………………………………..120
Methods…………………………………………………………………………………123
Results…………………………………………………………………………………..130
Discussion………………………………………………………………………………135
Conclusions……………………………………………………………………………..141
References………………………………………………………………………………143
CHAPTER FIVE:
QUANTIFYING CARBON SEQUESTRATION OF VARIOUS GREEN ROOF AND
LANDSCAPE SYSTEMS……………………………………………………………………...147
Abstract…………………………………………………………………………………148
Introduction……………………………………………………………………………..149
Methods…………………………………………………………………………………151
Results…………………………………………………………………………………..160
Discussion………………………………………………………………………………165
Conclusions……………………………………………………………………………..170
References………………………………………………………………………………172
vii
LIST OF TABLES
Table
Page
2.1
Planting and maintenance dates for vegetable production during the 2009-11 growing
seasons on the green roof, green roof platforms and in-ground ...………………………63
2.2
Composition of Lesco Professional Landscape and Ornamental All-Purpose Fertilizer 1414-14 (Lesco, Inc., Cleveland, OH)……………………………………………………..65
2.3
Harvest indicators for tomatoes (Lycopersicon esculentum) grown on the green roof,
green roof platforms and in-ground during the 2009-11 growing seasons ……………...69
2.4
Harvest indicators for beans (Phaseolus vulgaris) grown in the green roof, green roof
platforms and in-ground during the 2009-11 growing seasons …………………………71
2.5
Harvest indicators for cucumbers (Cucumis sativus) grown in the green roof, green roof
platforms and in-ground during the 2009-11 growing seasons …………………………72
2.6
Harvest indicators for Budapest hot banana peppers (Capsicum annuum) grown in the
green roof, green roof platforms and in-ground during 2011 growing season ………….73
2.7
Biomass fresh weight (g) of basil (Ocimum basilicum) for the 2009-11 growing seasons
on the green roof, green roof platforms and in-ground ……………………….................74
2.8
Marketable fresh weight yield (g) and marketable percent of biomass of basil (Ocimum
basilicum) for the 2010 growing season on the green roof, green roof platforms and inground …………………………………………………………………………………...74
2.9
Fresh weight yields (g) of chives (Allium schoenoprasm) for the 2009-11 growing
seasons on the green roof, green roof platforms and in-ground ………………………...74
2.10
Estimated yield in grams per plant for the highest yields achieved in this study and from
reported growing area and yield data reported by the USDA (2011) and recommended
planting densities based on row and plant spacing (Swiader and Ware, 2002)………….78
3.1
Initial physical and chemical properties of green roof substrate…………………….…..90
3.2
Composition of Lesco Professional Landscape and Ornamental All-Purpose Fertilizer 1414-14 (Lesco, Inc., Cleveland, OH)……………………………………………………...91
3.3
Important planting and maintenance dates for vegetable production during the 2010 and
2011 growing seasons on green roof platforms …………………………………………92
viii
3.4
Results for F tests for treatment interactions between mulch type, fertilizer application
and year. For all tests α = 0.05. * indicates p < 0.05, ** indicates p <0.01, and ***
indicates p < 0.001……………………………………………………………………....96
3.5
Quality indices of tomatoes (Lycopersicon esculentum) and pod and fruit size of beans
(Phaseolus vulgaris) and cucumbers (Cucumis sativus) in 2010 and 2010 under
2
treatments of no mulch, pine bark mulch, live Sedum mulch, and 25, 50, and 100 g/m
14-14-14 slow release fertilizer. Tomato sizes, colors and grades based on the USDA
standards for fresh tomatoes (USDA, 1991), bean sizes based on the USDA standards for
snap beans for processing (USDA, 1959), and cucumber sizes based on the USDA
standards for pickling cucumbers (USDA, 1936)……………………………………....99
3.6
Yield indices of tomatoes (Lycopersicon esculentum) in 2010 and 2011 under treatments
2
of no mulch, pine bark mulch, live Sedum mulch, and 25, 50, and 100 g/m 14-14-14
slow release fertilizer. Marketability based on USDA standards for fresh tomatoes
(USDA,
1991)……………………………………………………………………………………100
3.7
Harvest indices of beans (Phaseolus vulgaris) in 2010 and 2011 under treatments of no
2
mulch, pine bark mulch, live Sedum mulch, and 25, 50, and 100 g/m 14-14-14 slow
release fertilizer. Grade and marketability based on USDA standards for snap beans for
processing (USDA, 1959)……………..………………………………………………..101
3.8
Harvest indices of cucumbers (Cucumis sativus) in 2010 and 2011 under treatments of no
2
mulch, pine bark mulch, live Sedum mulch, and 25, 50, and 100 g/m 14-14-14 slow
release fertilizer. Grades and marketability based on the USDA standards for pickling
cucumbers (USDA,
1936)……………………………………………………………………………………103
3.9
Harvest indices of Budapest hot banana peppers (Capsicum annuum) for the 2011
growing season under treatments of no mulch, pine bark mulch, live Sedum mulch, and
2
25, 50, and 100 g/m 14-14-14 slow release fertilizer. Grades and marketability based on
the USDA standards for sweet peppers (USDA, 2005)……...…………………………104
3.10
Biomass fresh weight in grams of basil (Ocimum basilicum) for the 2010 and 2011
growing season and marketable fresh weight yield in grams and marketable percent of
biomass fresh weight for the 2010 growing seasons under treatments of no mulch, pine
2
bark mulch, live Sedum mulch, and 25, 50, and 100 g/m 14-14-14 slow release
fertilizer…………………………………………………………………………………105
3.11
Yield fresh weight in grams for chives (Allium schoenoprasum) in 2010 and 2011 under
2
treatments of no mulch, pine bark mulch, live Sedum mulch, and 25, 50, and 100 g/m
14-14-14 slow release fertilizer………………...………………………………………105
ix
3.12
Estimated yield in grams per plant for the highest yields achieved in this study, from
chapter 2 (Whittinghill, 2012), and from reported growing area and yield data reported by
the USDA (2011) and recommended planting densities based on row and plant spacing
(Swiader and Ware, 2002)……………………………………………………………...107
3.13
The nutrient recommendations (g/m ) for each vegetable and herb crop based on soil
testing perfumed by the Michigan State University Soil and Plant Nutrient Laboratory
(MSUSPNL) (East Lansing, MI) and nutrient application recommendations from
2
Warncke et al (2004) and the nutrients supplied (g/m ) by the applications of 25, 50 and
2
2
100 g/m of a Lesco Professional Landscape and Ornamental All-Purpose Fertilizer 1414-14……………………………………………………………………………………111
4.1
Initial physical and chemical properties of substrate…………………………………...125
4.2
Planting and maintenance dates of the vegetable green roofs for the growing seasons of
2009, 2010 and 2011……………………………………………………………………126
4.3
Composition of Lesco Professional Landscape and Ornamental All-Purpose Fertilizer 1414-14 (Lesco, Inc., Cleveland, OH)…………………………………………………….126
4.4
Nitrate concentrations (mg/L) in the first 125 mL of runoff water taken in 2011 from
extensive green roofs vegetated with a mix of Sedum species, a native prairie mix, and a
fertilized vegetable and herb garden before the first fertilizer application and 3 and 5
weeks after the first and second fertilizer applications………………………….……...133
4.5
Phosphorus concentrations (mg/L) in the first 125 mL of runoff water taken in 2011 from
extensive green roofs vegetated with a mix of Sedum species, a native prairie mix, and a
fertilized vegetable and herb garden before the first fertilizer application and 3 and 5
weeks after the first and second fertilizer applications...……………………………….134
4.6
Volumetric moisture content (m /m ) of extensive green roofs vegetated with a mix of
Sedum species, a native prairie mix, and a fertilized vegetable and herb garden………135
4.7
Nitrogen and Phosphorus supplied to the vegetable green roof treatment from the 25 g/m
Lesco Professional Landscape and Ornamental All-Purpose Fertilizer 14-14-14 applied
twice during the growing season as compared with nutrient recommendations based on
substrate tests and plant nutrient use (Warncke et al, 2004)……………………………137
4.8
Estimated total nitrate loads in milligrams for runoff from extensive green roofs
vegetated with a mix of Sedum species, a native prairie mix, and a fertilized vegetable
and herb garden during the growing seasons of 2009-11……………………………....139
3
3
2
x
4.9
Estimated total phosphorus loads in milligrams for runoff from extensive green roofs
vegetated with a mix of Sedum species, a native prairie mix, and a fertilized vegetable
and herb garden during the growing seasons of 2009-11………………………………140
5.1
Composition of Lesco Professional Landscape and Ornamental All-Purpose Fertilizer 1414-14 (Lesco, Inc., Cleveland, OH)……………………………………………...……..152
5.2
Planting and maintenance dates of vegetable and herb garden and vegetable green roof
plots for the 2009, 2010 and 2011 growing seasons……………………………………153
5.3
Initial physical and chemical properties of green roof substrate…………….…………154
5.4
Carbon content (kg/m ) of above ground of all landscape systems at the end of the 2010
and 2011 growing seasons……….………………………………………..……………162
5.5
Carbon content (kg/m ) of below ground biomass of all landscape systems at the end of
the 2010 and 2011 growing seasons.………...…………………………………………163
5.6
Carbon content of soil or substrate of all landscape systems at the beginning of the 2009
growing season and the end of the 2010 and 2011 growing seasons…...……………....164
5.7
Total carbon content (kg/m ) of all landscape systems at the end of the 2010 and 2011
growing seasons.………………………………………………………………………..165
2
2
2
xi
LIST OF FIGURES
Figure
Page
1.1
Vegetable production on a green roof in Lansing, MI…………………………………...33
1.2
A community garden in Detroit, MI……………………………………………………..37
1.3
An herb garden on a green roof in Grand Rapids, MI…………………………………...38
2.1
Weekly average maximum air temperature in Celsius and weekly total precipitation in
mm for the growing seasons of (A) 2009 (May 31-Oct 2) (B) 2010 (May 25-Oct 2) and
(C) 2011 (May 29-Oct 1)………………………………………………………………...68
3.1
Weekly average maximum air temperature in Celsius and weekly total precipitation in
mm for the growing seasons of (A) 2010 (May 25-Oct 2) and (B) 2011 (May 29-Oct 1)
…………………………………………………………………………………....………97
4.1
Weekly average maximum air temperature in Celsius and weekly total precipitation in
mm for the growing seasons of (A) 2009 (May 31-Oct 2) (B) 2010 (May 25-Oct 2) and
(C) 2011 (May 29).……………………………………………………………………..129
4.2
Runoff hydrographs of precipitation events of (A) light, (B) medium, and (C) heavy
rainfall. Rain events recorded at 15 min intervals. Values are averages of measurements
taken from all tipping buckets for each treatment within each precipitation event size
category over the growing seasons of 2009, 2010, and 2011…………………………..131
4.3
Multiple linear regressions of precipitation and runoff for the growing seasons of 2009,
2010 and 2011. Each marker represents a single observation…………………………132
5.1
Weekly average maximum air temperature in Celsius and weekly total precipitation in
mm for the growing seasons of (A) 2009 (May 31-Oct 2) (B) 2010 (May 25-Oct 2) and
(C) 2011 (May 29-Oct 1)……………………………………………………………….161
xii
A LITERATURE REVIEW:
Carbon Sequestration: A Potential Benefit of Landscape Systems
1
Introduction
Although the term “green” has come to be associated with many environmentally friendly
technologies, corporations, and policies, “green roof” refers specifically to a roofing technology
that enables the growth of vegetation on rooftops. The environmental benefits attributed to this
type of roofing include reduced air and noise pollution, increased habitat and biodiversity,
increased roof lifespan, stormwater retention, energy savings and mitigation of the urban heat
island effect (Alexandri and Jones, 2008; Barrio, 1998; Carter and Jackson, 2007; Getter and
Rowe, 2006; Getter et al., 2007; Loder and Peck, 2004; Rowe, 2011; Saiz et al., 2006; VanWoert
et al., 2005a; Wong et al., 2003). Green roofs have also been shown to provide a wide variety of
health benefits (Ulrich, 1981, 1984) and when open to the public can provide recreation
opportunities.
Green roofs are typically constructed of several layered materials that facilitate vegetative
growth. The specific makeup of these layers will depend on building load capacity, purpose of
the project (Getter and Rowe, 2006) and manufacturer. These layers include a root barrier to
protect the underlying structure from root penetration, a drainage layer to facilitate water
movement off the roof, a filter fabric to minimize the loss of substrate and subsequent clogging
of the drainage layer, an optional water retention fabric (Getter and Rowe, 2006), growing
substrate, and the vegetation itself. Substrate composition varies, but the primary component is a
light-weight, mineral based material, such as heat expanded slate (Getter and Rowe, 2006).
Substrate depth and the vegetation used vary between roofs. Extensive green roofs, those with
less than 15 cm of substrate, are low maintenance, usually rely on natural precipitation (Durhman
et al., 2006), and are typically planted with succulents, grasses, herbs, or other ground cover
species (Getter and Rowe, 2006). Intensive green roofs, those with more than 15 cm of
2
substrate, require greater structural support and often more maintenance than extensive roofs, but
can support a greater diversity of plants.
The History of Green Roofs
The first modern green roofs were constructed in Germany in the 1880’s during a period
of rapid urbanization (Kohler and Keely, 2005). A highly flammable, inexpensive tar was
commonly used on roofs at the time and in an effort to reduce this fire hazard, a roofer, H. Koch,
began covering such roofs with sand and gravel (Kohler and Keeley, 2005). These sanded roofs
were gradually colonized by local plant life and 50 of these original roofs were still intact and
waterproof in the 1980’s (Kohler and Keeley, 2005). Roof gardens on city department stores in
the early 1900’s (Firth and Gedge, 2005; Mikami, 2005) and the use of sod roofs for airfield
hanger camouflage during World War II (Firth and Gedge, 2005) are some examples of early,
intentional, modern green roofs. The first large-scale green roof project was constructed at the
Free University of Berlin by Reinhard Bornkamm after the rediscovery of Koch’s roofs in the
1960’s (Kohler and Keeley, 2005). Since then, interest in green roofs has increased in Europe
(Kohler and Keeley, 2005), although significant interest in the United States did not arise until
much later (Cheney, 2005; Lipton, 2005).
The current growth of the green roof industry in the United States (Getter and Rowe,
2006) has been enhanced through a number of incentive programs (Cheney, 2005; City of
Portland, 2009b; City of Chicago, 2005a, 2006; Kula, 2005; Lipton, 2005). Cities such as
Portland, Oregon, and Chicago, Illinois, have been applauded for their inclusion of green roofs in
stormwater management (Lipton, 2005), offering incentives for the use of green roofs in building
plans (City of Chicago, 2005b, City of Portland, 2009a), and making financial assistance for the
3
installation of green roofs available (City of Portland, 2009b; City of Chicago, 2005a, 2006).
The Leadership in Energy and Environmental Design (LEED) program provides further
incentive to builders and corporations. LEED was developed by the US Green Building Council
as a tool to promote and measure green building design, construction, operation, and
maintenance (USGBC, 2011). It employs a number of rating systems that award points (100
possible) in the categories of sustainable sites, water efficiency, energy and atmosphere,
materials and resources, and indoor environmental quality with additional bonus points (10
possible) in the categories of innovation in design and regional priority. Up to 15 points toward
certification can be earned by incorporating a green roof in the building design, with the possible
addition of points for innovative design (Kula, 2005).
Plant Selection for Green Roofs
There are many criteria for selecting plants for installation on a green roof, some of
which are interconnected. It is important to consider the hardiness of potential plant species,
because rooftop plants are exposed to harsh temperature and irradiance extremes, wind, and may
rely solely on precipitation for water (Getter and Rowe, 2006; Getter et al., 2007). Succulents,
such as sedums, are often used in extensive green roofs because of their ability to survive such
conditions. Deeper substrate and more intensive maintenance can enable the use of less drought
tolerant species through greater water retention (VanWoert et al., 2005a) and irrigation. Careful
selection of plant species can ensure survival of individual plants and self-sustaining populations
(Getter and Rowe, 2006), which influence the extent of benefits received from the green roof
(Getter and Rowe, 2006; VanWoert et al., 2005b; Wong et al., 2003).
4
Plant selection may also be dictated by the purpose of the roof project and the desired
roof appearance. If a more natural habitat is the goal of the project, native species may be
preferred. Irrigation is, however, usually necessary not only for establishment of native taxa on
the roof (Monterusso et al., 2005), but for long term survival of individual plants and the plant
community as a whole (Durham et al., 2006; Monterusso et al., 2005), resulting in greater
maintenance needs. Although the survival of native plants on a green roof will depend on local
climatic conditions, drought tolerance of selected species, depth of substrate, and management
practices, there are many examples of native vegetation on green roofs (Arpels et al., 2005). In
recent years, interest in the production of vegetables and herbs on green roofs has been
increasing (Arpels et al., 2005; Loder and Peck, 2004; Whittinghill and Rowe, 2011).
Traditional Uses of Green Roofs
Energy savings and mitigation of the urban heat island (UHI) are two benefits of green
roofs that make them particularly appealing in light of growing concerns about climate change
and greenhouse gas (GHG) emissions. Stormwater retention is also particularly important in
cities where it is difficult to manage the water flowing off impervious surfaces during a storm.
Although other benefits of green roofs are also important, these have had an impact on current
policies in the United States, such as stormwater utilities (Cater and Keeler, 2008, Lipton, 2005).
Green roofs have half the negative environmental impact of conventional roofs over the
lifetime of the roof (Kosareo and Ries, 2007). Energy savings of an extensive green roof as
compared with a conventional roof have been estimated between 1.2 and 8.5 % over the life of
the roof (Saiz et al., 2006; Wong et al., 2003), and between 6 and 87 % reduction in cooling load
during the summer depending on the building height and insulation, and distance of the floor
5
from the roof (Santamouris et al., 2007). Some of this reduction is due to insulation provided by
the additional layers in a green roof, which reduce heat flux through the building roof (Barrio,
1998; Wong et al., 2007). This reduction in heat flux through the roof has also been found on
inverted roofs, where the insulation is placed above the waterproofing membrane (Getter et al.,
2011). Reductions in building energy use result in less environmental loading, fewer SOx and
NOx emissions, and a smaller impact on human health (Kosareao and Ries, 2007).
To help building designers and owners better estimate the energy savings that could be
accomplished through the use of a green roof, researchers have developed an energy calculator
(Bass and Sailor, 2010; GBRL, 2011). The calculator estimates energy use for conventional,
white reflective, and green roofs based on building information. Currently, the calculator is only
calibrated for new construction, without irrigation (GBRL, 2011), but will soon include
retrofitted roofs and estimates of stormwater retention and UHI mitigation (Bass and Sailor,
2010). This calculator could be a valuable tool for maximizing the energy savings of a building.
Secondary energy reduction can also be achieved through mitigation of the UHI effect.
Green roofs have been shown to provide significant roof level temperature reductions (Wong et
al., 2007), which are projected to extend to urban canyon microclimates (Alexandri and Jones,
2008). When green walls are included in urban building designs, further air, surface, and nonvegetated surface temperature reductions are expected (Alexandri and Jones, 2008). These
cooling effects result in urban temperatures more comfortable for human inhabitants, further
reducing the need for indoor cooling (Alexandri and Jones, 2008). In some climates, it is
projected that the need for summer cooling could be eliminated entirely (Alexandri and Jones,
2008).
6
Both direct and indirect energy savings of green roofs contribute to a reduction in carbon
emissions produced during the generation of the electricity used to cool buildings in the summer.
Assuming a 2% reduction in electricity and 9-11% reduction in natural gas consumption (Sailor,
2008), a generic building, and the US average CO2 production for generating electricity and
-2
burning natural gas, one could expect annual savings of 2.3 to 2.6 kg CO2 m , and 0.24 to 0.97
kg CO2 m
-2
2
from electricity and natural gas, respectively (Rowe, 2011). If 1.1 km of flat roofs
(the flat roof area of the Michigan State University in East Lansing, MI, for example) were
greened it is estimated that 3.64 million kg CO2 emissions would be avoided each year (Rowe,
2011). This would greatly contribute to efforts to reduce anthropogenic CO2 emissions, which
were estimated to be 6,822 million mT CO2 Eq for the United States in 2010 (USEPA, 2012).
Stormwater management is a third benefit of green roofs that plays a role in policy and
construction decisions. The city of Portland, Oregon, for example, has incorporated the
stormwater management provided by green roofs into their municipal stormwater management
regulations (Lipton, 2005). Green roofs have been shown to retain between 52.4 and 100% of
precipitation depending on slope, substrate depth, substrate moisture content, evapotranspiration
rates, vegetation, and precipitation event size (Czerniel Berndtsson, 2010; Getter et al., 2007;
Hathaway et al., 2008; Rowe, 2011; VanWoert et al., 2005a). Green roofs also provide a delay
in peak runoff of at least 15 to 30 min, compared to gravel ballast roofs (Hathaway et al., 2008;
VanWoert et al., 2005a). These reductions and delays in stormwater runoff can reduce loads to
municipal stormwater systems, which are often overtaxed during precipitation events (Getter and
Rowe, 2006; Rowe, 2011).
7
Green roofs also improve the quality of stormwater runoff compared with conventional
roofs. Heavy metal concentrations in green roof runoff are lower than those from neighboring
conventional roofs and dependent on drain pipe composition and age of green roof (Czerniel
Berndtsson, 2010; Czemiel Berndtsson et al., 2006; Rowe, 2011). Czemiel Berndtsson (2010)
speculated that heavy metal concentrations in runoff are not reduced, but the total amount of
heavy metals coming off the roof are lower, because of water retention. However, there is some
disagreement about the impact of green roofs on nitrogen levels in runoff (Czemiel Berndtsson et
al., 2006; Hathaway et al., 2008). Some studies have also shown that green roofs increase
phosphorus concentrations in runoff (Czemiel Berndtsson et al., 2006; Hathaway et al., 2008).
Nitrogen and phosphorus levels in runoff are of greater concern on roofs that require the addition
of chemical fertilizers (Czemiel Berndtsson et al., 2006; Emilsson et al., 2007; Rowe et al.,
2006). The impacts of fertilizer addition on stormwater quality can be minimized, if controlled
release fertilizers are used instead of easily dissolved fertilizers (Emilsson et al., 2007).
Emerging Uses of Green Roofs
Carbon Sequestration. In the recent past, there has been growing concern surrounding
global climate changes, focused on temperature increases due to anthropogenic GHG emissions.
The Intergovernmental Panel on Climate Change (IPCC) has published several reports outlining
sources of anthropogenic GHG emissions, changes in emissions levels, and the impacts that
emissions levels have, and are projected to have, on climate. One of the most recent reports
states that CO2 levels have increased by 36% in the last 250 years, half of which took place after
1970 (IPCC, 2007). The National Research Council (2010) has recently stated that atmospheric
CO2 levels reached 396 ppm as of June 2012 (ESRL, 2012), higher than any levels estimated for
8
the last 800,000 years. This increase in GHG emissions has been shown to have a number of
actual and potential impacts on climate including rises in temperature, changes in precipitation
patterns and ice cover, as well as increases in extreme weather events (IPCC, 2007; NRC, 2010).
These reports led to the development of a number of policies regarding GHG emissions.
One of the most notable policies is the Kyoto Protocol, which calls for a reduction in greenhouse
gas emissions to 5% below 1990 emissions levels by the year 2012 (UNFCCC, 1997). The goals
of the Kyoto Protocol have been met with varying success. Annex 1 countries have reduced
emissions by as much as 54.7% *(Latvia) but have also increased emissions by as much as 119.1
% (Turkey) (UNFCCC, 2009). These policies have enabled the creation of emissions trading
markets in which organizations surpassing reduction requirements are able to sell credits to other
companies or organizations (UNFCCC, 1997). Currently, 192 nations and 1 regional economic
integration organization have ratified the Kyoto Protocol (UNFCCC, 2011). For some GHGs,
credits can be quite valuable in markets that trade them. Nitrogen oxide credits, for example,
sold for $2800 to $4000 per ton in 2004 (Clark et al., 2005). One example is the Chicago
Climate Exchange (CCX), an institution that oversees a legally binding cap and trade system for
GHGs in North America (CCX, 2009). The CCX was a two phase program requiring emissions
reductions of 1% per year in the first phase and emissions reductions to 6% below baseline for
all members by 2010 in the second phase. Those unable to meet the reductions must purchase
credits from institutions that surpass reductions (CCX, 2009). In 2011, the Chicago Climate
Exchange Offsets Registry Program was launched to develop a database of verified emissions
reductions (CCX, 2011). The total work of global carbon markets reached US$ 176 billion in
2011 with a total of 10.3 billion T CO2 Eq traded (Kossoy and Guigon, 2012). There are also
9
established trade programs for sulfur dioxide and nitrogen oxides in both Europe and the United
States (Clark et al., 2006).
If companies are to trade carbon credits, they must first have an understanding of the
ability of their landscapes (natural, agricultural, ornamental, at grade, or on rooftops) to sequester
carbon. Carbon sequestration rates on the landscape level can be highly variable and are
dependent on species diversity, plant physiological characteristics, species abundance, and
climate (Kucharik et al., 2003; Matamala et al., 2008; Sandermann and Amundson, 2009; Tilman
et al., 2006). Tilman et al. (2006) found that a low-input high-diversity mix of perennial
herbaceous grassland species provided a greater amount of soil carbon sequestration than mixes
with fewer species or monocultures. They also found increased root biomass with increased
species diversity. Soil carbon losses during the conversion from forests to pasture are lower
when a mix of species, rather than a monoculture, is used (Rhodes et al., 2000).
The type of plants present can impact above-ground biomass, litter quality and quantity,
and root distribution and biomass, which impact carbon concentrations and the role of dissolved
organic carbons (DOCs) in carbon sequestration in soils (Sanderman and Amundson, 2009). In
poplar trees, mean carbon concentrations range from 42% in leaves to 50% in stems (Fang et al.,
2007). This pattern of carbon content has also been shown in olive and peach trees (Sofo et al.,
2005). In a forest, litter is the largest source of DOC, but in a prairie, the biologically active root
zone is the dominant source of DOC (Sanderman and Amundson, 2009). There are also
differences in the ability of C3 and C4 plants to sequester carbon. C4 plants sequester less
carbon than C3 plants; higher proportions of C4 plants in an ecosystem result in less carbon
sequestration by that system (Rhodes et al., 2000).
10
These physiological differences contribute to landscape level changes in carbon
sequestration in both space and time. Variation in aboveground carbon has been attributed to
variations in species composition (Matamala et al., 2008). Increases in soil temperature and
prolonged drought have been shown to cause changes in species composition. Harte et al. (2006)
showed that, a shift from forbs to shrubs resulted in fewer carbon inputs to the soil, but also
lowered decomposition rates because litter shifted to more recalcitrant forms. This species shift
took place under both 2°C warming and drought conditions resulting in 22 and 15% reductions
in soil organic carbon (SOC), respectively. Balesdent et al. (2000) also reviewed studies that
suggested similar responses to warming and drought. These results may have implications for
future carbon sequestration as temperatures and precipitation patterns change because of climate
change.
The age of a landscape can also affect plant physiological characteristics, and therefore
carbon sequestration. Matamala et al. (2008) found that litter, microbial, and root biomass
increased with age in prairie landscapes. As woody plants age, more biomass is allocated to
stems, and less to leaves and roots (Fang et al. 2007), which leads to reductions in litter biomass
with age. Other site conditions, such as soil properties and soil moisture content also influence
carbon sequestration. Irrigation of arid lands has been shown to result in increases of both SOC
and soil inorganic carbon above levels found in native soils (Wu et al., 2008). This increase may
be due to differences between crop and native plant root density and structure. Drying and
rewetting cycles may also be a factor effecting the protection of carbon soil pools, in part
because of deabsorption of SOM (Balesdent et al., 2000). Clay contributes to the protection of
soil carbon pools, by holding SOM within its particles (Balesdent et al., 2000).
11
Management practices impact the ability of natural and agricultural and landscape
systems to sequester carbon (Balesdent et al., 2000; Fang et al., 2007; Wu, et al., 2008).
Numerous studies have shown that cultivation for agriculture causes a drop in soil organic matter
(SOM) (Balesdent et al., 2000). Agricultural land has a lower percent SOC than land that has
been returned to grassland for at least 8 yrs at soil depths of 0 to 5 cm, but not at depths between
5 and 25 cm (Kucharik et al. 2003). Total percent organic matter showed a similar pattern and
both SOC and total organic matter were lower than in adjacent pasture sites. This observation is
in agreement with other studies showing lower microbial and root biomass and SOC in cultivated
fields than in prairie ecosystems (Matamala et al., 2008), and a reduction in carbon when forests
are converted to agricultural lands (Rhodes et al., 2000). Tillage breaks up macroaggregates in
soil, which protect pools of SOM, increasing carbon mineralization (Balesdent et al., 2000). The
addition of crop resides, can add SOM (Wu, et al., 2008). In no-till systems some of this SOM
remains above the mineral soil, an effect which must be taken into account when studying soil
carbon pools (Balesdent et al., 2000). The decay rate of SOM under no-till systems is also
lower, because SOM remains protected within undisturbed soil aggregates (Balesdent et al.,
2000).
Soil organic and inorganic carbon can increase in agricultural soils over many decades;
however, these increases take place at depths of 10 to 60 cm and greater than 150 cm,
respectively (Wu et al., 2008). Restoration of agricultural lands to prairie can also return both
aboveground and SOCs to levels found in remnant prairies. The former can take as little as 14
years, but the latter requires several centuries (Matamala et al., 2008). Carbon residence times
do, however, vary with climate, ecosystem, soil depth, and soil properties. High clay content, for
example, promotes long-term carbon storage via adsorption to clay particles, while sandy soils
12
tend to promote downward transport of dissolved organic carbon (DOC) (Sandermann and
Amundson, 2009). In the short term, changes in SOC levels are minimal, 1 to 10 % of the
carbon pool size, making them difficult to detect. Kucharik et al. (2003) suggest that these
changes in carbon pools would be more easily detected using a soil surface layer between 5 and
7.5 cm, especially when comparing land use types. Organic carbon has also been shown to
increase in constructed landscapes. Getter et al. (2007) found and increase in organic carbon
from 2.33 to 4.35 % over five years in an extensive green roof.
The ornamental horticulture industry comprises a large portion of the United States
economy and a large area of land for both production and planting (Marble et al., 2011). Typical
ornamental planting substrate is pine bark based, with much greater carbon content than field
soils (Marble et al., 2011). This material is then transferred into the ground when planted and if
the plant biomass accumulation is greater than the rate of decomposition then it could be
considered a carbon sink (Getter et al., 2009). Carbon sequestration of urban trees and forests
has been well studied. Urban tree carbon sequestration depends on the tree species used, their
growth rates and tree ages (Stoffberg et al., 2010). Stoffberg et al (2010) estimated that the
115,200 street trees in Tshwane, South Africa, would sequester 200,492 tons of CO2 by 2032,
valued at about 3 million U.S. dollars. Using remote sensing to examine urban forest cover over
time in Syracuse, NY, USA, Myeong et al. (2006) estimated that148,659 tons of carbon were
stored in 1999. They also found a pattern of increased carbon storage between 1985 and 1992 of
1.79 %, but a decrease between 1992 and 1999 of 0.52 %. Myeong et al. (2006) concluded that
Syracuse had not undergone much urbanization in that time frame and was therefore showing
relatively stable carbon storage. When compared to nearby rural forests, urban forests are a
greater sink for CO2 during the growing season, but a greater source of it during the rest of the
13
year (Awal et al., 2010). This difference has been attributed at least in part to higher urban
temperatures (Awal et al., 2010). Forest understory carbon content has also been assessed to a
certain extent. Estimates of the forest understory carbon pool range from 1.0 to 4.8 t C/ha (Smith
et al., 2004). It is however not clear if this would be useful in assessing ornamental landscapes.
Some barriers to putting this knowledge about carbon sequestration into practice have
been discovered. These include a lack of a complete inventory of urban trees, carbon emissions
reductions are not yet a goal of some municipalities, a lack of familiarity with carbon trading
markets among municipal forest managers (Poudyal et al., 2010), inadequate updating of
inventories over time, and a lack of standardized sampling methods (Brown, 2002). There are
however guidelines available to help urban forest managers and those hoping to sequester CO2
though urban tree projects calculate both carbon sequestered and carbon emissions (eg.
McPherson and Simpson, 1999). Current carbon markets also complicate the issue of carbon
sequestration in forests and potentially urban trees and ornamental landscapes. Currently forest
carbon stocks are purchased outright, and considered permanent, limiting flexibility in
management practices and restricting entry into the market by small forest owners (Bigsby,
2009). Limiting management practices puts a large burden of liability on the forest owner in the
event that the land is shown not to contain the carbon sold, for any reason including natural
disaster (Bigsby, 2009). Small forests can change hands frequently and different owners have
differing ideas about what management practices are best (Bigsby, 2009). Management practices
also need to be a factor in the overall assessment of net carbon sequestered. Nowak et al (2002),
suggest a last positive point (LPP), a point in time where carbon emitted to manage a urban trees
exceeds that which is sequestered by those trees. The length of time it takes to reach this point
depends on the extent of management, the extent management relies on carbon emitting tools,
14
and the fate of removed tree material and therefore the rate at which that removed carbon is
returned to the atmosphere. More intensive management and faster return of removed carbon to
the atmosphere results in achieving the LPP in fewer years (Nowak et al., 2002). Ornamental
landscapes change hands frequently and are highly managed for aesthetics using carbon emitting
tools such as lawn mowers and leaf blowers. Extraneous detritus, such as grass clippings, fallen
leaves, and dead flowers, are often removed and changes in landscape design could drastically
alter the carbon stock.
Bibsby (2009) suggests the use of a carbon banking system to address forest management
issues, which could apply to other ornamental landscapes. Under such a system owners of
carbon stocks deposit carbon on a short term basis and carbon individuals or corporations
seeking to offset carbon emissions borrow carbon (Bigsby, 2009). A better understanding of
carbon sequestration in ornamental landscapes incorporating shrubs, perennials and other
ornamental species would be necessary to apply those carbon assets to any carbon market. There
is currently little such information available in the literature (Marble et al., 2000). Another
complicating factor has also been pointed out, who gets the credit for the carbon sequestered?
The pine bark mulch used as potting substrate is an industry byproduct of forestry operations and
Marble et al. (2000) suggest that credit will depend on what the alternative fate of the pine bark
would have been and if the forestry industry has already made a financial gain from the sale of
the pine bark to the ornamental horticulture industry.
Rooftops converted to green roofs offer spaces in which sequestration of CO2 and other
GHGs can take place. As these areas are relatively undisturbed and can have recorded roof life
spans of up to 50 years (Kohler and Keeley, 2005) they are potential short- to moderate-term
sinks. If just 10% of roofs in Chicago, Illinois were green, they are estimated to sequester
15
between 445 and 15267 tons NOx per year, depending on the plant species used (Clark et al.,
2005). Although this sequestration capacity represents only 0.46 to 15.89 % of total emissions
for the city of Chicago, it increases with increasing green roof area and could be used in
emissions trading programs (Clark et al., 2005). Economic benefit from such programs could
offset the high initial cost of installing a green roof, one barrier to their widespread construction
by enabling faster returns on the initial investment (Clark et al., 2005; Clark et al., 2006).
However, little research has been done to quantify the carbon sequestration potential of
green roofs. There has also been limited research on carbon sequestration in ornamental planted
landscapes, which could be a closer approximation to green roof landscapes than the agricultural
or forested landscapes discussed in the literature. This, in combination with the highly variable
ability of species to sequester GHGs (Clark et al., 2005), illustrates the need for further research
that not only quantifies sequestration by a whole roof, but also sequestration by individual
species, in order to develop of species mixes that optimize sequestration.
2
Getter et al. (2009) found that extensive green roofs store an average of 162 g C/m (1.62
t C/ha) in aboveground biomass, with variation due to roof age and substrate depth. Further
examination of belowground biomass and substrate showed that the whole extensive green roof
2
system examined sequestered 375 g C/m (3.75 t C/ha) (Getter et al., 2009). This was in
addition to the initial carbon content of the green roof substrate. Although substrate carbon
content was found to be comparable to that of other ecosystems, the above-ground and root
biomass was found to be lower than expected based on the literature on vascular plants. Getter et
al. (2009) speculated that this may be due to the age of the plants in the study, and differences
between the physiology of succulents used on green roofs and the plants used in previous
16
research on forests and agriculture. Higher temperatures on rooftops may also contribute to
faster oxidation, affecting carbon sequestration. Rugh et al. (2010), have taken another approach
to estimate the carbon sequestration of a large green roof over time. Using the results of Getter
et al. (2009), they grouped the plant species found on the Ford Dearborn truck plant and
estimated whole roof aboveground biomass sequestration from species abundance and coverage
2
data generated in surveys taken in 2009 and 2010. They estimated 194.8 and 195.1 g C/m (1.95
t C/ha,respectively) for spring 2009 and summer 2010, respectively stored in aboveground
2
biomass (Rugh et al., 2010), which were very similar to the 196 g C/m (1.96 t C/ha), value
sampled in 2006 by Getter et al (2009). This suggests that the carbon content of a green roof is
stable over time (Rugh et al., 2010), and there is a limit on the amount of carbon a roof is able to
sequester.
The ability of green roofs to sequester carbon will also be affected by the embodied
energy or carbon of materials used in the construction, and how that might differ from the
conventional roof alternative. Getter et al. (2009) estimated the embodied carbon content of
2
green roofs to be 6.6 kg C/m . This includes typical root barrier, drainage layer, and substrate of
2
heat expanded slate and sand and was 6.5 kg C/ m larger than the estimated embodied carbon
for a traditional roof. This embodied carbon must be accounted for when considering the value
of green roof carbon sequestration, especially considering that it is much greater than the amount
of carbon contained in substrate and plant biomass (Getter et al., 2009; Rugh et al., 2010). Those
estimates do not, however, take into account annual emissions avoided through the energy saving
benefit of green roofs. According to Getter et al. (2009) it will take 9 yrs to offset the carbon
debt of green roof materials using emissions savings.
17
There is also the issue of carbon credit quality. The quality of a carbon credit depends on
a set of criteria, which also vary with organization. Poudyal et al. (2011) compiled several lists
of criteria and found additionally, baseline establishment, use of real or actual emission
reduction, quantification and monitoring, verification, ownership, leakage, permanence,
regionalist, and co-benefits were of importance. The authors found that urban forests often met
these requirements. In ornamental landscapes, permanence is of particular importance. In the
case of urban forests, these trees meet the requirements as they are not harvested and many
municipalities have plans in place to manage tree loss due to disease or natural disaster (Poudyal
et al., 2011). Green roofs have an estimated lifespan of 40 to 60 yrs (Carter and Keeler, 2008;
Lee, 2004). Compare this to the 30 to 40 yr time frame of studies on urban tree survival
(Stoffberg et al., 2010) and green roofs may also be considered to meet the permanence
requirement of a quality credit. Other ornamental landscapes, may not however meet the criteria
of permanence. This may depend on the lifespan of ornamental species used, the ability of
annuals to self-seed and maintain their populations, and the frequency that ornamental areas are
re-landscaped. These issues draw into question the quality of potential carbon credits from
ornamental landscapes and highlight areas of needed research.
Urban Agriculture. Global population, currently at 6.99 billion (USCB, 2012), is
continuing to rise, with a projected increase to 9.3 billion in 2050 (USCB, 2009). This estimate
has resulted in speculation about whether agricultural food production will be able to keep up
with a growing demand (Peters at al., 2009). In 2007 the United Nations Department of
Economic and Social Affairs, Population Division predicted that 50 % of the world’s population
would live in urban areas, and that this percentage would continue to increase (UNDESA, 2007).
The growth of urban centers raises concern about ensuring access to food, both in the ability of a
18
region to produce and ship adequate food supplies to the urban center and in the ability of urban
residents to afford adequately nutritious food (Enete and Achike, 2008; Graefe et al., 2008;
Peters et al., 2009; Vagneron, 2007; van Averbeke, 2007). Many turn to urban agriculture to
address these issues. Urban agriculture is, however, not without problems (Agbenin et al., 2009;
Enete and Achike, 2009; Graefe et al., 2009; Thornton, 2009; Vagneron, 2007; van Averbeke,
2007). Green roof technology is one possible method of urban food production that would
address some of these problems as is discussed further in chapter 1.
19
REFERENCES
20
REFERENCES
Agbenin, J.O., M. Danko, and G.Welp. 2009. Soil and vegetable compositional relationships of
eight potentially toxic metals in urban garden fields from northern Nigeria. Journal of the
Science of Food and Agriculture 89:49–54.
Alexandri, E. and P. Jones. 2008. Temperature decreases in an urban canyon due to green walls
and green roofs in diverse climates. Building and Environment 43:480-493.
Arpels, M., S. Chrisman, H. Sommerfield, J. Towers, E. Berkowitz, G. Brainard, and L. Hickey
(Eds.). 2005. Building Case Studies. p. 24-102 In Earthpledge. Green roofs: Ecological
design and construction. Schiffer Books, Atglen, Pa.
Awal, M.A., T. Ohta, J. Matsumoto, T. Toba, K. Daikoku, S. Hattori, T. Hiyama, and H. Park.
Comparing the carbon sequestration capacity of temperate deciduous forests between
urban and rural landscapes in central Japan. Urban Forestry & Urban Greening 9:261270.
Balesdent, J., C. Chenu, and M. Balabane. 2000. Relationship of soil organic matter dynamics to
physical protection and tillage. Soil & Tillage Research 53:215-230.
Barrio, E.P.D. 1998. Analysis of green roofs cooling potential in buildings. Energy and
Buildings (27):179-193.
Bass, B. and D. Sailor. 2010. Introduction to the new energy calculator. In: Proc. of 8th North
American Green Roof Conference: Greening Rooftops for Sustainable Communities,
Vancouver, BC. 30 Nov-3 Dec 2010. The Cardinal Group, Toronto.
Bigsby, H. 2009. Carbon banking: creating flexibility for forest owners. Forest Ecology and
Management 257:378-383.
Brown, S. 2002. Measuring carbon in forests: current status and future challenges.
Environmental Pollution 116:363-372.
Carter, T. and C.R. Jackson. 2007. Vegetated roofs for stormwater management at multiple
spatial scales. Landscape and Urban Planning 80:84-94.
Carter, T. and A. Keeler. 2008. Life-cycle cost-benefit analysis of extensive vegetated roof
systems. Journal of Environmental Management 87:350-363.
CCX. 2009. Chicago Climate Exchange. Accessed 01 May 2009.
.
21
CCX. 2011. Chicago Climate Exchange. Accessed 15 Nov 2011.
.
Cheney, C. 2005. New York City: Greening Gotham’s Rooftops, p. 130-133. In Earthpledge.
Green roofs: Ecological design and construction. Schiffer Books, Atglen, Pa.
City of Chicago, Department of Environment. 2005a. City Launches Green Roof Grants
Program. Accessed 1 April 2009. .
City of Chicago, Department of Environment and City of Chicago Department of Planning and
Development. 2005b. Chicago: Towards a new standard of green building. P 124-126 In
Earthpledge. Green roofs: Ecological design and construction. Schiffer Books, Atglen,
Pa.
City of Chicago, Department of Community Development. 2006. Green roof improvement fund.
Accessed 1 April 2009.
.
City of Portland, Environmental Services. 2009a. Ecoroof floor area ration bonus option. 1 April,
2009. .
City of Portland, Environmental Services. 2009b. Ecoroof incentive program. 1 April, 2009.
.
Clark, C., P. Adriaens, and B. Talbot. 2006. Probabalistiv economiv analysis of green roof
benefits for policy design. In: Proc. of 4th North American Green Roof Conference:
Greening Rooftops for Sustainable Communities, Boston, MA. 11-12 May 2006. The
Cardinal Group, Toronto.
Clark, C., B. Talbot, J. Bulkley, and P. Adriaens. 2005. Optimization of green roofs for air
pollution mitigation. In: Proc. of 3rd North American Green Roof Conference: Greening
Rooftops for Sustainable Communities, Washington, DC. 4-6 May 2005. The Cardinal
Group, Toronto.
Czerniel Berndtsson, J. 2010. Green roof performance towards management of runoff water
quantity and quality: A review. Ecological Engineering 36:351-360.
Czerniel Berndtsson, J., T. Emilsson, and L. Bengtsson. 2006. The influence of extensive
vegetated roofs on runoff water quality. Science of the Total Environment 335:48-63.
Durhman, A.K., D.B. Rowe, and C.L. Rugh. 2006. Effect of watering regimen on chlorophyll
fluorescence and growth of selected green roof plant taxa. HortScience 41(7):1623-1628.
22
Emilsson, T., J. Czerniel Berndtsson, J.E. Mattsson, and K. Rolf. 2007. Effect of using
conventional and controlled release fertilizer on nutrient runoff from various vegetated
roof systems. Ecological Engineering 29:260-271.
Enete, A.A. and A. I. Achike. 2008. Urban agriculture and urban food insecurity/poverty in
Nigeria: The case of Ohafia, south-east Nigeria. Outlook on Agriculture 37(2):131-134.
Earth Systems Research Laboratory (ESRL). 2012. Trends in atmospheric carbon dioxide.
National Oceanic and Atmospheric Administration. 24 July 2012.
http://www.esrl.noaa.gov/gmd/ccgg/trends/.
Fang, S. J. Xue, and L. Tang. 2007. Biomass production and carbon sequestration potential in
poplar plantations with different management patterns. Journal of Environmental
Management 85:672-679.
Firth, M. and D. Gedge. 2005. London: The wild roof renaissance, p. 117-120. In Earthpledge.
Green roofs: Ecological design and construction. Schiffer Books, Atglen, Pa.
Getter, K.L. and D.B. Rowe. 2006. The role of extensive green roofs in sustainable development.
HortScience 41(5):1276-1285.
Getter, K.L, D.B. Rowe, and J.A. Andresen. 2007. Quantifying the effect of slope on extensive
green roof stormwater retention. Ecological Engineering 31:225-231.
Getter, K.L., D.B. Rowe, J.A. Andresen, and I.S. Wichman. 2011. Seasonal heat flux properties
of an extensive green roof in Midwestern U.S. climate. Energy and Building 43:35483557.
Getter, K.L., D.B. Rowe, G.P. Robertson, B.M. Cregg, and J.A. Andresen. 2009. Carbon
sequestration potential of extensive green roofs. Environmental Science and Technology
43(19):7564-7570.
Graefe, S., E. Schlecht, and A. Buerkert. 2008. Opportunities and challenges of urban and periurban agriculture in Niamey, Niger. Outlook on Agriculture 37(1):47-56.
Green Building Research Laboratory (GBRL). 2011. Green roof energy calculator (v1.1).
Accessed 3 Nov 2011. .
Harte, J., S. Saleska, and T. Shih. 2006. Shifts in plant dominance control carbon-cycle responses
to experimental warming and widespread drought. Environmental Research Letters 1.
DOI 10.1088/1748-9326/1/1/014001.
Intergovernmental Panel on Climate Change. 2007. Climate Change 2007: The Physical Science
Basis. Cambirdge University Press, Cambrige, UK, New York.
23
Kohler, M. and M. Keeley. 2005. Berlin: Green roof technology and development, p. 108-112 In
Earthpledge. Green roofs: Ecological design and construction. Schiffer Books, Atglen,
Pa.
Kosareo, L. and R. Ries. 2007. Comparative environmental life cycle assessment of green roofs.
Building and Environment 42:2606-2613.
Kossoy, A. and P. Guigon. 2012. State and trends of the carbon market 2012. Carbon Finance at
The World Bank. Accessed 25 July 2009.
http://siteresources.worldbank.org/INTCARBONFINANCE/Resources/State_and_Trends
_2012_Web_Optimized_19035_Cvr&Txt_LR.pdf.
Kucharik, C.J., J.A. Roth, and R.T. Nabielski. 2003. Statistical assessment of a paired-site
approach for verification of carbon and nitrogen sequestration on Wisconsin
Conservation Reserve Program land. Journal of Soil and Water Conservation 58(1):5867.
Kula, R. 2005. Green roofs and the LEEDTM green building rating system. P 141-153 In Proc. of
3rd North American Green Roof Conference: Greening rooftops for sustainable
communities, Washington, D.C. 4-6 May 2005. The Cardinal Group, Toronto.
Lee, A. 2004. Life cycle cost analysis- green roofs from an investment perspective
Liptan, T. 2005. Portland: A new kind of stormwater management, p. 121-123. In Earthpledge.
Green roofs: Ecological design and construction. Schiffer Books, Atglen, Pa.
Loder, A. and S.W. Peck. 2004. Green roofs’ contribution to smart growth implementation, p 824. In Proc. of 2nd North American Green Roof Conference: Greening rooftops for
sustainable communities, Portland, OR. 2-4 June 2004. The Cardinal Group, Toronto.
Marble, S.C., S.A. Prior, G.B. Runion, H.A. Torbert, C.H. Gilliam, and G.B. Fain. 2011. The
importance of determining carbon sequestration and greenhouse gas mitigation potential
in ornamental horticulture. HortScience 46 (2):240-244.
Matamala, R., J.D. Jastrow, R.M. Miller, and C.T. Garten. 2008. Temporal changes in C and N
stocks of restored prairie: implications for C sequestration strategies. Ecological
Applications 18(6):1470-1488.
McPherson, G.E. and J.R. Simpson. 1999. Carbon dioxide reduction through urban forestry:
guidelines for professional and volunteer tree planters. United States Department of
Agriculture, Forest Service Pacific Southwest Research Station, General Technical
Report PSW-GTR-171.
Myeong, S., B.J. Nowak, and M.J. Duggin. 2006. A temporal analysis of urban forest carbon
storage using remote sensing. Remote Sensing of Environment 101:277-282.
24
Mikami, T. 2005. Tokyo: Cooling rooftop gardens, p. 113-115. In Earthpledge. Green roofs:
Ecological design and construction. Schiffer Books, Atglen, Pa.
Monterusso, M.A., D.B. Rowe, and C.L. Rugh. 2005. Establishment and persistence of Sedum
spp. and native taxa for green roof applications. HortScience 40(2):391-396.
National Research Council (NRC). 2010. Advancing the Science of Climate Change. The
National Academies Press, Washington DC.
Nowak, D.J., J.C. Stevens, S.M. Sisinni, and C.J. Luley. 2002. Effects of urban tree management
and species selection on atmospheric carbon dioxide. Journal of Arboriculture 28(3):113122.
Peters, C.J., N.L. Bills, A.J. Lembo, J.L. Wilkins and G.W. Fick. 2009. Mapping potential
foodsheds in New York state: A spatial model for evaluating the capacity to localize food
production. Renewable Agriculture and Food Systems 24(1):72-84.
Poudyal, N.C., J.P. Siry, and J.M. Bowker. 2010. Urban forests’ potential to supply marketable
carbon emission offsets: A survey of municipal governments. Forest Policy and
Economics 12: 432-438.
Poudyal, N.C., J.P. Siry, and J.M. Bowker. 2011. Qualty of urban forest carbon credits. Urban
Forestry & Urban Greening 10:223-230.
Rhodes, C.C., G.E. Eckert, and D.C. Coleman. 2000. Soil carbon differences among forest,
agriculture and secondary vegetation in lower montane Ecuador. Ecological Applications
10(2):497-505.
Rowe, D.B. 2011. Green roofs as a means of pollution abatement. Environmental Pollution
159(8–9):2100–2110.
Rowe, D.B., M.A Monterusso, and C.L. Rugh. 2006. Assessment of heat-expanded slate and
fertility requirements in green roof substrates. HortTechnology 16(3):471–477.
Rugh, C.L., K.Y. Liu, K.L. Getter, and D.B. Rowe. 2010. Green roof carbon dynamics: seasonal
biomass and sequestration flux. In Proc. of 8th North American Green Roof Conference:
Greening rooftops for sustainable communities, Vancouver, B.C. 30 Nov-3 Dec 2010.
The Cardinal Group, Toronto.
Saiz, S., C. Kennedy, B. Bass, and K. Pressnail. 2006. Comparative life cycle assessment of
standard and green roofs. Environ. Sci. Technol., 40(13):4312-4316.
Sanderman, J. and R. Amundson. 2009. A comparative study of dissolved organic carbon
transport and stabilization in California forest and grassland soils. Biogeochemistry
92:41-59.
25
Santamouris, M., C. Pavlou, P. Doukas, G. Mihalakakou, A. Synnefa, A. Hatzibiros, and P.
Patargias. 2007. Investigating and analyzing the energy and environmental performance
of an experimental green roof system installed in a nursery school building in Athens,
Greece. Energy 32:1781-1788.
Smith, J.E., L.S. Heath, and P.B. Woodbury. 2004. How to estimate forest carbon for large areas
form inventory data. Journal of Forestry 102(5):25-31. Accessed 18 March 2012.
.
Sofo, A., V. Nuzzo, A.M. Palese, C. Xiloyannis, G. Celano, P. Zukowskyj, and B. Dichio. 2005.
Net CO2 storage in Mediterranean olive and peach orchards. Scientia Horticulturae
107:17-24.
Stoffberg, G.H., M.W. van Rooyen, M.J. van der Linde, and H.T. Groeneveld. 2010. Carbon
sequestration estimates of indigenous street trees in the city of Tshwane, South Africa.
Urban Forestry & Urban Greening 9:9-14.
Thornton, A. 2009. Pastures of plenty?: Land rights and community-based agriculture in Peddie,
a former homeland town in South Africa. Applied Geography 29:12–20.
Tilman, D., J. Hill, and C. Lehman. 2006. Carbon-negative biofuels from low-input highdiversity grassland biomass. Science 314:1598-1600.
Ulrich, R.S. 1981. Natural versus urban scenes: Some psychophysiological effects. Environment
and Behavior 13:523-556.
Ulrich, R.S. 1984. View through a window may influence recovery from surgery. Science
224:420-421.
UNDESA. 2007. World urbanizations prospects, 2007 Edition: United Nations Department of
Economic and Social Affairs (DESA) Population Division -Population estimates and
projections section. 29 Sept 2009. .
United Nations Framework Convention on Climate Change (UNFCCC). 1997. Kyoto Protocol.
Accessed 31 March 2009. .
United Nations Framework Convention on Climate Change (UNFCCC). 2009. National
greenhouse gas inventory data for the period 1990-2007. Accessed 25 July 2012.
.
United Nations Framework Convention on Climate Change (UNFCCC). 2011. Status of
ratification of the Kyoto Protocol. Accessed 15 Nov 2011.
.
26
U.S. Census Bureau (USCB). 2009. International data base: Total midyear population for the
world: 1950-2050. Accessed 3 Oct. 2009.
.
U.S. Census Bureau (USCB). 2012. U.S. and world population clocks. Accessed 1 Feb 2012.
.
U.S. Environmental Protection Agency (USEPA). 2012. Inventory of U.S. greenhouse gas
emissions and sinks: 1990-2010. EPA 430-R-12-001. Accessed 25 July 2012.
.
US Green Building Council (USGBC). 2011. LEED. Accessed 19 Dec 2011.
.
Vagneron, I. 2007. Economic appraisal of profitability and sustainability of peri-urban
agriculture in Bangkok. Ecological Economics 61:516-529.
van Averbeke, W. 2007. Urban farming in the informal settlements of Atteridgeville, Pretoria,
South Africa. Water SA 33(3):337-342.
VanWoert, N.D., D.B. Rowe, J.A. Andresen, C.L. Rugh, R.T. Fernandez, and L. Xiao. 2005a.
Green roof stormwater retention: Effects of roof surface, slope, and media depth. J.
Environ. Qual. 34:1036-1044.
VanWoert, N.D., D.B. Rowe, J.A. Andresen, C.L. Rugh, and L. Xiao. 2005b. Watering regime
and green roof substrate design affect Sedum plant growth. HortScience 40(3):659-664.
Whittinghill, L. J. and D.B. Rowe. 2011. The role of green roof technology in urban agriculture.
Renewable Agriculture and Food Systems. DOI 10.1017/S174217051100038X.
Wong N.H., S.F. Tay, and Y. Chen. 2007. Study of thermal performance of extensive rooftop
greenery systems in the tropical climate. Building and Environment 42:25-54.
Wong N.H., S.F. Tay, R. Wong, C.L. Ong, and A. Sai. 2003. Life cycle cost analysis of rooftop
gardens in Singapore. Building and Environment 38:499-509.
Wu, L., Y. Wood, P. Jiang, L. Li, G. Pan, J. Lu, A.C. Chang, and H.A. Enloe. 2008. Carbon
sequestration and dynamics of two irrigated agricultural soils in California. SSSAJ
72(3):808-814.
27
CAPTER ONE
The role of green roof technology in urban agriculture, by Leigh J. Whittinghill and D. Bradley
Rowe Renewable Agriculture and Food Systems, FirstView Article (August 2011), pp. 1-9
Copyright © 2011 Cambridge University Press. Reprinted with permission.
28
Abstract
Urban agriculture is a global and growing pursuit that can contribute to economic
development, job creation, food security, and community building. It can, however, be
limited by competition for space with other forms of urban development, a lack of
formalized land use rights, and health hazards related to food contamination. The use of
green roof technology in urban agriculture has the potential to alleviate some of these
problems, without adversely affecting the benefits provided by urban agriculture. It
would not only enable the use of land for development and agriculture, but may facilitate
the formation of formal space and water use agreements and enable redistribution of
ground level resources among urban farmers. This could decrease the use of
contaminated land and water at ground level and alleviate health concerns. Before green
roof technology can be incorporated into urban agriculture on a larger scale, installation
costs must be reduced, roof weight limitations should be assessed, and appropriate
management practices developed which will ensure that the benefits of green roofs, such
as energy savings and storm water management, are still provided to urban communities.
Introduction
In recent years, the importance of green space in urban areas has been changing. A new
vision of urban centers incorporates more green space, such as parks (Nowak, 2006), and a more
human friendly environment mixing traditional urban centers of industry, commerce and
residence with food production (Nugent, 2002; de Zeeuw et al., 1999). Implementation of this
vision has been facilitated by the introduction of more environmentally friendly technologies and
the introduction of policies and programs which that promote their use (Cheney, 2005; City of
29
Chicago, 2006; City of Portland, 2009; Kula, 2005; Lipton, 2005). One such technology being
incorporated into development is green roofing.
Definition of green roofs. Green roofing is a technology enabling growth of vegetation on
rooftops, effectively replacing green space lost during building construction. Conventional green
roofs generally consist of a number of layers including a root barrier to prevent damage to the
underlying structure; a drainage layer to facilitate the removal of excess water; a filter fabric to
prevent the drainage layer from clogging with media; growing media; and vegetation (Getter and
Rowe, 2006). Others may be composed of growing modules or vegetated mats. Green roofs
vary in depth of growing media and vegetation, but are generally broken up into two categories:
extensive (those with less than 15 cm of media) and intensive (those with more than 15 cm of
media) (Kula, 2005). Extensive roofs are usually planted with ground cover or succulent species
that require little maintenance after establishment, while intensive roofs can support herbaceous
perennials, shrubs and even trees (Dvorak and Voder, 2010; Getter and Rowe, 2006) but
typically require continued inputs. Design and plant selection depend on the purpose of the
project and the environmental benefits to be achieved. Uses of green roofs range from functional
storm water and energy management roofs, to park like amenities open to the public, to food
production.
Factors determining plant survival include media composition and depth (Getter and
Rowe, 2009; Rowe et al., 2006), incoming solar radiation (Getter et al., 2009), climate, and most
importantly soil moisture (Durhman et al., 2006; Emilason, 2008; Monterusso et al., 2005).
Insufficient moisture can be remedied by altering media composition and depth or with irrigation
if available. Growing media used on commercial green roofs are often engineered and
comprised mostly of lightweight materials such as heat expanded slate or shale (Getter and
30
Rowe, 2006). Media composition depends on manufacturer and available materials, but it is
designed to be lightweight while maintaining the ability to support plant life. Organic matter
content may vary and is beneficial for plant growth, but when it decomposes it may leach
nutrients resulting in runoff water quality issues and is inconvenient to replace. Economics
dictate that substrate composition will depend on materials that are locally available and can be
formulated for the intended plant selection, climatic zone, and anticipated level of maintenance.
Benefits of green roofs. Use of green roofs in urban development has been shown to
provide a number of benefits including reduced air and noise pollution, carbon sequestration,
increased habitat and biodiversity, increased roof lifespan, storm water retention, energy savings
and mitigation of the urban heat island (Alexandri and Jones, 2008; Barrio, 1998; Czemiel
Berndtsson, 2010; Getter and Rowe, 2006, Getter et al., 2007; Getter et al., 2009; Loder and
Peck, 2004; Monter. Computer modeling predicts that an increase in green roof area, as could
take place with the technology’s incorporation into urban agriculture, will amplify these benefits
(Bass et al., 2003). Moreover, the introduction of green roofs to city development plans could
promote infill development (the redevelopment of vacant areas within urban centers) and reduce
spending on the development of new infrastructure such as roads and sewer lines (Loder and
Peck, 2004). Although infill development largely benefits municipalities and companies,
expansion of the green roof industry would increase employment and economic growth in urban
centers benefiting populations which typically turn to urban agriculture.
Energy savings (Fang, 2008; Sailor, 2008; Santamouris et al., 2007; Wong et al., 2007)
and mitigation of the urban heat island (Alexandri and Jones, 2008; Bass et al., 2003; Harlan et
al, 2006; Johnson and Wilson, 2009; Memon et al., 2008; Wong et al., 2003) in particular have
great practical benefit to poorer communities who are most likely to benefit from urban
31
agriculture. Green roofs can reduce energy consumption of a building from 2 to 39% (Sailor,
2008; Santamouris et al., 2007) and between 12 and 87% for the top floor (Santamouris et al.,
2007) during the summer. These reductions are primarily due to reducing the amount of direct
solar radiation that reaches the roof and the amount of heat transferred into the building (Fang,
2008; Wong et al., 2007). The extent of the reduction is dependent on the extent of vegetative
cover on the roof (Fang, 2008; Sailor, 2008; Wong et al., 2003), the existence of other insulation
(Santamouris et al., 2007), the thickness of the green roof media, irrigation of the roof and
climate (Sailor, 2008). Energy savings during the winter are negligible in warm climates, but can
be seen in cooler climates (Sailor, 2008; Santamouris et al., 2007). Energy savings in very
humid climates are also expected to be lower due to reduced evapotranspiration and the
associated cooling effects (Alexandri and Jones, 2008).
Green roofs have been shown to reduce ambient temperatures (Wong et al., 2007), an
effect which is projected to increase with increasing roof surface area and the inclusion of green
walls (plants grown on walls using a variety of training and planting systems) in urban
development (Alexandri and Jones, 2008; Bass et al., 2003). Modeling predicts a corresponding
reduction in urban temperatures and increase in thermal comfort (Alexandri and Jones, 2008;
Bass et al., 2003). The end result of reduced temperatures and therefore energy use is a
reduction in the use of conventional air-conditioning (Alexandri and Jones, 2008; Santamrouris
et al., 2007). This would benefit low income neighborhoods for two reasons. Urban
neighborhoods with higher summer temperatures than surrounding urban areas are correlated
with lower incomes, higher poverty rates (Harlan et al., 2006) and impoverished individuals aged
65 and older (Johnson and Wilson, 2009) in cities such as Phoenix, AZ and Philadelphia, PA.
These individuals are more at risk to environmental hazards such as extreme heat events due to a
32
lack of resources enabling them to cope with the hazards, such as air-conditioning (Harlan et al.,
2006). Implementation of green roofs would not only reduce energy bills in such neighborhoods,
but could also reduce the occurrence of extreme heat and the need to air-condition in the summer
time, freeing up limited funds for other uses and improving the quality of life of individuals in
these at risk areas and populations.
Figure 1.1. Vegetable production on a green roof in East Lansing, MI
Food production in an urban setting. Food production can be added to the benefits
provided by green roofs (Loder and Peck, 2004; Arpels et al., 2005) (Figure 1.1) and expanded
through the technologies incorporation into urban agriculture. Urban agriculture is defined as
horticultural, agricultural, or farming activities carried out on plots of land in and around urban
33
centers (Enete and Achike, 2008; Graefe et al.., 2009; Vagneron, 2007). Individuals in urban
centers around the world participate in urban agriculture for reasons, such as poverty,
unemployment, food insecurity (Nugent, 2002; van Averbeke, 2007), high prices of market food,
income or asset diversification and supplementary employment (Nugent, 2002). These
motivational factors (Enete and Achike, 2008; Graefe et al., 2009; Hu and Ding, 2009; Nugent,
2002; Peters et al., 2009; Vagneron, 2007; van Averbeke, 2007; de Zeeuw et al., 1999);
limitations such as land availability, land use and ownership rights, physical and economic
access to inputs and potential food contamination (Agbenin et al., 2009; Enete and Achike, 2008;
Graefe et al., 2009; Thornton, 2009; Vagneron, 2007; van Averbeke, 2007); and geographic and
climatic factors unique to each urban center shape urban agriculture (Enete and Achike, 2008;
Nugent, 2002; Vagneron, 2007; de Zeeuw et al., 1999). It is likely that a number of these
limitations could be alleviated through the use of green roofs, while maintaining the benefits
expected by urban farmers. Currently, policy is under reform in a number of cities worldwide
resulting in accommodations for urban agriculture (de Zeeuw et al., 1999). These policy changes
present an opportunity and could be guided to include farming on green roofs and expedite the
inclusion of green roofs and other alternative technologies in urban farming with substantial
benefit to urban populations.
Despite the possible benefits from incorporating green roof technology into urban
agriculture, there are a number of potential issues which must be addressed. These include
installation and maintenance costs, weight limitations, media composition and depth, cultural
practices, potential water quality issues of effluent, and how food production would influence the
other known benefits attributed to green roofs. These are not only factors that may limit the use
of green roofs, but would also limit their viability for widespread use in urban agriculture.
34
Further research and innovation may present solutions to these problems. The goal of this review
is to examine some of the possible benefits and barriers to incorporating green roof technology
into urban agriculture. In doing so we shall first examine the current state of urban agriculture
and then introduce potential benefits or limitations of using green roof technology. Due to the
lack of published studies on the subject we will also discuss future research needs. We have
therefore broken the review up into sections containing common themes; economic benefits and
food security, economic barriers, access to resources and policy, human health, and
environmental health concerns.
Economic improvement and food security
Economic development brought about by participation in urban agriculture comes in a
variety of forms including the supplementation of family income, job creation, and freeing up
funds previously used to purchase food. Crop choice and scale affect the extent to which urban
agriculture contributes to the income of a household (Graefe et al., 2009; Nugent, 2002;
Vagneron, 2007; van Averbeke, 2007). Rice, for example, is a staple in many parts of the world
and can provide income security for an urban farmer’s household (Vagneron, 2007), but
production of vegetables may yield higher market prices (Graefe et al., 2009; Vagneron, 2007).
Animal husbandry, another form of urban agriculture, can provide high profits (Graefe et al.,
2009; Nugent, 2002; Vagneron, 2007), but may require much higher investments (Vagneron,
2007). In some cases social capital can be generated by a household, by giving away food that
could not be sold (Nugent, 2002). The impact of urban agriculture on employment is highly
variable, depending on the economic status of urban farmers (Graefe et al., 2009; Nugent, 2002).
For households in both developing and developed countries that do not produce food for sale, or
35
sell only their excess produce, urban agriculture frees up funds for other uses (Enete and Achike,
2008; Nugent, 2002; Vagneron, 2007; van Averbeke, 2007). The prevalence of urban agriculture
increases when poverty increases and when costs of purchasing food surpass that of growing it
(Nugent, 2002). This can be an important measure in stretching household budgets, allowing for
the purchase of other items (Nugent, 2002; van Averbeke, 2007) or some economic freedom for
women where household budgets are male-controlled as was found in Pretoria, South Africa (van
Averbeke, 2007). In addition, economic concerns are an incentive for consumers who assume
that purchasing local produce increases economic returns to local farmers through shortened
supply chains and better market accessibility (Hu and Ding, 2009; Peters et al., 2009).
Food security, the second major driver of urban agriculture, is affected by both quantity
and quality of food available to a household. Even in locations where urban agriculture does not
contribute significantly to employment, food security is of major concern to urban farmers
(Nugent, 2002). Food in the U.S. is shipped 2080 km (1300 mi) from to consumer, but this
could be reduced to 49 km with re-localization of the food (Peters et al., 2009). It has been
estimated that under ideal conditions the agricultural products of the entire state of New York
could not supply the agricultural needs of more than 55% of New York City (Peters et al., 2009).
This suggests that providing adequate food supplies for urban centers with growing populations
may not even be possible on a regional scale and costs associated with shipping may affect the
food security of the urban poor. Producing agricultural goods within urban centers is one
method of reducing the ecological footprint of urban centers (Peters et al., 2009; de Zeeuw et al.,
1999) and ensuring urban dwellers access to food. Food insecurity, or the lack of access to
adequate food for an active and healthy life (Alexandri and Jones, 2008) is not just a problem in
the developing world, but in the United States as well (Enete and Achike, 2008; Nugent, 2002;
36
Widome et al., 2009) (Figure 1.2). Food insecurity can be temporary or chronic (de Zeeuw et al.,
1999) and is associated with a variety of problems in adolescents, who are at higher risk than
young children (Widome et al., 2009). A perceived or actual need to improve food security and
a lack of ability to rely on food from rural areas can result in the use of urban agriculture (Graefe
et al., 2009; de Zeeuw et al., 1999), which has been shown to improve the quantity and quality of
food available to low income urban households under a variety of conditions (Enete and Achike,
2008; Graefe et al., 2009; Nugent, 2002; Widome et al., 2009; de Zeeuw et al., 1999).
Figure 1.2. . A community garden in Detroit, MI.
37
Figure 1.3. An herb garden on a green roof in Grand Rapids, MI.
Green roofs are already utilized to improve the economic circumstances and food security
of urban farmers (Figure 1.3). EcoHouse, in St. Petersburg, Russia is an example of a rooftop
garden project which provides jobs to and increases cash flow among individuals living within
the apartment complex (Arpels et al., 2005). The project also provides those residents with a
reliable source of vegetables (Arpels et al., 2005). Another example is the green roof of the
Fairmont Hotel in Vancouver, a portion of which is devoted to a kitchen garden, saving the hotel
approximately $30,000 a year (Loder and Peck, 2004). The rooftop garden on Earth Pledge’s
New York office is used not only as a source of food, but as a promotional tool for the group’s
organic local produce campaigns (Cheney, 2005; Loder and Peck, 2004). Similar community
garden projects have been developed in other cities, such as the Multnomah County Green Roof
Project in Portland, OR (King, 2004) and several community scale gardens in Chicago, IL
38
(Coffman and Martin, 2004). A green roof at Trent University in Peterborough, Ontario is
producing vegetables for a local restaurant, The Seasoned Spoon Café, which was started as a
healthy fast food alternative (Blyth and Menagh, 2006).
Economic barriers
Economic barriers to urban agriculture include inadequate access to or knowledge about
markets (Nugent, 2002) and insufficient labor (Bass et al., 2003; Nugent, 2002), inputs such as
fertilizers (Vagneron, 2007), quality seeds (Graefe et al., 2009), and credit or subsidy for startup
costs or inputs (Enete and Achike, 2008; Graefe et al., 2008; Nugent, 2002; Vagneron, 2007),
the latter three of which are of more concern in developing nations. These barriers are due to
limited resources of urban farmers and will not be affected by the introduction of green roof
technology to urban agriculture. An additional barrier introduced by the use of green roof
technology is the cost of green roof installation and maintenance.
2
Installation of a green roof can be $32/m more expensive than a conventional roof for
roof structure alone (Wong et al., 2003). Installation of green roof systems can vary from two to
six times more expensive than conventional roof systems depending on the design of the roof
system (Wong et al., 2003). Factors that impact the cost of a green roof include ease of access
for installation, structural integrity of the building, type of drainage system, depth and
composition of media, inclusion of an irrigation system and the use of a modular, mat or
conventional built-up continuous roof system (Rowe and Getter, 2010). Maintenance costs of a
green roof also depend on roof design, as intensive roofs tend to require more care than extensive
roofs. Maintenance of the roofing layers themselves is comparable to that of a conventional roof
due to the longer life span of green roofs (Wong et al., 2003).
39
Tapping into incentive programs such as those used in Portland, OR (City of Portland,
2009) and Chicago, IL (City of Chicago, 2006) which provide reductions in storm water removal
fees and grants to help subsidize installation may help. Other programs, such as those geared
toward improving the availability of fresh fruits and vegetables in urban centers and improving
healthy eating habits in urban youth may also be sources of funding. It is also possible that
locally made materials could be used to construct green roofs, reducing the cost of installation,
but generation of policy promoting the use of green roofs and subsidizing their installation will
be of greater importance in low income areas than in high income areas. Evaluation of locally
available materials would also be necessary to determine both their suitability and their impact
on green roof installation costs.
Access to resources and policy
Availability of land, especially land of adequate quality, is the main obstacle affecting
urban agriculture (Nugent, 2002; de Zeeuw et al., 1999). Land scarcity and uncertainty in
maintaining access to available land are due to competition with other development uses,
primarily building construction (Graefe et al., 2009; Nugent, 2002; Vagneron, 2007). These
other uses are often more economically profitable and are therefore preferred by land owners
(Thornton, 2009; van Averbeke, 2007; de Zeeuw et al., 1999). Land use and investments in
urban agriculture by urban farmers are impacted by resource use rights. Under current systems
there are often no formal leasing agreements between land owners and urban farmers cultivating
vacant lots (Nugent, 2002). Rights of urban farmer are often minimal (de Zeeuw et al., 1999),
uncertain (Thornton, 2000), and frequently transient due to changing land uses and termination
of informal use agreements (Thornton, 2009; van Averbeke, 2007). Lack of formal agreements
40
over water use rights has led to conflict between municipalities and urban farmers (van
Averbeke, 2007).
Incorporation of green roofs into new development would increase the potential
agricultural area and remove competition with urban development that reduces willingness of
urban farmers to invest in urban agriculture (Nugent, 2002; Vagneron, 2007). Currently, flat
rooftops comprise as much as 85% of the roof area in downtown and commercial areas (Carter
and Jackson, 2007). In larger urban areas this could add up to a great deal of space and potential
for productive use if these existing roofs were retrofitted into green roofs. Buildings must,
however have the structural integrity to support the added weight in a worst case scenario,
regardless of whether roofs are designed new or retrofitted (Kortright, 2001). Some existing
roofs may not be suitable for retrofitting without considerable costs incurred in structural
2
2
support. Many flat roofs have load capacities of only 146 kg/m (30/ft ), which could be
exceeded by as little as 7.6 cm (3 in) of growing media (Dillion, 2010). This means that the flat
roof area of existing buildings which could be used for urban agriculture is not accurately
represented by the flat roof area of a city. More information on what roofs can support the
additional weight of a green roof and the minimum depth of media necessary for agricultural
production will enable more accurate estimates of how much roof area could be used. Despite
these limitations, land owners could take advantage of this potential, enabling them to utilize
more profitable development and then generate secondary profits through rental agreements with
urban farmers.
Development of flat roof space into agriculturally productive areas could facilitate
formalization and standardization of rental, leasing, or use agreements between land owners and
urban farmers. Access to roof space is limited and would require urban farmers to negotiate with
41
building owners to gain access to the green roof space. This would be a reversal of the current
use of vacant lots for urban agriculture, whose absent owners are unaware of agricultural
activities or unwilling to take measures to keep urban farmers off the land (Nugent, 2002; de
Zeeuw et al., 1999). Although this could create problems for urban farmers if green roof owners
are unwilling to rent the space due to zoning or building code issues that might arise (Sutton,
2009, UNDESA, 2007), it could also empower urban farmers. Formal, legally binding use
arrangements would grant urban farmers recourse should the green roof owner break the
agreement. Such formal and empowering leasing agreements could also encourage farmers to
increase investments in urban agriculture, increasing productivity and food security. Urban
farmers with informal arrangements do not currently have this level of power and security
(Thornton, 2009; van Averbeke, 2007; de Zeeuw et al., 1999). Formalized rental and leasing
agreements could easily be extended to include access to the buildings water supply. This would
be greatly beneficial to those farmers who have expressed willingness to pay for clean water
where no clean water source currently exists (Graefe et al., 2009), but will increase the costs of
farming for most urban farmers. Rainwater capture from an unused portion of the roof may also
provide an added source of clean water for irrigation.
There are however, two additional potential outcomes of rental agreements for the use of
green roof space. First is the exclusion from farming and water resources of resource poor urban
farmers unable to pay rent for green roof space. Second is the reallocation of ground level space
and water sources currently used by farmers able to pay for rental agreements. The former could
result in greater problems associated with poverty and food security in urban areas, but the latter
could grant a larger number of urban dwellers access to land and water and therefore the
economic opportunities and additional food security provided by urban agriculture. If the latter
42
is the outcome, it would mean a better quality of life for a greater number of urban dwellers.
This will be of particular importance as populations become increasingly urban both worldwide
and in the United States (USCB, 2009; van Averbeke, 2007; Zhuang et al., 2009) and doubts
about the ability of rural areas to agriculturally support these growing urban populations also
increase (Peters et al., 2009; van Averbeke, 2007; Widome et al., 2009).
Human health concerns
Access to fertilizers is especially important as space limitations in urban agriculture
require more intensive farming and greater fertilizer use per area than rural areas (Enete and
Achike, 2008). Often resource poor urban farmers will use inexpensive and easily accessible
fertilizers, such as manures or municipal wastes, which can lead to an increase in soil heavy
metal and pathogen concentrations (Agbenin et al., 2009; Enete and Achike, 2008; Graefe et al.,
2009; Sharma et al., 2009; Srinivas et al., 2009). Heavy metal and pathogen contamination of
food is the primary human health concern associated with urban agriculture. Sources of
contamination include soils in which crops are grown, water used for irrigation, and air
pollutants. In many cases the land most readily available to urban farmers is contaminated with
heavy metals from a variety of industrial and mining sources, which can lead to contamination of
the agricultural products (Agbenin et al., 2009; Hu and Ding, 2009; Srinivas et al., 2009). In
addition, resource poor urban farmers often cannot afford to pay for clean irrigation water even if
a source exists (Graefe et al., 2009; Nugent, 2002; Vagneron, 2007). Atmospheric deposition of
contaminants during production, transportation and marketing of produce also leads to elevated
levels of heavy metals (Hu and Ding, 2009; Sharma et al., 2009; Vousta et al., 1996; Yang et al.,
2009).
43
The extent to which heavy metal contamination affects safety of vegetables depends on
several different factors, including the vegetable species, the part of the plant which is eaten, and
the type of heavy metal (Arora et al., 2008; Sharma et al., 2009; Srinivas et al., 2009; Yang et al.,
2009). On average, fruit vegetables accumulate lower quantities of heavy metals than leafy or
root vegetables (Arora et al., 2008; Srinivas et al., 2009; Yang et al., 2009). Leafy vegetables are
a major source of heavy metal dietary intake (Sharma et al., 2009), due to high rates of
translocation, transpiration and growth as well as high surface area in close proximity to
contaminated soil and irrigation splash (Srinivas et al., 2009). High surface area vegetables, such
as cauliflower (Vousta et al., 1996), and those that spend more time in the field (Yang et al.,
2009) are also known to accumulate higher concentrations of heavy metals through atmospheric
deposition. Dietary intake of heavy metals can lead to accumulation in the human body because
they are non-biodegradable (Chambria and Moyo, 2009), causing their negative effects to
become apparent only after years of exposure (Chambria and Moyo, 2009; Sharma et al., 2009).
Among resulting health problems are a variety of cognitive disruptions (Chambria and Moyo,
2009; Emilsson et al., 2007), nervous, cardiovascular (Emilsson et al., 2007; Srinivas et al.,
2009; Vousta et al., 1996), kidney, bone and liver (Srinivas et al., 2009; Vousta et al., 1996)
diseases as well as cancer (Chambria and Moyo, 2009). Although these are more common
problems in developing countries, contamination of food produced in urban areas by heavy
metals, such as lead, can take place in developed countries.
The use of green roofs in urban agriculture also has the potential to reduce health
concerns. Green roofs have the potential to reduce use of contaminated land in urban agriculture
due to the nature of their construction. In most cases, the media in which vegetables would be
grown on green roofs is engineered instead of using local soils, so initial contamination will be
44
minimal. Green roof media are also less likely to accumulate heavy metals than ground soils due
to high permeability and low cation exchange capacity which results in leaching of nutrients
(Czemiel Berndtsson et al., 2006) and heavy metals (FLL, 1995) into runoff water. This
tendency for leaching would reduce the likelihood of vegetable contamination on green roofs if
contaminated sources of water or fertilizers are used. Rental and water use agreements will also
facilitate more wide-spread use of uncontaminated water sources for irrigation. Atmospheric
deposition of contaminants may also be reduced during the production phase. It has been
suggested that distance from the source of pollutants impacts the extent of heavy metal
contamination due to atmospheric deposition (Vousta et al., 1996). Most green roofs are several
stories high, increasing the distance between crop production and such sources of pollution as
major roadways and highways.
In addition to food contamination, concerns about urban agriculture include health
problems due to improper handling of agrochemicals and urban waste, a potential increase in
pests such as rodents and flies which can contribute to the spread of diseases and the
transmission of diseases from livestock to humans due to improper animal husbandry techniques
(de Zeeuw et al., 1999). Although these concerns may be avoidable through proper practices,
they promote negative perceptions of urban agriculture. Formalized leasing agreements may
provide greater oversight of agrochemicals and urban waste used in urban agriculture. Leasing
agreements could include specifications for what, if any, agrochemicals or urban wastes can be
used on the green roof, how they should be stored and used, and the consequences for the urban
farmer if the specifications are not followed resulting in human injury or a health hazard. In
some countries organizations may already be in place which could monitor such agreements,
such as the Occupational Safety and Health Administration (OSHA) in the United States. It is
45
unlikely that the use of green roofs in urban agriculture will affect the keeping of large livestock
which would be impractical on rooftops.
Environmental health concerns
Despite the costs, urban farmers are currently producing vegetables on green roofs in
natural soils and composts at a media depth 17.8 to 45.7 cm (7 to 18 in) deep (GRC, 2011). This
practice potentially creates several problems including the added weight to the roof, consistency
of growing media, potential nutrient loads polluting effluent that discharges into our waterways
from fertilizers and as compost decomposes, and the logistical practicality of adding compost
every year on a roof several stories above the ground.
Water quality of runoff is another concern as nutrient leaching could cause problems
downstream. Composition of the growing media is one aspect of this problem. Most commercial
green roof media are formulated within the guidelines of the German FLL
(Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau) standards (Hathaway et al.,
2008). Years of experience has resulted in media that possess the chemical and physical
properties to support plants, yet are light weight, coarse enough to allow drainage at shallow
depths, can be replicated, and limit nutrient runoff when guidelines are followed. Natural soil is
always variable and the addition of compost can lead to runoff quality issues. Even so,
commercial green roof media generally contain about 15 to 20% organic matter which can result
in nutrient loading (Czemiel Berndtsson, 2010; Rowe, 2011; USEPA, 2009). Researchers in
North Carolina (USEPA, 2009) found that concentrations of both N and P decreased with
decreasing percentages of compost in the media. These results emphasize the point that growing
media can have an immense effect on the quality of effluent. In addition, applications of
46
fertilizers and pesticides to ensure plant growth can be very detrimental to water quality
(Czemiel Berndtsson et al., 2006; Rowe, 2011). This is especially true for soluble fertilizers
applied in liquid form (Czemiel Berndtsson et al., 2006).
Studies performed on green roofs with conventional media have shown that nutrient
leaching can initially be a problem, but overall, they can have a positive effect on water quality
(Heilman et al., 1996; Rowe, 2011). The initial nutrient load likely is due to decomposition of
organic matter that was incorporated into the original mix. Once established, and the organic
matter reaches an equilibrium, vegetation and substrates can improve water quality of runoff by
absorbing and filtering pollutants (Rowe, 2011). The effect of plants and their root systems was
evident when effluent from an unplanted green roof containing media had higher concentrations
and totals of N and P than effluent from planted roofs (Heilman et al., 1996).
There is also the question of how much fertilizer is necessary to maintain agricultural
productivity in green roof media. Experimentation has shown that non succulents grown on
green roofs require either additional organic matter in the media, or fertilization (Rowe et al.,
2006). The relatively high levels of fertilizer that may be necessary to produce vegetables could
lead to high levels of nutrient leaching. Which begs the question, are we trading the benefits of
local food production for decreased water quality? If nutrient loading does turn out to be a
problem, then green roofs could be coupled with other low impact development practices such as
rain gardens and bioswales (landscaping techniques designed to manage storm water), although
these practices are not always possible in dense urban settings. This highlights the need to
develop and use green roof growing media and cultural practices that minimize leaching of
nutrients while still providing adequate physical and chemical properties for plant growth.
47
Conclusions
In addition to previously mentioned research needs, there are several areas where
research on the use of green roof technology in urban agriculture is necessary before wide scale
use of the technology can be implemented. First, determination of what crops are suited to
growth in green roof media will be necessary. Little is known about how growing vegetables on
green roofs will impact the environmental benefits provided by green roofs. Many of the benefits
are directly related to the amount of coverage achieved by the vegetation and the leaf area of the
vegetation (Heilman et al., 1996). The coverage that would be achieved by vegetables will be
very different than that of the ground covers and perennials traditionally used on green roofs
because they are typically cultivated in rows. In addition, vegetable gardens would be replanted
every year, whereas, typical green roofs are populated with perennial species. Research on how
this difference will impact energy savings and storm water retention, for example, will enable
better assessment of this use of green roofs in areas where these benefits are of particular
importance.
The effects of other environmental factors on crops, such as exposure to higher winds,
should be determined for optimum crop selection. Although pollinators have been seen and kept
on green roofs, an understanding of the efficiency and quality of pollination of vegetable plants
on green roofs would enable better decision making about which crops to grow and the necessity
of bee keeping on vegetable growing green roofs. Finally, economic evaluation of different
crops may generate more information on how much economic impact this form of food
production could have on both a small and large scale.
The incorporation of green roof technology into urban agriculture maintains the
economic and food security benefits of urban agriculture while eliminating some of the many
48
difficulties faced by urban farmers around the world. The ideal case, where formalized use
agreements with building owners and oversight by municipal authorities ensures greater space
availability and healthier produce is however only possible through the cooperation of all parties
involved, something which may be difficult in areas where urban agriculture is viewed in a
particularly negative light. The formalization of use rights required by use of green roof space
by urban farmers represents an opportunity for farmers to achieve guaranteed access to quality
land and irrigation water, providing security for their agricultural pursuits. For land and building
owners, the formalization of use rights represents an opportunity to achieve greater economic
success and some degree of oversight over the activities taking place. This combination of
economic opportunity and oversight may have the added benefit of improving land and building
owners’ attitudes toward urban agriculture, which could expedite policy reform. Municipal
involvement will enable new insights into the benefits of urban agriculture and understanding the
ways in which its negative impacts can be minimized.
The process, though difficult, could be made easier through the establishment of policy
friendlier to urban agriculture, incentive and subsidy programs for the installation of green roofs,
and research into reducing the initial cost green roofs and minimizing the inputs necessary for
productive agriculture on green roofs. The resolution of these issues will further enable a future
in which urban areas are greener and healthier places to live. This future could utilize ideas
about development that incorporate green space in the forms of green roofs, parks or agricultural
plots, enabling a closer connection with nature and the production of food with the benefit of
increased food security, especially for the urban poor.
49
REFERENCES
50
REFERENCES
Alexandri, E. and P. Jones. 2008. Temperature decreases in an urban canyon due to green walls
and green roofs in diverse climates. Building and Environment 43:480-493.
Agbenin, J.O., M. Danko, and G. Welp. 2009. Soil and vegetable compositional relationships of
eight potentially toxic metals in urban garden fields from northern Nigeria. Journal of the
Science of Food and Agriculture 89:49-54.
Arora, M., B. Kiran, S. Rani, A. Rani, B. Kaur, and N, Mittal. 2008. Heavy metal accumulation
in vegetables irrigated with water from different sources. Food Chemistry 111:811-815.
Arpels, M., S. Chrisman, H. Sommerfield, J. Towers, E. Berkowitz, G. Brainard, and L. Hickey
(Eds.). 2005. Building Case Studies. p. 24-102 In Earthpledge. Green roofs: Ecological
design and construction. Schiffer Books, Atglen, Pa.
Barrio, E.P.D. 1998. Analysis of green roofs cooling potential in buildings. Energy and
Buildings (27):179-193.
Bass, B., E.S. Krayenhoff, A. Martilli, R.B. Stull, and H. Auld. 2003. The impact of green roofs
on Toronto’s urban heat island. In Proc 1st North American Green Roof Conference:
Greening Rooftops for Sustainable Communities, Chicago, IL. 29 -30 May 2003. The
Cardinal Group, Toronto.
Blyth, A. and L. Menagh. “From rooftop to restaurant – A university café fed by a rooftop
garden.” City Farmer, Canada’s Office of Urban Agriculture. 16 Nov 2006. Accessed 19
April 2010. .
Carter, T. and C.R. Jackson. 2007. Vegetated roofs for stormwater management at multiple
spatial scales. Landscape and Urban Planning 80:84-94.
Cheney, C. 2005. New York City: Greening Gotham’s Rooftops, p. 130-133. In Earthpledge.
Green roofs: Ecological design and construction. Schiffer Books, Atglen, Pa.
Chimbira, C. and D.Z. Moyo. 2009. The effect of single and mixed treatment of lead and
cadmium on soil bioavailability and yield of Brassica napus irrigated with sewage
effluent: a potential human risk. African Journal of Agricultural Research 4(4):359-364.
City of Chicago, Department of Community Development. 2006. Green roof improvement fund.
Accessed 1 April 2009.
.
51
City of Portland, Environmental Services. 2009. Ecoroof incentive program. Accessed 1 April,
2009. .
Coffman, R.R. and J.F. Martin. 2004. The sustainability of an agricultural roof garden. In Proc.
2nd North American Green Roof Conference: Greening Rooftops for Sustainable
Communities, Portland, OR. 2 -4 June 2004. The Cardinal Group, Toronto.
Czemiel Berndtsson, J. 2010. Green roof performance towards management of runoff water
quantity and quality: a review. Ecological Engineering 36:351-360.
Czemiel Berndtsson, J., T. Emilsson, and L. Bengtsson. 2006. The influence of extensive
vegetated roofs on runoff water quality. Science of the Total Environment 355:48-63.
Dillion, R. M. 2010 Urban gardeners versus zoning laws. Associated Press 16 Feb 2010.
Accessed 15 April 2010. .
Durhman, A.K., D.B. Rowe, and C.L. Rugh. 2006. Effect of watering regimen on chlorophyll
fluorescence and growth of selected green roof plant taxa. HortScience 41:1623-1628.
Dvorak, B. and A. Volder. 2010. Green roof vegetation for North American ecoregions: a
literature review. Landscape and Urban Planning 96:197-213.
Emilsson, T. 2008. Vegetation development on extensive vegetated green roofs: Influence of
substrate composition, establishment method and species mix. Ecological Engineering
33:265-277.
Emilsson, T., J. Czerniel Berndtsson, J.E. Mattsson, and K. Rolf. 2007. Effect of using
conventional and controlled release fertilizer on nutrient runoff from various vegetated
roof systems. Ecological Engineering 29:260-271.
Enete, A.A., and A.I. Achike. 2008. Urban agriculture and urban food insecurity poverty in
Nigeria: The case of Ohafia, south-east Nigeria. Outlook on Agriculture 37(2):131-134.
Fang, C.-F. 2008. Evaluating the thermal reduction effect of plant layers on rooftops. Energy and
Buildings 40:1048-1052.
FLL (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau). 1995. Guidelines for
the Planning, Execution and Upkeep of Green-Roof Sites. Forschungsgesellschaft
Landschaftsentwicklung Landschaftsbau. Bonn, Germany.
Getter, K.L. and D.B. Rowe. 2006. The role of extensive green roofs in sustainable development.
HortScience 41(5):1276-1285.
52
Getter, K.L., and D.B. Rowe. 2009. Substrate depth influences sedum plant community on a
green roof. HortScience 44(2):401-407.
Getter, K.L, D.B. Rowe, and J.A. Andresen. 2007. Quantifying the effect of slope on extensive
green roof stormwater retention. Ecological Engineering 31:225-231.
Getter, K.L., D.B. Rowe, and B.M. Cregg. 2009. Solar radiation intensity influences extensive
green roof plant communities. Urban Forestry and Urban Greening 8(4):269-281.
Graefe, S. E. Schlecht and A. Buerkert. 2009. Opportunities and challenges of urban and periurban agriculture in Niamey, Niger. Outlook on Agriculture 37(1):47-56.
Greenroofs.com (GRC). 2011. The green roof and wall project database. Accessed 2 March,
2012. .
Harlan, S.L., A.J. Brazel, L. Prashad, W.L. Stefanov, and L. Larsen. 2006. Neighborhood
microclimates and vulnerability to heat stress. Social Science & Medicine 63:2847-2863.
Hathaway, A.M., W.F. Hunt, and G.D. Jennings. 2008. A field study of green roof hydrologic
and water quality performance. Transactions of Amer. Soc. Agricultural and Biological
Engineers 51(1):37-44.
Heilman, J.L., K.J. McInnes, R.W. Gesh, R.J. Lanscano, and M.J. Savage. 1996. Effects of
trellising on the energy balance of a vineyard. Agricultural and Forest Meteorology
81:79-93.
Hu, X., Z. Ding. 2009. Lead/Cadmium contamination and lead isotopic ratios in vegetables
grown in peri-urban and mining/smelting contaminated sites in Nanjing, China. Bulletin
of Environmental Contamination and Toxicology 82:80-84.
Johnson, D.P. and J.S. Wilson. 2009. The socio-spatial dynamics of extreme urban heat events:
The case of heat-related deaths in Philadelphia. Applied Geography 29:419-434.
King, J. 2004. Multnomah County’s Green Roof Project: A Case Study. In Proc. of 2nd North
American Green Roof Conference: Greening rooftops for sustainable communities,
Portland, OR. 2-4 June 2004. The Cardinal Group, Toronto.
Kortright, R. 2001. Evaluating the potential of green roof agriculture. Urban Agriculture Notes.
City Farmer, Canada’s Office of Urban Agriculture. Accessed 21 Dec 2010.
.
Kula, R. 2005. Green roofs and the LEEDTM green building rating system. P 141-153 In Proc. of
3rd North American Green Roof Conference: Greening rooftops for sustainable
communities, Washington, D.C. 4-6 May 2005. The Cardinal Group, Toronto.
53
Liptan, T. 2005. Portland: A new kind of stormwater management, p. 121-123. In Earthpledge.
Green roofs: Ecological design and construction. Schiffer Books, Atglen, Pa.
Loder, A. and S.W. Peck. 2004. Green roofs’ contribution to smart growth implementation, p 824. In Proc. of 2nd North American Green Roof Conference: Greening rooftops for
sustainable communities, Portland, OR. 2-4 June 2004. The Cardinal Group, Toronto.
Memon, R.A., D.Y.C., Leung, and L. Chunho. 2008. A review on the generation, determination
and mitigation of Urban Heat Island. Journal of Environmental Sciences 20:120-128.
Monterusso, M.A., D.B. Rowe, C.L. Rugh. 2005. Establishment and persistence of Sedum spp.
and native taxa for green roof applications. HortScience 40(2):391-396
Nowak, D.J. 2006. Institutionalizing urban forestry as a “biotechnology” to improve
environmental quality. Urban Forestry & Urban Greening 5:93-100.
Nugent, R. 2002. The impact of urban agriculture on the household and local economies. RUAF
Foundation International Workshop of Urban Agriculture: Growing Cities, Growing
Food. Accessed 31 Jan 2009 .
Peters, C.J., N.L. Bills, A.J. Lembo, J.L. Wilkins, and G.W. Fick. 2009. Mapping potential
foodsheds in New York State: A spatial model for evaluating the capacity to localize food
production. Renewable Agriculture and Food Systems 24(1):72-84.
Rowe, D.B. 2011. Green roofs as a means of pollution abatement. Environmental Pollution
159(8-9):2100-2110.
Rowe, D. B. and K.L. Getter. 2010. Green roofs and roof gardens. p. 391-412. In J.
Aitkenhead-Peterson and A. Volder (ed.). Urban Ecosystems Ecology. Agronomy
Monograph. 55. American Society of Agronomy. Crop Science Society of America.
Soil Science Society of America, Madison, WI.
Rowe, D.B. M.A. Monterusso, and C.L. Rugh. 2006. Assessment of heat-expanded slate and
fertility requirements in green roof substrates. HortTechnology 16(3):471-477.
Rugh, C.L. E-mail to author 5 Feb 2010.
Sailor, D.J. 2008. A green roof model for building energy simulation programs. Energy and
Buildings 40:1466-1478.
Saiz, S, C Kennedy, B Bass, and K Pressnail. 2006. Comparative life cycle assessment of
standard and green roofs. Environmental Science Technology 40(13):4312-4316.
54
Santamouris, M., C. Pavlou, P. Doukas, G. Mihalakakou, A. Synnefa, A. Hatzibiros, and P.
Patargias. 2007. Investigating and analyzing the energy and environmental performance
of an experimental green roof system installed in a nursery school building in Athens,
Greece. Energy 32:1781-1788.
Sharma, R.K., M. Agrawal, and F.M. Marshall. 2009. Heavy metals in vegetables collected from
production and market sites of a tropical urban area of India. Food and Chemical
Toxicology 47:583-591.
Srinivas, N., S.R. Rao, and K.S. Kumar. 2009. Trace metal accumulation in vegetables grown in
industrial and semi-urban areas- a case study. Applied Ecology and Environmental
Research 7(2): 131-139.
Sutton, S. 2009. City in the sky: Harnessing the potential of urban rooftops. Presentation at the
7th North American Green Roof Conference: Greening Rooftops for Sustainable
Communities, Atlanta, GA. 3 -5 June 2009.
Thornton, A. 2009. Pastures of plenty?: Land rights and community-based agriculture in Peddie,
a former homeland town in South Africa. Applied Geography. 29:12-20.
UNDESA. 2007. World urbanizations prospects, 2007 Edition: United Nations Department of
Economic and Social Affairs (DESA) Population Division -Population estimates and
projections section. 29 Sept 2009. .
U.S. Census Bureau (USCB). 2009. International data base: Total midyear population for the
world: 1950-2050. Accessed 3 Oct. 2009.
.
U.S. Environmental Protection Agency. 2009. Green roofs for stormwater runoff control. EPA600-R-09-026. USEPA, Washington, DC.
Vagneron, I. 2007. Economic appraisal of profitability and sustainability of peri-urban
agriculture in Bangkok. Ecological Economics 61:516-529.
van Averbeke, W.. 2007. Urban farming in the informal settlements of Atteridgeville Pretoria,
South Africa. Water SA 33(3):337-342.
VanWoert, N.D., D.B. Rowe, J.A. Andresen, C.L. Rugh, R.T. Fernandez, and L. Xiao. 2005.
Green roof stormwater retention: Effects of roof surface, slope, and media depth. Journal
of Environmental Quality 34:1036-1044.
Voutsa, D., A. Grimanis, and C. Samara. 1996. Trace elements in vegetables grown in an
industrial area in relation to soil and air particulate matter. Environmental Pollution
94(3):325-334.
55
Widome, R., D. Neumark-Sztainer, P.J. Hannah, J. Haines, and M. Story. 2009. Eating when
there is not enough to eat: Eating behaviors and perceptions of food among food-insecure
youths. Research and Practice. 99(5):822-828.
Wong N.H., S.F. Tay, and Y. Chen. 2007. Study of thermal performance of extensive rooftop
greenery systems in the tropical climate. Building and Environment 42:25-54.
Wong N.H., S.F. Tay, R. Wong, C.L. Ong, and A. Sai. 2003. Life cycle cost analysis of rooftop
gardens in Singapore. Building and Environment 38:499-509.
Yang, Y., F.S. Zhang, H.F. Li, and R.F. Jiang. 2009. Accumulation of cadmium in the edible
parts of zix vegetable species grown in Cd-contaminated soils. Journal of Environmental
Management 90:1117-1122.
de Zeeuw, H., S. Guendel, and H. Waibel. 1999. The integration of agriculture in urban policies.
RUAF Foundation International Workship on Urban Agriculture: Growing Cities,
Growing Food- Urban agriculture on the policy agenda. Accessed 31 Jan 2009.
.
Zhuang, P., M.B. McBride, H. Xia, N. Li, and Z. Li. 2009. Health risk from heavy metals via
consumption of food crops in the vicinity of Dabaoshan mine, South China. Science of
the Total Environment 407:1551-1561.
56
CHAPTER TWO:
Evaluation of vegetable production on extensive green roofs
57
Abstract
Rooftop vegetable gardening is a production system in urban agriculture based on green
roof technology. In order to broaden the scope of this practice, the use of relatively shallow
substrate depths must be explored since most existing urban flat roofs are not structurally strong
enough to support much added weight. In this study, vegetables grown on a roof in 10 cm of
substrate, on raised green roof platforms in 10 cm of substrate, and in-ground were evaluated
over three growing seasons (2009-11) to determine the practicality of using an extensive green
roof system for food production. Tomatoes (Lycopersicon esculentum), green beans (Phaseolus
vulgaris), cucumbers (Cucumis sativus), peppers (Capsicum annuum), basil (Ocimum basilicum)
and chives (Allium schoenoprasum) were studied because of their common use in home gardens.
All plants survived and produced biomass on the green roof, on green roof platforms and inground. All vegetable species yielded crops large enough for statistical analysis except for
pepper in 2009 and 2010. Overall, yields were higher and of better quality in-ground during
2009 when plots were irrigated, and there were similarities in yields between the green roof and
roof platforms. Variability in success of the vegetables was due in part to annual variation in
weather conditions with weather having the greatest impact on cucumber. Biomass of basil was
the same in the roof and platform plots two out of three years and was higher in the ground only
during the first year when the ground plots were irrigated. After one year of growth there was no
difference in chive yield among growing systems. Results suggest that with proper management,
vegetable and herb production in an extensive green roof system is not only possible, but
productive.
58
Introduction
In light of population growth trends (UNDESA, 2007; USCB, 2009) and concerns over
food deserts in urban centers, malnutrition (van Averbeke, 2007; Peters et al., 2009; Windome et
al. 2009) and the availability of land for agriculture (van Averbeke, 2007; Peters et al., 2009),
alternatives to conventional agriculture need to be investigated. Urban agriculture, the
production of agricultural products (vegetables, staple grains, dairy and meat) within the
boundaries of an urban center, has received a great deal of interest (Enete and Achike, 2008;
Graefe et al., 2009; Vagneron, 2007). Urban agriculture varies greatly in scale and amount of
investment, in part due to the resources available to urban farmers (van Averbeke, 2007; Enete
and Achike, 2008; Graefe et al., 2009; Nugent, 2002; Vagneron, 2007). Geography and climate
also impact the forms of urban agriculture practiced (Enete and Achike, 2008; Nugent, 2002;
Vagneron, 2007; de Zeeuw et al., 1999).
One of the biggest challenges facing urban agriculture is the availability of land (Nugent,
2002; de Zeeuw et al., 1999). There are numerous competing uses of land in urban centers
including building development, which is highly profitable to land owners (Graefe et al., 2009;
Nugent, 2002; Vagneron, 2007). Urban agriculture is generally not profitable to land owners, as
farmers infrequently have formal use arrangements with land owners (Nugent, 2002; Thornton,
2009; de Zeeuw et al., 1999). Even when use agreements do exist, there can be conflict between
the land owners and urban farmers because the development and use goals of the land owners
may change (van Averbeke, 2007). This competition often forces urban farmers onto marginal
vacant lots and brownfields. These lands are often found in industrial areas and may be
contaminated by heavy metals, which results in contaminated food, and in turn creates a health
hazard (Agbenin et al., 2009; Hu and Ding, 2009; Zhuang et al., 2009). If urban agriculture is to
59
be a viable solution to increasing urban food issues, the conundrum of land availability and
safety must be addressed.
In Germany, a country regarded as a leader in green roof technology, only 14 % of flat
roofs are green (Kohler and Keeley, 2005). This leaves the remaining 86 % of flat roofs as a
space resource with potential for productive use, which could enable the consumption of locally
grown fresh produce by urban populations. The available space is even higher in the U.S., which
lags Germany in green roof development. Some other possible benefits of utilizing green roof
space to produce food include improved economic and food security, formalization of use rights,
increased oversight of the use of agrichemicals and other fertilizers, and improved food safety
(Whittinghill and Rowe, 2011). Although vegetables and herbs are now being grown on some
green roofs (Arpels et al., 2005; Loder and Peck, 2004), there is currently very little information
in the literature on how roof-top gardening (or growing conditions) affects vegetable yields and
quality. Such information is necessary when examining the potential use of green roof space for
larger scale vegetable production. It would also provide information useful in designing
management programs to maximize both the quality and quantity of vegetables produced.
A wide variety of agricultural products have been produced in limited quantities on green
roofs worldwide. These typically include many different herb species, peppers (hot and
sweet)(Capsicum anuum), tomatoes (Licopersicum esculentum), carrots (Daucus carota), fennel
(Foeniculum vulgare), beets (Beta vulgaris), beans (Phasiolus vulgaris), peas (Pisum sativum),
pumpkin (Cucurbita spp), zucchini (Cucurbita pepo), squash (Cucurbuta spp),, eggplant
(Solanum melongena), turnip (Brassica rapa), broccoli (Brassica oleracea), ground cherries
(Physalis spp), sweet potato (Ipomoea batatas), artichoke (Cynara cardunculus), and radish
(Raphanus sativus) as well as other greens, berries, and melons (GRC, 2011). In some cases tree
60
fruits such as cherry (Prunus spp), pear (Prunus spp), apple (Malus spp) (GRC, 2011),
mushrooms, and rice are grown (Arpels, et al., 2005). Many green roof agricultural products are
produced on intensive green roof systems in substrate depths greater than 15 cm (Coffman and
Martin, 2004; GRC, 2011). These are typically 17.8 to 45.7 cm (7 to 18 in) deep (GRC, 2011)
2
2
and the corresponding weight of 146 kg/m (30 lb/ft ) is heavier than most existing flat roofs can
support (Dillion, 2010). This brings into question the scale at which green roof agriculture could
be employed, unless adequate crop production can be achieved using less substrate.
The objective of this study was to evaluate the survival and productivity of vegetable and
herb production in an extensive green roof system. Six vegetable and herb species were selected
based on their common use in home gardens and their growth patterns. Each species was
evaluated over three growing seasons for crop yield, crop quality, and biomass production on a
rooftop and, green roof platforms in typical green roof substrate, and in-ground in natural soil.
We hypothesized that there would be differences between the quantity and quality of produce
among the three growing systems. Crops that perform poorly in the extensive green roof and
green roof platforms are unlikely to be good candidates for urban agricultural production using
green roof technology. Comparison of crop performance in the two green roof systems with
production in-ground was used as an indicator for the suitability of extensive green roof
agriculture as an alternative to urban agriculture in-ground.
Methods
Plot Locations and Preparation. Four vegetable and two herb cultivars were planted in
four replicate plots in each of three growing systems: on a green roof, on green roof platforms,
and in-ground. These growing systems were compared over three growing seasons, 2009, 2010,
61
and 2011. Green roof plots were located on the Michigan State University (MSU) Plant and Soil
Sciences Building (PSSB) in East Lansing, MI. Roof platform and ground plots were located at
the MSU Horticultural Teaching and Research Center (HTRC) in Holt, MI, 6.3 km (4 mi) from
the PSSB. The ground plots were located in Copac loam (USDA, 2012). The green roof and
green roof platforms both contained 10.5 cm of green roof substrate laid over a XeroFlor XF-105
drainage mat (XeroFlor America, Durham, NC). Substrate consisted of 25% each of Haydite A
and Haydite B heat-expanded shale, 35% 2NS sand, and 15% leaf compost (Renewed Earth,
Kalamazoo, MI) and was installed May 29, 2009 and June 10, 2009, respectively. The depth of
10.5 cm was selected due to load capacity limitations of the PSSB roof. Green roof platforms
were constructed according to VanWoert et al. (2005) and each 2.4 x 2.4 m platform was divided
into three 0.8 x 2.4 m sections. In order to control for edge effect, vegetable plots were
alternated between the three sections in the four replicate platforms. Plots on the PSSB
measured 2.3 m x 1.4 m. Ground level plots were prepared by first treating them with
glyphosate (Roundup®, Monsanto, St. Louis, MO). Once the existing vegetation was dead the
area was deep tilled by spading on May 25, 2009. Four 0.8 x 2.4 m plots were created from the
prepared area to simulate plots on the green roof platforms. Weather data was compiled from the
Michigan Automated Weather Network (MAWN) station East Lansing/MSUHORT located at
the HTRC adjacent to the platforms and ground plots.
Plant Selection and Maintenance. Vegetables and herbs selected were Roma (VF)
tomatoes (Lycopersicon esculentum), early contender bush beans (Phaseolus vulgaris), bush
®
pickle hybrid cucumbers (Cucumis sativus), Sweetheart hybrid sweet peppers (Capsicum
annuum) in 2009 and 2010, large-leaf Italian basil (Ocimum basilicum), chives (Allium
schoenoprasum) (Gurney’s Seed and Nursery Co., Greendale, IN), and Budapest hot banana
62
Table 2.1. Planting and maintenance dates for vegetable production during the 2009-11 growing
seasons on the green roof, green roof platforms and in-ground.
Activity
2009
2010
2011
May 9
May 3 and 14
May 6
Seeds sown
Plugs planted:
May 26
May 15
June 18
Green roof
May 29
May 15
June 16
Green roof platforms
May 29
May 27
June 19
In-ground
Pepper plugs planted:
May 26
July 1
July 6
Green roof
May 29
May 29
June 30
Green roof platforms
May 29
May 29
July 6
In-ground
May 29
June 30
June 7
Irrigation started
October 2
October 3
October 1
Irrigation ended
Fertilizer application 1
May 30
July 1
July 22
Green roof
June 13
July 1
July 22
Green roof platforms
June 13
July 1
July 22
In-ground
Fertilizer application 2
N/A
August 9
September 2
Green roof
N/A
August 9
August 26
Green roof platforms
N/A
August 12
August 26
In-ground
Biomass harvest:
October 1 and 2
October 2 and 3
October 1 and 2
Tomato
October 1 and 2
September 20
August 29
Bean
October 1 and 2
August 24
August 29
Cucumber
October 1 and 2
October 2
October 1 and 2
Pepper
October 1 and 2
August 22- October 2
October 1 and 2
Basil
October 1 and 2
October 2
October 1 and 2
Chives
Tomatoes are Roma (VF) tomatoes (Lycopersicon esculentum), beans are early contender bush
beans (Phaseolus vulgaris), cucumbers are bush pickle hybrid cucumbers (Cucumis sativus),
®
peppers are Sweetheart hybrid sweet peppers in 2009 and 2010 and Budapest hot banana
peppers in 2011 (Capsicum annuum), basil is large-leaf Italian basil (Ocimum basilicum), and
chives (Allium schoenoprasum)
peppers (C. annuum) (Seedway, Hall, NY) in 2011. They were selected because of their
availability, growth habit (determinate or bush variety), and common use in home gardens.
Vegetables and herbs were sown from seed into 48-cell plug trays in early May of each year
(Table 2.1) and grown outdoors, in East Lansing, MI. Budapest hot banana peppers were planted
63
from seed into 98-cell plug trays on April 6, 2011 and grown in a greenhouse until May 24,
2011, then transferred to a lath house at the HTRC.
Due to the limited space available two plants of each type were planted from plugs into
the green roof, green roof platform, and ground plots (Table 2.1). Plants within each plot were
evenly spaced in two rows of six. The specific location of each plant was selected randomly in
2009. Plant locations remained the same for each of the three growing seasons because of the
perennial nature of chives. Plots on the PSSB roof were watered three times daily for 20 min
using a sprinkler throughout the growing season of 2009, and using micro-emitters throughout
the growing seasons of 2010 and 2011 (Table 2.1). Throughout the 2009 growing season
platforms were watered three times daily for 20 min using overhead sprinklers and three times
daily for 5 min using micro-emitters throughout the 2010 and 2011 growing seasons (Table 2.1).
Ground plots were watered daily with overhead sprinklers during establishment, then as needed
for the 2009 growing season only. A comparison of production in each growing system using
minimal management was the goal of this study so irrigation was stopped in-ground after 2009,
because scheduled irrigation of back-yard gardens with clay soils in Michigan is not usually
necessary. Occasional manual irrigation was provided to the ground plots during periods of
drought in 2010 and 2011. Irrigation in the platforms was however necessary to maintain
survival of the vegetable plants because herbaceous plants do not survive well in shallow green
roof substrate without irrigation (Monterusso et al., 2005). Lesco Professional Landscape and
Ornamental All-Purpose Fertilizer 14-14-14 (Lesco, Inc., Cleveland, OH) (Table 2.2) was
2
applied to all plots at a rate of 25 g/m once in 2009, and twice in 2010 and 2011 (Table 2.1).
64
Table 2.2. Composition of Lesco Professional Landscape and Ornamental All-Purpose Fertilizer
14-14-14 (Lesco, Inc., Cleveland, OH).
Nutrient
Percent Content by Weight (%)
14.00
Total Nitrogen
5.48
Ammoniacal Nitrogen
8.55
Urea Nitrogen
14.00
Phosphate (P2O5)
14.00
Soluble Potash (K O)
2
19.40
14.40
5.00
0.45
0.005
0.45
0.05
2.00
Total Sulfur
Free Sulfur
Combined Sulfur
Total Iron
Water soluble Iron
Total Manganese
Water soluble manganese
Chlorine Max
Data collection and Analysis. Tomatoes, beans, cucumbers, and peppers were harvested
as they ripened until just before the first frost when all remaining fruit were harvested, combined
for each plot, and weighed. Sizes of individual vegetables were measured on the two longest
axes using a caliper. Vegetables were divided in to size categories based on USDA standards
(USDA, 1991; USDA, 1959; USDA 1936) and size categories were assigned numbers for
analysis with 1 representing the smallest size category for each vegetable and 5, 6, and 4
representing the largest size category for tomatoes, beans and cucumbers, respectively. Tomato
color was also determined based on USDA standards (USDA, 1991; USDA, 1959; USDA 1936)
and the color category number (eg. 1 = green, 6 = red) was used for analysis. Potential
acceptability of the vegetables to the consumer was rated according to USDA standards (USDA,
1991; USDA, 1959; USDA 1936; USDA, 2009). Reasons for unacceptability, such as insect or
disease damage, discoloration, or scarring, were recorded. Quality ratings were used to
determine crop quality according to USDA standards with lower numbers representing higher
quality and 4 indicating a cull (USDA, 1991; USDA, 1959; USDA 1936; USDA, 2009). The
65
number of fruit, total yield in grams and marketable yield in grams based of fruit quality were
also recorded at the time of harvest. Marketable percent of the yield was calculated from total
and marketable yield data.
After the first frost in 2009 and just before the first frost in 2010 and 2011, whole basil
and chive plants were harvested at the soil or substrate surface (Table 2.1). In 2010, basil leaves
were evaluated for marketability by removing all leaves and sorting based on the presence of
insect damage, discoloration, sun scalding or disease. Basil and chive plant material was
weighed fresh, dried at 60 ˚C for 1 wk and weighed again to determine biomass. At the end of
each growing season, tomato, bean, cucumber and pepper plants were also cut even with the soil
or substrate surface, and their biomass was determined (Table 2.1). Biomass dry weight was also
measured but will not be discussed because it is correlated to biomass wet weight.
Data were analyzed using SAS (Version 9.1, SAS Institute, Cary, NC). Correlations
2
were found between number of fruit and total yield, marketable yield and plant biomass (R from
2
0.166 to > 0.8); total yield and marketable yield and plant biomass (R from 0.233 to > 0.8); and
2
marketable yield and plant biomass (R from 0.1816 to 0.8288). Due to these high correlations
results for fruit number, marketable yield and plant biomass will be presented in tables, but not
discussed in the text. All data were checked for normality prior to analysis of variance. Nonnormal data were analyzed after applying a logarithmic transformation for tomato number, total
yield and marketable yield, bean number, total yield, and biomass wet weight, basil marketable
yield, and chive fresh weight yield and a square root transformation for cucumber total weight.
All values are presented as back-transformed data. Influence diagnostics were used to identify
outliers, which were removed if they were deemed unrepresentative. Mean fruit size, fruit
66
quality, total yield, marketable yield and biomass of each species were analyzed using an
ANOVA model with growing system as a fixed effect. Significant differences among treatments
were determined using multiple comparisons by LSD with an alpha of 0.05 (PROC MIXED,
Version 9.1, SAS Institute). No comparisons among species were made.
Results
Weather. During 2009, maximum ambient air temperatures in May, July and August
were lower than the following two growing seasons and similar maximum temperatures were
recorded during 2010 and 2011 (Figure 2.1). Minimum ambient air temperatures were also
lower in May, September, and October in 2009, than those recorded in 2010 and 2011. Total
precipitation was greatest during 2011, however, 2009 experienced more precipitation during
June and August than 2010 and 2011 but less in September and July (Figure 2.1). Incoming
solar radiation was similar for all three growing seasons with totals for June, July, August and
2
September of 2.24, 2.16, and 2.21 million kJ/m for 2009, 2010, and 2011, respectively.
67
precipitation
A
air temperature
25.00
20.00
15.00
10.00
5.00
0.00
B
Temperature (°C)
35.00
Total precipitation = 303.26 mm
30.00
25.00
20.00
15.00
10.00
5.00
0.00
Temeprature (°C)
C
35.00
Total precipitation = 367.50 mm
30.00
25.00
20.00
15.00
10.00
5.00
0.00
Precipitation (mm)
Temperature (°C)
30.00
45.00
40.00
35.00
30.00
25.00
20.00
15.00
10.00
5.00
0.00
180.00
160.00
140.00
120.00
100.00
80.00
60.00
40.00
20.00
0.00
Precipitation (mm)
90.00
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
Precipitation (mm)
Total precipitation = 307.34 mm
35.00
1 2 3 4 5 6 7 8 9 101112131415161718
Time (weeks)
Figure 2.1. Weekly average maximum air temperature in Celsius and weekly total precipitation
in mm for the growing seasons of (A) 2009 (May 31-Oct 2) (B) 2010 (May 25-Oct 2) and (C)
2011 (May 29-Oct 1).
68
Table 2.3. Harvest indicators for tomatoes (Lycopersicon esculentum) grown on the green roof,
green roof platforms and in-ground during the 2009-11 growing seasons.
Variable
Growing System
2009
2010
2011
3.1 Ab
1.3 Bb
1.2 Ba
Fruit Size
Green roof
1.9 ABc
2.2 Aa
1.4 Ba
Green roof platform
3.5 Aa
1.0 Bb
1.5 Ba
In-ground
2.5 Aa
1.7 Bb
2.2 Ab
Fruit Color
Green roof
1.3 Bb
3.0 Aa
3.0 Aa
Green roof platform
1.4 Bb
1.8 ABb
2.0 Ab
In-ground
2.0 Aa
3.4 Bb
2.1 Aa
Fruit Grade
Green roof
2.9 Bb
2.7 Ba
1.8 Aa
Green roof platform
2.9 Bb
3.4 Cb
2.1 Aa
In-ground
18.0 ABb
36.6 Aa
13.1 Bab
Number of Fruit
Green roof
10.0 Bb
22.6 Aa
17.3 ABa
Green roof platform
132.4 Aa
4.9 Bb
7.3 Bb
In-ground
401.6 Ab
534.1 Aa
143.0 Ba
Total Yield (g)
Green roof
102.6 Bc
571.0 Aa
238.3 ABa
Green roof platform
3273.4 Aa
57.1 Cb
147.2 Ba
In-ground
401.6 Ab
281.3 ABa
136.0 Ba
Marketable Yield
Green roof
(g)
87.3 Bc
458.5 Aa
235.2 Aa
Green roof platform
1597.3 Aa
43.7 Cb
146.7 Ba
In-ground
100.0 Aa
52.9 Bb
95.2 Aa
Marketable
Green roof
Percent of Yield
85.7 Aa
81.5 Aa
98.7 Aa
Green roof platform
(%)
49.6 Bb
82.4 Aa
99.7 Aa
In-ground
142.1 Bb
551.2 Aa
197.3 Ba
Biomass Wet
Green Roof
Weight (g)
93.4 Bb
305.1 Ab
95.6 Ba
Green roof platform
1280.3 Aa
188.5 Bb
246.6 Ba
In-ground
Sizes, colors and grades based on USDA standards for fresh tomatoes (USDA, 1991).
Marketable yield based on fresh weight of all fruit of a marketable grade according to USDA
standards. Each mean represents four observations. Means were separated using LSD with an
alpha of 0.05. Capital letters in rows indicate differences among years within growing systems
and lower case letters in columns indicate differences among growing systems within years for
each variable. Letters for number, total yield, and marketable yield data indicate significant
difference in the transformed data.
Tomato. Results for all variables measured for tomato are listed in Table 2.3. The largest
fruit were produced in –ground in 2009 and in platforms in 2010. In 2011, growing system did
not affect fruit size. Fruit harvested from the roof and ground were larger in 2009 than in later
growing seasons. Fruit color was closest to red at harvest on the roof in 2009 and on the
platforms in 2010 and 2011. There was no consistent pattern of fruit color across years. The
69
lowest grade fruit were produced on the roof in 2009 and the platforms in 2010. Growing system
had no effect on fruit grade in 2011. Fruit grown in–ground and in platforms in 2011 graded
lower than any other year.
In 2009, plants grown in-ground produced the highest total yield, but the lowest
marketable percent of yield. A reversal of this was seen in 2010 when plants on the roof and
platforms produced the highest total yield but plants on platforms and ground produced the
highest marketable percent of yield. Growing system had no effect total yield or marketable
percent of yield in 2011 and year had no effect on marketable percent of yield from the
platforms.
Bean. Results for all variables measured for beans are listed in Table 2.4.Growing
system had no effect on pod size in 2009 or 2010. Pods were smallest in 2011 for all three
growing systems and lower in-ground than the roof and platform that year. Pods were of lowest
grade in 2009, when growing system had no effect on pod grade. In 2010, plants on platforms
produced the lowest grade pods and in 2011 plants on the roof produced the lowest grade pods.
In 2009, total yield was highest in-ground, but growing system had on effect on
marketable percent of yield. Plants growing on platforms had higher total yield in 2010 than the
other growing systems, but had lower marketable percent of yield. In 2011, the total yield from
the platforms did not differ from either growing system and the marketable percent of yield did
not differ from the ground but was higher than that of roof plants. Total yields were highest in
2009, 2010 and 2011 for the ground, platform and roof, respectively. Marketable percent of
yield was highest in 2009.
70
Table 2.4. Harvest indicators for beans (Phaseolus vulgaris) grown in the green roof, green roof
platforms and in-ground during the 2009-11 growing seasons.
Variable
Growing System
2009
2010
2011
3.7 Aa
3.3 ABa
2.8 Ba
Pod Size
Green roof
4.1 Aa
3.5 Aa
2.0 Ba
Green roof platform
4.4 Aa
3.6 Ba
1.8 Cb
In-ground
1.8 Aa
3.2 Cb
3.1 Ba
Pod Grade
Green roof
1.8 Aa
2.5 Ba
3.5 Cb
Green roof platform
1.8 Aa
3.1 Bb
3.7 Bb
In-ground
4.4* Bb
4.3 Bb
13.7 Aa
Number of Pods
Green roof
7.0 Bb
31.3* Aa
5.1* Ba
Green roof platform
20.1 Aa
6.5 Bb
5.9 Ba
In-ground
6.7* Bb
5.0 Bb
22.8 Aa
Total Yield (g)
Green roof
14.9 Bb
81.2* Aa
12.9 Bab
Green roof platform
81.1 Aa
12.6 Bb
3.7 Bb
In-ground
1.8* Aa
3.2 Ca
3.1 Bb
Marketable Yield
Green roof
(g)
1.8 Aa
2.5 Bb*
3.5 Ca
Green roof platform
1.8 Aa
3.1 Ba
3.7 Ba
In-ground
93.3* Aa
38.7 Ca
22.6 Bb
Marketable
Green roof
Percent of Yield
87.1 Aa
68.6 Bb*
73.7 Ca
Green roof platform
(%)
84.7 Aa
55.6 Ba
48.0 Ba
In-ground
4.7 Ab
3.4 Ab
12.7 Aa
Biomass Wet
Green roof
Weight (g)
11.0 Bb
47.5 Aa
13.4 Ba
Green roof platform
54.3 Aa
8.3 Bb
6.5 Ba
In-ground
* Based on 3 observations due to the exclusion of outliers.
Sizes, and grades based on USDA standards for snap beans for processing (USDA, 1959).
Marketable yield based on fresh weight of all fruit of a marketable grade according to USDA
standards. Each mean represents four observations. Means were separated using LSD with an
alpha of 0.05. Capital letters in rows indicate differences among years within growing systems
and lower case letters in columns indicate differences among growing systems within years for
each variable. Letters for number, total yield, and biomass data indicate significant difference in
the transformed data.
Cucumber. Results for all variables measured for cucumber are listed in Table 2.5.There
was no clear pattern of variation in fruit size among years. During 2009 fruit harvested from the
platforms were smaller than those harvested from the other growing systems and during 2010
fruit harvested in-ground were smaller. Growing system had no effect on fruit size in 2011 or
fruit grade in 2010 and year had no effect on grade of fruit harvested from the roof or platforms.
71
During 2011, fruit harvested from the roof were of lower grade than those harvested from any
other growing system that year.
Table 2.5. Harvest indicators for cucumbers (Cucumis sativus) grown in the green roof, green
roof platforms and in-ground during the 2009-11 growing seasons.
Variable
Growing System
2009
2010
2011
2.3 ABa
2.9 Aa
1.6 Ba
Fruit Size
Green roof
1.0 Bb
2.6 Aa
2.0 ABa
Green roof platform
2.8 Aa
1.3 Bb
2.0 ABa
In-ground
3.2 Aab
3.7 Aa
3.1 Aa
Fruit Grade
Green roof
4.0 Ab
3.5 Aa
4.0 Ab
Green roof platform
3.0 Aa
4.0 Ba
3.5 ABb
In-ground
1.5 Bb
3.0 Aa
0.5 Bb
Number of Fruit
Green roof
0.7 Bb
4.2 Aa
2.0 Ba
Green roof platform
4.2 Aa
0.7 Bb
3.2 Aa
In-ground
56.2 Bb
380.4 Aa
14.9 Bb
Total Yield (g)
Green roof
8.0 Cc
426.1 Aa
139.4 Ba
Green roof platform
403.7 Aa
13.5 Cb
227.9 Ba
In-ground
52.2 ABb
120.9 Aa
28.8 Bab
Marketable Yield
Green roof
(g)
0.0 Bb
109.9 Aa
100.9 Aa
Green roof platform
362.1 Aa
0.0 Bb
0.0 Bb
In-ground
44.4 Ab
29.2 Aa
25.0 Ab
Marketable
Green roof
Percent of Yield
0.0 Bc
25.2 Ba
70.2 Aa
Green roof platform
(%)
90.4 Aa
0.0 Bb
0.0 Bb
In-ground
10.7 Bb
75.9 Aa
18.4 Ba
Biomass Wet
Green roof
Weight (g)
13.5 Bb
34.1 Ab
22.8 ABa
Green roof platform
45.7 Aa
7.5 Bc
19.6 Ba
In-ground
Sizes and grades based on the USDA standards for pickling cucumbers (USDA, 1936).
Marketable yield based on fresh weight of all fruit of a marketable grade according to USDA
standards. Each mean represents four observations. Means were separated using LSD with an
alpha of 0.05. Capital letters in rows indicate differences among years within growing systems
and lower case letters in columns indicate differences among growing systems within years for
each variable. Letters for total yield data indicate significant difference in the transformed data.
Total yields and marketable percent of yield exhibited similar patterns in 2009, with the
ground performing best and in 2010, with the roof and platforms performing better than inground plants. In 2011, there was no difference between the total yield of platform and inground plants and marketable percent of yield was highest in the platforms. Plants grown on the
72
roof and platforms had higher total yields in 2010 than any other year. Year had no effect on
marketable percent of yield of roof plants. Marketable percent of yield of the platforms was
highest in 2011 and the total yield and marketable percent of yield of in-ground plants was
highest in 2009.
Pepper. Yields from sweet peppers in 2009 and 2010 were extremely limited (data not
shown). Statistical analysis was not performed because we believe that this lack of success was
the result of cultivar choice and not the experimental treatments. Results for all variables
measured for Budapest hot banana pepper are listed in Table 2.6. The size of peppers harvested
from the roof did not differ from any growing system in 2011. Growing system had no effect on
grade of fruit in 2011. Total yields of roof and platforms plants were higher than that from the
ground. Marketable percent of was higher from the roof than either of the other growing
systems.
Table 2.6. Harvest indicators for Budapest hot banana peppers (Capsicum annuum) grown in the
green roof, green roof platforms and in-ground during 2011 growing season.
Variable
Green roof
Green roof platform
In-ground
2.0 ab
2.53 a
1.1* b
Fruit Diameter (cm)
2.7 a
2.7 a
4.0* a
Fruit Grade
3.0 a
3.0 a
0.5 b
Number of Fruit
25.2 a
31.0 a
0.2 b
Total Yield (g)
23.1 a
20.8 a
0b
Marketable Yield (g)
72.2 a
18.8 b
0b
Marketable Percent of Yield (%)
15.1 a
26.0 a
14.6 a
Biomass Wet Weight (g)
* Mean based on only two observations
Sizes and grades based on USDA standards for sweet peppers (USDA, 2005).
Marketable yield based on fresh weight of all fruit of a marketable grade according to the USDA
standards. Each mean represents four observations. Means were separated using LSD with an
alpha of 0.05. Letters in rows indicate differences among growing systems for each variable.
Basil. Basil biomass fresh weight was higher in-ground in 2009, in platforms in 2010
and lower in-ground in 2011 (Table 2.7). Biomass fresh weight in 2009 was greater than any
73
other year for in-ground plants (Table 2.7). Marketable yield of basil was lower on the roof
(Table 2.8) and the marketable percent of biomass was higher in-ground (Table 2.8).
Table 2.7. Biomass fresh weight (g) of basil (Ocimum basilicum) for the 2009-11 growing
seasons on the green roof, green roof platforms and in-ground.
Growing System
2009
2010
2011
133.3 Bb
266.1 ABb
467.3 Aa
Green roof
62.7 Bb
605.1 Aa
694.7 Aa
Green roof platform
458.4 Aa
227.5 Bb
205.6 Bb
In-ground
Each mean represents four observations. Means were separated using LSD with an alpha of
0.05. Capital letters denote differences among years within growing systems; lower case letters
denote differences among growing systems within years.
Table 2.8. Marketable fresh weight yield (g) and marketable percent of biomass of basil
(Ocimum basilicum) for the 2010 growing season on the green roof, green roof platforms and inground.
Growing System
Marketable yield (g)
Percent of marketable biomass
0.2 b
0.8 b
Green roof
3.7 a
6.8 b
Green roof platform
4.0 a
27.8 a
In-ground
Each mean represents four observations. Means were separated using LSD with an alpha of
0.05. Letters denote differences among growing systems. Differences among growing systems
for marketable yield are for transformed data.
Table 2.9. Fresh weight yields (g) of chives (Allium schoenoprasm) for the 2009-11 growing
seasons on the green roof, green roof platforms and in-ground.
Growing System
2009
2010
2011
1.1 Bc
272.1 Aa
638.3 Aa
Green roof
439.1 Aa
601.7 Aa
Green roof platform 25.0 Ba
5.2 Bb
239.3 Aa
475.3 Aa
In-ground
Each mean represents four observations. Means were separated using LSD with an alpha of
0.05. Capital letters denote differences among years within growing systems and lower case
letters denote differences among growing systems within years for transformed data.
Chives. At the end of the 2009 growing season, platform plants had a higher fresh weight
yield than roof or in-ground plants (Table 2.9). Growing system had no effect on fresh weight
yield in 2010 or 2011. Fresh weight yields were lower in 2009 with no difference between 2010
and 2011 (Table 2.9).
74
Discussion
In 2009, tomato, bean, cucumber and basil plants grew better in-ground than either of the
green roof systems with a few exceptions. This was most likely due to the irrigation that was
supplied to the ground plots in 2009 but not 2010 or 2011. Supplemental irrigation enabled
larger fruit, a greater number of fruit, and more biomass than the other growing systems that year
and greater than the other years in that growing system. The exceptions include fruit color and
grade and marketable percent of yield of tomato (Table 2.3), fruit size and grade, marketable
yield and percent of marketable yield of bean (Table 2.4), and fruit grade of cucumber (Table
2.5). Although rooftop temperatures were not measured in this study, a previous study using the
PSSB roof compared measured green and ballast roof temperatures to those at ground level from
the MAWN weather station used in this study (Getter et al, 2011). The authors found that
temperatures on the green roof were warmer than nearby ground-level temperatures throughout
the course of their study. This combined with fewer observed pests on the roof likely resulted in
the higher number of ripe fruit and higher quality tomato fruit in that growing system (Table
2.3). For beans, the only measured variables that did not show plants from in-ground plots
outperforming the roof and platform plants showed no effect of growing system (Table 2.4).
There was also no difference between the fruit grade of cucumber from in-ground and roof level
plots (Table 2.5).
When comparing all three growing seasons there was no consistent pattern for all of the
vegetable and herb species examined. Bean was the only species to perform better in 2009 than
either of the two following growing seasons (Table 2.4). This may have been because of the
larger amount of precipitation early in the season and shorter periods of time without rain than
the other two growing seasons. Lower temperatures in July when there was less rain may have
75
also contributed to this success. There was no clear difference in performance in 2010 and 2011
for either tomato (Table 2.3) or basil (Table 2.6). Although 2011 was warmer, both years were
within the range of temperatures required for tomato production and the irrigation supplied in
2011 adequately made up for rain during the dry periods in June and August. The same may be
true of basil. Drastic difference between the results of cucumber from 2010 and 2011, suggests
that it was more susceptible to the long periods of warm dry weather that took place in 2011,
affecting performance in all growing systems. It should be noted that although fruit grades for
cucumber were high for all three growing season in all growing systems, this was at least in part
due to size restrictions in the USDA grades (USDA, 1936). USDA grade 3 cucumbers (the
lowest grade before cull (grade 4) has a listed maximum length of 15.2 cm (6 in) and maximum
diameter of 5.7 cm (2.25 in) (USDA, 1936) and many fruit harvested grew to lengths or
diameters greater than these restrictions but did not show flaws that would warrant culling.
Although the fruit grade of Budapest hot banana pepper was not very high (Table 2.6), we noted
some bitterness flavor in the fruit. This could influence consumer appeal and further research
could determine the cause and suggest a remedy. Chive, the only perennial species, exhibited its
lowest yields in 2009 (Table 2.9), the first year of the growth for this perennial species. This
suggests that it may be beneficial to wait for the second growing season to harvest.
Recommendations for harvesting chives state that harvesting every 4 to 6 wks throughout the
growing season stimulates multiplication (Swaider and Ware, 2002), which would result in
greater yields that a single harvest at the end of each growing season. This may however require
prohibitive labor inputs if implemented on rooftops on a larger scale than set harvests that
correspond with other crop harvest dates.
76
After the 2009 growing season, there were fewer clear differences in the performance of
vegetable and herb species among the three growing systems. Many similarities in performance
of vegetables in green roof and green roof platforms suggest that green roof platforms are and
adequate proxy for actual roof tops for experimentation. Performance of all vegetable and herb
species with the exception of sweet pepper on the roof and platforms was as good as if not better
than their performance in-ground. It should, however be noted that this coincides with the
discontinuation of irrigation for the in-ground plots. Sweet pepper had poor germination in
2010, and yielded few fruit in either 2009 or 2010. This resulted in the removal of this variety
from the study and suggests that it may not be well suited to production in an extensive green
roof system. Results suggest that tomato, bean, cucumber, Budapest hot banana pepper, basil
and chive are good candidates for production in an extensive green roof system. Allium species
have been considered good candidates for growth on green roofs for some time.
A comparison of yields form this study and those estimated based on United States
production area and yields for 2011 is also instructive. First it should be noted that the overall
2
planting density in this study, 6.35 plants/m , is higher than those calculated from recommended
2
plant and row spacing for tomatoes and peppers (2.4 to 4.34 and 3.25 to 4.10 plants/ m ,
2
respectively), at the high end of normal for cucumbers (1.74 to 6.6 plants/ m ) and low for beans
2
(25.06 to 38.65 plants/ m ) (Swiader and Ware, 2002). Reported yields for tomato, bean,
2
cucumber and hot pepper are 3,233, 574, 1,857, and 2,316 g/m , respectively (USDA, 2011). In
order to make those yields comparable with the yields reported here, yield per plant was
estimated to account for differing planting densities. The highest yields achieved in this study
were lower than those estimated from USDA reports with the exception of tomatoes and beans
77
(Table 2.10). The highest yields for tomato and beans were however much higher than any other
yields for those vegetables and occurred in the in-ground plots in 2009 when irrigation was
supplied to those plots. The next highest yields, from the green roof platforms in 2010, were
lower than those estimated from USDA reports (Table 2.10). This could be an indication of
water needs not being met. The issue of low yields, and problems with successfully producing
quality, good looking and tasting fruit from pepper cultivars could be resolved through different
management practices.
Table 2.10. Estimated yield in grams per plant for the highest yields achieved in this study and
from reported growing area and yield data reported by the USDA (2011) and recommended
planting densities based on row and plant spacing (Swiader and Ware, 2002).
Vegetable
Estimated from this study
Estimated yield
(g/plant) from USDA
Growing System and Yield (g/plant)
(2011) and Swaider
Year
and Ware (2002)
In-ground, 2009
1636.7
Tomato
Green roof platform,
285.5
744.93-1347.08
2010
In-ground, 2009
40.55
Bean
Green roof platform,
40.6
14.85-22.90
2010
Green roof, 2011
11.4
Green roof platform,
213.05
Cucumber
281.36-1067.24
2010
Green roof platform,
15.49
Pepper
712.62-564.88
2011
Estimated yields from this study derived by dividing the indicated mean by 2 plants. USDA
2
(2011) yield estimates derived by dividing the USDA reported yield/m by estimated plant
densities derived from Swiader and Ware (2002).
Conclusions
Although we expected that there would be differences between production on the green
roof, green roof platform and in- ground based on differences between the green roof and green
roof platform and in-ground growing conditions, this was generally not the case. Yearly
78
variation in weather seemed to have a larger impact on some of the species examined, especially
when irrigation and fertilizer were used to manage issues of substrate moisture and nutrient
availability in the green roof substrate. This study has shown that it is possible to produce
tomato, bean, cucumber, pepper, basil, and chive in an extensive green roof on a small scale in
Michigan with irrigation and minimal fertilizer inputs. A more sophisticated management
strategy could enable production of yields similar to those produce in-ground in the United
States. This study represents a potentially significant starting point in the literature of green roof
agriculture, which is currently limited. More research on irrigation efficiency, nutrient
management, pest and pollinator management and cultivar choice are needed. Not only to
expand the species and cultivars that are known to do well on extensive green roofs, but to make
their production more efficient, and to understand and minimize negative impacts on the benefits
already provided by green roofs, such as stormwater retention, energy savings and mitigation of
the urban heat island (Getter and Rowe, 2006).
Acknowledgements
Funding for this study was provided by Ford Motor Company, Dearborn, MI;
ChristenDETROIT Roofing Contractors, Detroit, MI; XeroFlor America, Durham, NC; Renewed
Earth, Kalamazoo, MI; and MSU AgBioResearch. Special thanks to the Michigan State
University Horticultural Teaching and Research Center for their support, and to Saloni Dagli
and Dan Finks for their field assistance.
79
REFERENCES
80
REFERENCES
Agbenin, J.O., M. Danko, and G. Welp. 2009. Soil and vegetable compositional relationships of
eight potentially toxic metals in urban garden fields from northern Nigeria. Journal of the
Science of Food and Agriculture 89:49–54.
Arpels, M., S. Chrisman, H. Sommerfield, J. Towers, E. Berkowitz, G. Brainard, L. Hickey
(Eds.). 2005. Building Case Studies. p. 24-102 In Earthpledge. Green roofs: Ecological
design and construction. Schiffer Books, Atglen, Pa.
Coffman, R.R. and Martin, J.F. 2004. The sustainability of an agricultural roof garden. In
Proceedings 2nd North American Green Roof Conference: Greening Rooftops for
Sustainable Communities, Portland, OR, June 2-4, 2004. The Cardinal Group, Toronto.
Dillion, R. M. 2010 Urban gardeners versus zoning laws. Associated Press 16 Feb 2010.
Accessed 15 April 2010. .
Enete, A.A. and Achike, A.I. 2008. Urban agriculture and urban food insecurity poverty in
Nigeria: The case of Ohafia, south-east Nigeria. Outlook on Agriculture 37(2):131–134.
Getter, K.L. and DB Rowe. 2006. The role of extensive green roofs in sustainable development.
HortScience 41(5):1276-1285.
Getter, K.L., D.B. Rowe, J.A. Andresen, and I.S. Wichman. 2011. Seasonal heat flux properties
of an extensive green roof in Midwestern U.S. climate. Energy and Building 43:35483557.
Getter, K.L., D.B. Rowe, G.P. Robertson, B.M. Cregg, and J.A. Andresen. 2009. Carbon
sequestration potential of extensive green roofs. Environ. Sci. Technol. 43:7564-7570.
Graefe, S., Schlecht, E., and Buerkert, A. 2009. Opportunities and challenges of urban and periurban agriculture in Niamey, Niger. Outlook on Agriculture 37(1):47–56.
Greenroofs.com (GRC). 2011. The green roof and wall project database. Accessed 2 March
2012. .
Hu, X. and Ding, Z. 2009. Lead/cadmium contamination and lead isotopic ratios in vegetables
grown in peri-urban and mining/smelting contaminated sites in Nanjing, China. Bulletin
of Environmental Contamination and Toxicology 82:80–84.
81
Kohler M and M Keeley. 2005. Berlin: Green roof technology and development, p. 108-112 In
Earthpledge. Green roofs: Ecological design and construction. Schiffer Books, Atglen,
Pa.
Loder, A. and Peck, S.W. 2004. Green roofs’ contribution to smart growth implementation. In
Proceedings of 2nd North American Green Roof Conference: Greening rooftops for
sustainable communities, Portland, OR, June 2-4, 2004. The Cardinal Group, Toronto, p.
8–24.
Monterusso, M.A., D.B. Rowe, and C.L. Rugh. 2005. Establishment and persistence of Sedum
spp and native taxa for green roof applications. HortScience 40(2):391-396.
Nugent, R. 2002. The impact of urban agriculture on the household and local economies. RUAF
Foundation International Workshop of Urban Agriculture: Growing Cities, Growing
Food. Accessed 31 Jan 2009. .
Peters, C.J., Bills, N.L., Lembo, A.J., Wilkins, J.L., and Fick, G.W. 2009. Mapping potential
foodsheds in New York State: A spatial model for evaluating the capacity to localize food
production. Renewable Agriculture and Food Systems 24(1):72–84.
Swiader, J.M. and G.W. Ware. 2002. Producing vegetable crops 5th Ed. Interstate Publishers, Inc.
Danville, Il. Pp 253, 346, 424, 448, and 547.
Thornton, A. 2009. Pastures of plenty?: Land rights and community-based agriculture in Peddie,
a former homeland town in South Africa. Applied Geography 29:12–20.
United States Department of Agriculture (USDA). 1991. United States standards for grades of
fresh tomatoes. Agricultural marketing service. Accessed 3 Oct 2009.
.
United States Department of Agriculture (USDA). 1959. United States standards for grades of
snap beans for processing. Agricultural marketing service. Accessed 3 Oct 2009.
.
United States Department of Agriculture (USDA). 1936. United States standards for grades of
pickling cucumbers. Agricultural marketing service. Accessed 3 Oct 2009.
.
United States Department of Agriculture (USDA). 2005. United States standards for grades of
sweet peppers. Agricultural marketing service. Accessed Oct 3, 2009.
.
United States Department of Agriculture (USDA). 2011. Vegetables 2011 summary. National
Agricultural Statistics Service. ISSN: 0884-6413. Accessed 18 March 2012.
.
82
Unites States Department of Agriculture (USDA). 2012. Natural Resources Conservation Service
web soil survey. Accessed 3 Jun 2012.
.
UNDESA. 2007. World urbanizations prospects, 2007 Edition. United Nations Department of
Economic and Social Affairs (DESA) Population Division – Population estimates and
projections section. Accessed 29 Sept 2009.
.
U.S. Census Bureau (USCB). 2009. International database: Total midyear population for the
world: 1950–2050. Accessed 3 Oct 2009. .
Vagneron, I. 2007. Economic appraisal of profitability and sustainability of peri-urban
agriculture in Bangkok. Ecological Economics 61:516–529.
van Averbeke, W. 2007. Urban farming in the informal settlements of Atteridgeville Pretoria,
South Africa. Water SA 33(3):337–342.
VanWoert, ND, DB Rowe, JA Andresen, CL Rugh, RT Fernandez, and L. Xiao. 2005. Green
roof stormwater retention: Effects of roof surface, slope, and media depth. J. Environ.
Qual. 34:1036-1044.
Whittinghill, L.J. and D.B. Rowe. 2011. The role of green roof technology in urban agriculture.
Renewable Agriculture and Food Systems. DOI 10.1017/S174217051100038X.
Widome, R., Neumark-Sztainer, D., Hannah, P.J., Haines, J., and Story, M. 2009. Eating when
there is not enough to eat: Eating behaviors and perceptions of food among food-insecure
youths. Research and Practice 99(5):822–828.
de Zeeuw, H., S. Guendel, and H. Waibel. 1999. The integration of agriculture in urban policies.
RUAF Foundation International Workship on Urban Agriculture: Growing Cities,
Growing Food- Urban agriculture on the policy agenda. Accessed 31 Jan 2009.
.
Zhuang, P., McBride, M.B., Xia, H., Li, N., and Li, Z. 2009. Health risk from heavy metals via
consumption of food crops in the vicinity of Dabaoshan mine, South China. Science of
the Total Environment 407:1551–1561.
83
CHAPTER THREE:
Evaluation of nutrient management and mulching strategies for vegetable production on an
extensive green roof
84
Abstract
Substrate nutrient and moisture management are two major concerns in green roof
agriculture. These concerns are amplified when using extensive green roof systems for food
production. Currently no recommendations or best management practices exist to guide rooftop
farmers in dealing with these issues. The purpose of this study was to explore three mulching
strategies (no mulch, pine bark mulch, and a living Sedum mulch) and three fertilization
2
regimens (25, 50, and 100 g/m of a 14-14-14 N-P-K slow release fertilizer applied twice during
each growing season) over two growing seasons to determine their possible benefits to rooftop
vegetable and herb production. Tomatoes (Lycopersicon esculentum), beans (Phaseolus
vulgaris), cucumbers (Cucumis sativus), sweet peppers (Capsicum annuum), basil (Ocimum
basilicum), and chives (Allium schoenoprasum) were included in this study because of their
common use in home gardens. Crops performed better in 2010 than 2011 because of more
extreme temperature and precipitation variations during 2011. When there were differences
among mulch treatments, pine bark mulch treatment usually resulted in higher productivity than
the live Sedum mulch. Mixed effects of live Sedum mulch on crop production are consistent
with other literature on the use of living mulches in vegetable production. Among fertilizer
2
treatments, 100 g/m treatment outperformed lower rates in most cases. Instances of higher
performance of the other fertilizer treatments are likely because those treatments adequately
supplied the crops with phosphorus and potassium. Further research into more types of mulch
and the effect those mulches have on the green roof microclimate could provide a better
understanding of the role that mulching could play in green roof agriculture. More research into
different types of fertilizers with different compositions could also help in the development of an
efficient and productive nutrient management practice.
85
Introduction
Substrate fertility and vegetation water needs are two major concerns on both green roofs
and in agriculture. Fertility and nutrient management are issues on green roofs because most
green roof substrates are engineered with less than 20% organic matter, which is usually
combined with coarse heat expanded materials such as slate or shale. This and the remaining
components of green roof substrates have high permeability and low cation exchange capacity
(Emilsson et al., 2007). Substrate organic matter breaks down over time (Emilsson, 2004;
Hathaway et al., 2008) and must either be replaced to maintain substrate depth, which could be a
cumbersome process on a rooftop, or managed with ferilizers, especially for plants with high
nutrient demands such as most vegetables. Both the breakdown of organic matter and the use of
fertilizers have been shown to lead to nutrient leaching (Czemiel Berndtsson, 2010; Emilsson,
2004; Hathaway et al., 2008).
Examination of the literature on green roof water quality reveal mixed results on the
extent to which nitrogen (N) is increased in runoff (Rowe, 2011). Czemiel Berndtsson et al
(2006) examined runoff from several green roofs in southern Sweden and found that some
experienced reduced nutrient loading compared to control roofs, while others showed increased
nutrient loading. Hathaway et al. (2008) and MacMillan (2004) observed higher runoff nutrient
loading from green roofs than from their respective control roofs. There is however a reduction
in runoff nutrient content with increasing roof age and decreasing losses of nutrients such as
phosphorus (P) from the green roof substrate (Czemiel Berndtsson, 2010). The addition of
fertilizers also results in nutrient leaching into runoff water, especially if those fertilizers are
highly water soluble (Czemiel Berndtsson, 2010; Emilsson, 2004; Emilsson et al., 2007). The
use of municipal water supply (MacMillan, 2004) and amendments such as biochar (Beck et al.,
86
2011), can increase or decrease runoff nutrient loading, respectively. This stresses the
importance of initial nutrient content of the substrate and subsequent fertilization and irrigation
practices on runoff quality.
A balance between meeting the nutritional needs of the vegetation and adversely
impacting the quality of runoff from the roof must be met. This balance may be particularly
difficult to achieve when considering vegetable production on green roofs and it is the major
environmental concern of rooftop gardening (Whittinghill and Rowe, 2011). The fertilizer
2
recommendation for typical extensive green roofs is 5 g N/m (FLL, 1995). Nitrogen
2
recommendations for agricultural vegetable crops grown in soil range from 4.5 g/m for snap
2
beans and peas to 22.4 g/m for celery (Warncke et al., 2004). These are much higher values
than those recommended for typical green roofs, but could still be low estimates for N needs for
roof top vegetable production because of the differences between soil and green roof substrate.
Whittinghill (2012) showed that it was possible to produce vegetables on an extensive green roof
2
with a minimal fertilizer input of 25 g/m of a 14-14-14 N-P-K slow release fertilizer, however
yields were much lower than those expected based on ground level soil production in the United
States. Yield improvements are possible with larger quantities of nutrient amendments, but there
is the potential for increasing nutrient concentrations in runoff water. One method of achieving
this balance between plant nutrient needs and runoff quality would be to use the lowest quantity
of fertilizer input possible while still achieving acceptable production results. With the use of
slow release fertilizers this would increase the probability that more of the fertilizer can be taken
up by plants before it is leached from the substrate (Emilsson et al., 2007).
87
A second practice to reduce nutrient leaching into runoff water is simply to limit runoff.
This could be achieved by utilizing substrate moisture management practices to reduce the
amount of irrigation that is applied to the production roof. Mulching is a common method of
managing soil moisture content and plant water needs in agriculture (Ham, et al., 1993; Masiunas
et al., 2003; Ngouajio and Ernest, 2001, Tanner, 1974; Tarara, 2000). Mulch has been shown to
affect the energy balance in the field, by reducing evaporation through the substrate surface and
by altering surface and near surface temperatures (Gruda, 2008; Law et al., 2006; Monks et al.,
1997; Olsen and Gounder, 2001; Warnick et al., 2006). A variety of plastic and organic mulches
can be used to achieve these effects. Some organic mulches, such as wood fiber, can however
lead to N immobilization because of their high carbon to N ratio and adversely affect fruit yields
(Gruda, 2008). Mulching may also enhance the ability of a green roof to support vegetable and
herb plants while minimizing the necessity of irrigation. On unmulched extensive green roofs,
many herbaceous native plants exhibit poor survival and health without irrigation (Monterusso et
al., 2005), which makes managing irrigation and irrigation efficiency on vegetable producing
green roofs important for production.
A third option which addresses nutrient runoff would be to collect runoff and recycle on
the roof, thus developing a closed system. However, there are issues regarding the buildup of
salts in the irrigation water and substrate. Therefore, the recycling option is not considered in
this study.
The use of live vegetative cover is also a practice which impacts the energy balance, and
therefore water loss from an agricultural setting. Its effects can, however, vary depending on the
plant used and the crop being produced (Abdul-Baki and Teasdale 1993) and can either be
positive (Abdul-Baki et al., 1996; Abdule-Baki and Teasedale, 1993) or negative due to
88
competition with the crop plants (Roberts and Anderson, 1994). The use of live plants as a
mulch had been explored on green roofs. Butler and Orians (2011) suggested that the use of
Sedum album as nurse plants for more sensitive herbaceous plants could improve survival and
appearance of those plants. This suggestion was based on previous studies showing that Sedum
reduced evaporative water loss from the substrate (Durhman et al., 2006; Wolf and Lundholm,
2008). It has also been established that green roof substrate covered by plants, regardless of
species, contains more moisture than uncovered substrate (VanWoert et al., 2005). Butler and
Orians (2011) found that the use of a living Sedum mulch reduced substrate temperatures and
improved herbaceous plant health during periods of drought. During periods where water was
not limiting however, the Sedum acted as a competitor with the herbaceous plants. It is possible
that such a mulching strategy could be beneficial in green roof agriculture.
The two main objectives of this study were to develop a mulching strategy and a
fertilization regimen that would improve crop production in extensive green roof agriculture.
Two types of mulch, pine bark and live Sedum mulch, and a no mulch control were examined. It
was hypothesized that the two mulches would reduce evaporation from the substrate surface and
improve plant productivity. A secondary goal of this study was the evaluation of Sedum as a
viable living mulch solution for vegetable or herb production on green roofs. Three fertilization
treatments were examined to determine if a low rate of fertilizer application can produce quality
crops and reduce potential impacts on runoff water quality.
89
Table 3.1. Initial physical and chemical properties of green roof substrate.
Component
Unit
86
Total Sand (%)
52.02
Extremely coarse sand (>2 mm) (%)
11.33
Very coarse sand (1-2 mm) (%)*
8.94
Coarse sand (0.5 -1 mm) (%)*
0
Medium sand (0.25-0.5 mm) (%)*
8.32
Fine sand (0.10-0.25 mm) (%)*
0
Very find sand (0.07-0.10 mm) (%)*
Extremely fine sand ( < 0.07 mm) (%)* 5.37
Silt (%)
Clay (%)
11
3
Bulk density (g/cm3)
Pore space (%)
Air filled porosity (%)
Water holding capacity at 0.01 MPa (%)
1.20
18.97
21.33
15.86
pH
Conductivity (EC) (m mho/cm)
7.2
1.59
65
Nitrate (mg/kg)
4.5
Phosphorus (mg/kg)
71
Potassium (mg/kg)
793
Calcium (mg/kg)
95
Magnesium (mg/kg)
65
Sodium (mg/kg)
94
Sulfur (mg/kg)
0.7
Boron (mg/kg)
24
Iron (mg/kg)
5.2
Magnesium (mg/kg)
4.3
Zinc (mg/kg)
0.6
Copper (mg/kg)
Analysis per A&L Great Lakes Laboratories, Inc., Ft. Wayne, IN.
* Analysis per Renewed Earth, Inc., Kalamazoo, MI.
Methods
Plot Location and Preparation. This experiment was conducted on green roof platforms
at the Michigan State University Horticultural Teaching and Research Center (HTRC) in Holt,
MI. Platforms were constructed as described by VanWoert et al. (2005). Platform plots
90
measured 1.2 x 1.2 m and 0.8 x 2.4 m. Green roof platforms were constructed as per VanWoert
et al. (2005a). Green roof substrate (Renewed Earth, Kalamazoo, MI) (Table 3.1) was installed
over a XeroFlor XF-105 drainage layer (XeroFlor America, Durham, NC) on June 10, 2009 for
the 1.2 x 1.2 m plots and May 5, 2010 for the 0.8 x 2.4 m plots to a depth of 12.7 cm (5 in).
Table 3.2. Composition of Lesco Professional Landscape and Ornamental All-Purpose Fertilizer
14-14-14 (Lesco, Inc., Cleveland, OH).
Nutrient
Percent Content by Weight (%)
14.00
Total Nitrogen
5.48
Ammoniacal Nitrogen
8.55
Urea Nitrogen
14.00
Phosphate (P2O5)
14.00
Soluble Potash (K O)
2
19.40
14.40
5.00
0.45
0.005
0.45
0.05
2.00
Total Sulfur
Free Sulfur
Combined Sulfur
Total Iron
Water soluble Iron
Total Manganese
Water soluble manganese
Chlorine Max
Three mulching strategies (no mulch, pine bark mulch, and a ground cover of Sedum
2
album) in combination with three fertilization rates (two applications of 25, 50, and 100 g/m of
Lesco Professional Landscape and Ornamental All-Purpose Fertilizer 14-14-14 (Table 3.2))
(Lesco, Inc, Cleveland, OH) were examined in a three by three factorial design layed out in a
randomized complete block with four replicates. Pine bark mulch was spread on June 29, 2010,
to a depth of 2.5 cm (1 in). Sedum album cuttings were taken from the green roof on the
Michigan State University Plant and Soil Sciences Building in East Lansing, MI, and 517 g
spread on each of the appropriate platform plots on June 24, 2010. These plots received some
supplemental watering during the establishment of the S. album living mulch. Fertilizer
91
application dates are listed in table 3.3. Weather data was compiled from the Michigan
Automated Weather Network (MAWN) station East Lansing/MSUHORT located at the HTRC
adjacent to the platforms.
Table 3.3. Important planting and maintenance dates for vegetable production during the 2010
and 2011 growing seasons on green roof platforms.
Activity
2010
2011
May 3 and 14
May 6
Seeds planted
May 15 and 17
June 16
Plugs planted
May 29
June 30
Pepper plugs planted
June 30
June 7
Irrigation started
October 3
October 1
Irrigation ended
July 1
July 22
Fertilizer application 1
August 9
August 26
Fertilizer application 2
Biomass harvest:
October 2 and 3
October 1
Tomatoes
September 20
August 29
Beans
August 24
August 29
Cucumbers
October 2
October 1
Peppers
October 1
Basil August 22- October 2
October 2
October 1
Chives
Tomatoes are Roma (VF) tomatoes (Lycopersicon esculentum), beans are early contender bush
beans (Phaseolus vulgaris), cucumbers are bush pickle hybrid cucumbers (Cucumis sativus),
®
peppers are Sweetheart hybrid sweet peppers in 2009 and 2010 and Budapest hot banana
peppers in 2011 (Capsicum annuum), basil is large-leaf Italian basil (Ocimum basilicum), and
chives (Allium schoenoprasum).
Plant Selection. Vegetable and herbs selected were Roma (VF) tomatoes (Lycopersicon
esculentum), early contender bush beans (Phaseolus vulgaris), bush pickle hybrid cucumbers
®
(Cucumis sativus), Sweetheart hybrid sweet peppers (Capsicum annuum) in 2009 and 2010,
large-leaf Italian basil (Ocimum basilicum), chives (Allium schoenoprasum) (Gurney’s Seed and
Nursery Co., Greendale, IN), and Budapest hot banana peppers (C. annuum) (Seedway, Hall,
NY) in 2011. They were selected because of their availability, growth habit (determinate or bush
variety), and common use in home gardens. These vegetable and herb cultivars were also shown
92
to be successful in an extensive green roof environment in 2009 (Whittinghill, 2012).
Vegetables and herbs were planted from seed into 48-cell plug trays in early May of each year
(Table 3.3) and grown outdoors, in East Lansing, MI. Budapest hot banana peppers were planted
from seed into 98-cell plug trays on April 6, 2011 and grown in a greenhouse until May 24,
2011, then transferred to a lath house at the HTRC.
Due to the limited space available two plants of each type were planted from plugs into
green roof platforms (Table 3.3). Plants within each plot were evenly spaced in two rows of six
and four rows of three in the 0.8 x 2.4 and 1.2 x 1.2 m plots, respectively. The specific location
of each plant was selected randomly in 2010 and remained the same for the 2011 growing season
because of the perennial nature of chives. Throughout the growing seasons of 2010 and 2011, all
platforms were watered three times daily for 5 min using micro-emitters (Table 3.3).
Data collection and analysis. Tomatoes, beans, cucumbers, and peppers were harvested
as they ripened until just before the first frost when all remaining fruit were harvested, combined
for each plot, and weighed. The sizes of individual vegetables were measured on the two longest
axes using a caliper. Vegetables were divided in to size categories based on USDA standards
(USDA, 1991; USDA, 1959; USDA 1936) and size categories were assigned numbers for
analysis with 1 representing the smallest size category for each vegetable and 5, 6, and 4
representing the largest size category for tomatoes, beans and cucumbers, respectively. Color of
tomatoes was also determined based on USDA standards (USDA, 1991) and color category
number (eg. 1 = green, 6 = red) was used for analysis. Potential acceptability of the vegetables
to the consumer was rated according to USDA grade standards (USDA, 1991; USDA, 1959;
USDA 1936; USDA, 2005). Reasons for unacceptability, such as insect or disease damage,
discoloration, or scarring, were recorded. Quality ratings were used to determine crop quality
93
according to USDA standards with lower numbers representing higher quality and 4 indicating a
cull (USDA, 1991; USDA, 1959; USDA 1936; USDA, 2005). Number of fruit, total yield in
grams and marketable yield in grams based of fruit quality were also recorded at time of harvest.
Marketable percent of yield was calculated from total and marketable yield data.
Just before the first frost in 2010 and 2011, whole basil and chive plants were harvested
at the substrate surface (Table 3.3). In 2010, basil leaves were evaluated for marketability by
removing all leaves and sorting based on the presence of insect damage, discoloration, sun
scalding or disease. Basil and chive plant material was weighed fresh, dried at 60 ˚C for 1 wk
and weighed again to determine biomass. At the end of each growing season, tomato, bean,
cucumber, and pepper plants were also cut even with the soil or substrate surface, and their
biomass was determined (Table 3.3). Biomass dry weight was also measured but will not be
discussed because it is correlated to biomass wet weight.
Substrate moisture was measured using a theta meter HH1 (Delta-T Devices, Cambridge,
England). Three measurements were taken from each plot in the afternoons between September
15 and September 20, 2011. These measurements were taken at the center of the north end,
middle and south end of the plots.
Data were analyzed using SAS (Version 9.1, SAS Institute, Cary, NC). Correlations
were found between number of fruit and total yield, marketable yield and plant biomass for all
2
vegetables (R from 0.5477 to 0.7147); total yield and marketable yield and plant biomass in all
2
2
vegetables (R > 0.8); and marketable yield and plant biomass (R > 0.8). Due to these high
correlations results for fruit number, marketable yield and plant biomass will be presented in
tables, but not discussed in the text. All data were checked for normality prior to analysis of
variance. Non-normal data were analyzed after applying a logarithmic transformation for tomato
94
total yield and pepper total yield, marketable yield and biomass fresh weight and a square root
transformation for bean number, total yield, and marketable yield and cucumber biomass fresh
weight. Normality issues for the marketable yield data were difficult to resolve due to a large
number of zero values across mulch and fertilizer treatments in 2011 so data for each year were
analyzed separately. All values are presented as back-transformed data.
Crop yield variables and volumetric moisture content were analyzed using an ANOVA
model with mulch treatment, fertilization treatment, and year as fixed effects (PROC MIXED,
SAS version 9.1, SAS Institute, Cary, NC). The significance of treatment interactions was
determined using a F tests and those interactions that did not have an effect were removed from
the analytical model (Table 3.4). Significant differences among treatments were separated using
multiple comparisons by LSD with an alpha of 0.05 (PROC MIXED, Version 9.1, SAS
Institute). No comparisons among species were made.
95
Table 3.4. Results for F tests for treatment interactions between mulch type, fertilizer
application and year. For all tests α = 0.05. * indicates p < 0.05, ** indicates p <0.01, and ***
indicates p < 0.001
Crop
Variable
MulchMulchMulch-Year FertililzerSpecies
Fertilizer- Fertilizer
Year
Year
F = 2.36
F = 3.97** F = 1.21
F = 3.16*
Tomato
Size
F = 0.80
F = 0.87
F = 1.55
F = 5.55**
Number
F = 0.52
F = 1.65
F = 1.52
F = 2.22
Total Yield
F = 1.62
F = 1.87
F = 0.3
F = 0.45
Marketable Yield
F = 2.24
F = 0.37
F = 0.68
F = 0.43
Marketable Percent of
Yield
F = 0.98
F = 0.78
F = 0.43
F = 1.57
Biomass Fresh Weight
F = 1.73
F = 1.00
F = 0.33***
F = 3.79*
Bean
Grade
F = 0.05
F = 0.95
F = 3.61*
F = 0.28
Number
F = 0.33
F = 1.25
F = 0.78
F = 0.78
Total Yield
F = 0.46
F = 0.61
F = 2.17
F = 1.05
Marketable Yield
F = 0.69
F = 0.97
F = 1.31
F = 1.29
Marketable Percent of
Yield
F = 0.33
F = 0.33
F = 5.17**
F = 0.69
Biomass Fresh Weight
F = 0.46
F = 0.4
F = 0.07
F = 0.16
Cucumber Grade
F = 0.55
F = 1.80
F = 0.28
F = 0.30
Number
F = 0.89
F = 0.77
F = 1.66
F = 1.22
Total Yield
F = 1.79
F = 2.18
F = 3.59*
F = 5.54**
Marketable Yield
F = 0.55
F = 1.13
F = 0.09
F = 0.70
Marketable Percent of
Yield
F = 1.79
F = 0.69
F = 2.86
F = 0.28
Biomass Fresh Weight
NA
F = 0.73
NA
NA
Pepper
Diameter
NA
F = 1.39
NA
NA
Grade
NA
F = 0.30
NA
NA
Number
NA
F = 2.04
NA
NA
Total Yield
NA
F = 1.65
NA
NA
Marketable Yield
NA
F = 0.52
NA
NA
Marketable Percent of
Yield
NA
F = 0.68
NA
NA
Biomass Fresh Weight
F = 0.19
F = 0.74
F = 3.32
F = 0.19
Basil
Biomass Fresh Weight
F = 0.83
F = 0.81
F = 11.87*** F=21.17***
Chives
Total Yield
Tomatoes are Roma (VF) tomatoes (Lycopersicon esculentum), beans are early contender bush
beans (Phaseolus vulgaris), cucumbers are bush pickle hybrid cucumbers (Cucumis sativus),
®
peppers are Sweetheart hybrid sweet peppers in 2009 and 2010 and Budapest hot banana
peppers in 2011 (Capsicum annuum), basil is large-leaf Italian basil (Ocimum basilicum), and
chives (Allium schoenoprasum).
96
precipitation
35.00
air temperature
Total precipitation = 303.26 mm
40.00
30.00
35.00
25.00
30.00
20.00
25.00
15.00
20.00
15.00
10.00
10.00
5.00
0.00
Temeprature (°C)
B
5.00
0.00
35.00
Total precipitation = 367.50 mm
180.00
160.00
30.00
140.00
25.00
120.00
Precipitation (mm)
Temperature (°C)
45.00
Precipitation (mm)
A
20.00
100.00
15.00
80.00
60.00
10.00
40.00
5.00
20.00
0.00
0.00
1 2 3 4 5 6 7 8 9 101112131415161718
Time (weeks)
Figure 3.1. Weekly average maximum air temperature in Celsius and weekly total precipitation
in mm for the growing seasons of (A) 2010 (May 25-Oct 2) and (B) 2011 (May 29-Oct 1).
Results
Weather. Maximum ambient air temperatures in 2010 and 2011 were similar (Figure
3.1). Minimum ambient air temperatures in 2011 were lower than those in 2010 in September
and October but higher in June, July and August. 2011 had more total precipitation that 2010
(Figure 3.1). 2010 had more precipitation in June and September, while 2011 had more
precipitation in July and August (Figure 3.1B). Incoming solar radiation was similar for both
97
growing seasons with totals for June, July, August and September of 2.16 and 2.21 million kJ/m
2
for 2010 and 2011, respectively.
Tomatoes. Results for tomato fruit size, color and grade are presented in table 3.5.
2
Mulching had no effect on fruit size in the 100 g/m fertilizer treatment in 2011, fruit color for
2
the 25 and 50 g/m fertilizer treatments in 2010, and fruit grade for any fertilizer treatment in
2011. Fertilizer treatment had no effect on fruit size in the pine bark and live Sedum mulch
treatments in 2011, fruit color in the no mulch and pine bark mulch in treatments in 2010, and
2
fruit grade for any mulch treatment in 2011. The 2010 25 g/m fertilizer treatment was the only
treatment to exhibit a difference in size between the no mulch treatment and both of the other
mulch treatments. The live Sedum mulch treatment produced redder fruit in the 25 g/m
2
fertilizer treatment than in other fertilizer treatments in 2010. In 2011, however, tomato fruit had
2
2
redder color in the no mulch 25 g/m and 50 g/m treatments than any other mulch treatment for
those fertilizer treatments. In 2010, fruit from the pine bark mulch treatment had lower grade
than any other mulch treatment for all fertilizer treatments. Fruit size and color were greater in
2
2010 than 2011, except for the no mulch and pine bark mulch and 25 g/m treatment
2
combinations and the no mulch 25 g/m treatment combination, respectively.
98
Table 3.5. Quality indices of tomatoes (Lycopersicon esculentum) and pod and fruit size of beans
(Phaseolus vulgaris) and cucumbers (Cucumis sativus) in 2010 and 2010 under treatments of no
2
mulch, pine bark mulch, live Sedum mulch, and 25, 50, and 100 g/m 14-14-14 slow release
fertilizer. Tomato sizes, colors and grades based on the USDA standards for fresh tomatoes
(USDA, 1991), bean sizes based on the USDA standards for snap beans for processing (USDA,
1959), and cucumber sizes based on the USDA standards for pickling cucumbers (USDA, 1936).
2
2
2
Crop
Variable Year Treatment
25 g/m
50 g/m
100 g/m
2.1 BaA
2.2 BabA
2.4 AaA
No mulch
1.7 BbA
2.3 AaA
2.5 AaA
Pine bark
1.9 BbA
2.0 ABbA
2.1 AbA
Live Sedum
1.9 AaA
1.6 BbB
1.8 BaB
2011 No mulch
1.7 AaA
1.9 AaB
1.7 AaB
Pine bark
1.6 AbB
1.5 AbB
1.6 AaB
Live Sedum
4.3 AaA
4.2 AaA
4.3 AaA
Fruit
2010 No mulch
Color
4.4 AaA
4.3 AaA
4.1 AaA
Pine bark
4.5 AaA
4.1 BaA
3.7 CbA
Live Sedum
4.3 AaA
3.7 BaB
3.7 BaB
2011 No mulch
3.4 AbB
2.9 ABbB
2.6 BaB
Pine bark
2.6 AbB
2.4 AbB
1.8 BbB
Live Sedum
3.0 AbB
3.3 BcB
3.0 AbB
Fruit
2010 No mulch
Grade
2.8 ABaB
2.6 AaB
2.9 BaB
Pine bark
2.9 AabB
3.0 ABbB
3.1 BcB
Live Sedum
2.0 AaA
2.0 AaA
2.1 AaA
2011 No mulch
1.7 AaA
1.8 AaA
2.0 AaA
Pine bark
1.7 AaA
1.9 AaA
2.1 AaA
Live Sedum
2.4 ABaB
2.2 BaB
2.6 AaB
Bean
Pod Size 2010 No mulch
2.5 AaB
2.3 AaB
2.5 AbB
Pine bark
2.3 AaA
2.4 AaB
2.1 BbB
Live Sedum
3.4 AaA
3.3 AaA
3.2 AaA
2011 No mulch
3.3 ABaA
3.5 BaA
3.1 AaA
Pine bark
3.0 AaA
3.7 AaA
3.4 AaA
Live Sedum
2.8 AaA
2.7 AaA
2.9 AaA
Cucumber Fruit
2010 No mulch
Size
2.7 AaA
2.8 AaA
3.0 AaA
Pine bark
2.6 BaA
2.7 ABaA
2.9 AaA
Live Sedum
2.9 AaA
2.1 BaB
2.4 ABabA
2011 No mulch
2.7 ABabA
2.1 BaB
3.1 AaA
Pine bark
1.3 AbB
1.3 AaB
1.9 AbB
Live Sedum
Each mean represents observations from 4 replicates. Means were separated using LSD with an
alpha of 0.05. Non-bold capital letters in rows indicate differences among fertilizer treatments
within mulch treatments and lower case letters in columns indicate differences mulch treatments
within fertilizer treatments for each year. Bold capital letters indicates differences between
years.
Tomato
Fruit
Size
2010
99
Table 3.6. Yield indices of tomatoes (Lycopersicon esculentum) in 2010 and 2011 under
2
treatments of no mulch, pine bark mulch, live Sedum mulch, and 25, 50, and 100 g/m 14-14-14
slow release fertilizer. Marketability based on USDA standards for fresh tomatoes (USDA,
1991)
Treatment
Number of Fruit
Total
Marketable Marketable Biomass
Yield (g)
Yield (g)
Percent of
Fresh
Yield (%)
Weight (g)
2010
2011
75.2 Aa 41.2 Ba 1833.8 a
1277.1 a
73.2 b
615.3 a
No mulch
57.4 Aa 36.0 Ba 1522.8 a
1283.3 a
87.1 a
594.9 a
Pine bark
74.4 Aa 27.2 Ba 1446.1 a
1012.1 a
76.5 ab
683.2 a
Live Sedum
2
46.4 Ab 29.1 Ba 1084.4 b
925.7 b
88.6 a
444.1 b
25 g/m
2
54.5 Ab
50 g/m
31.3 Ba
1310.0 b
978.1 b
74.7 b
471.0 b
2
106.1
44.0 Ba 2408.3 a
1668.8 a
73.4 b
978.3 a
Aa
N/A
N/A
2296.3 a
1592.0 a
70.9 b
786.5 a
2010
N/A
N/A
905.5 b
789.7 b
87.0 a
475.8 b
2011
Treatment means represent 24 observations, year means represent 48 observations except for
number of fruit where treatment means represent 12 observations. Means were separated using
LSD with an alpha of 0.05. Letters denote differences among treatments within each factor,
capital letters denote differences between years within treatment and are for the transformed data
for total yield, marketable yield and biomass fresh weight.
100 g/m
Results for the remaining tomato yield indices are presented in Table 3.6. Fruit number
and total yield were not affected by mulching. Fruit number was also not affected by fertilizer
treatment in 2011. Application of 100 g/m2 resulted in higher total, marketable yield, and fresh
2
biomass compared to lower rates. Surprisingly, the 25 g/m fertilizer treatment resulted in a
higher marketable percent of yield than treatments with higher fertilizer rates. Marketable
percent of yield was higher in the pine bark mulch treatment than the no mulch treatment. Fruit
were of a lower grade in 2011 than 2010. Fruit number and total yields were higher in 2010 than
2011, but marketable percent of yield was higher in 2011.
100
Table 3.7. Harvest indices of beans (Phaseolus vulgaris) in 2010 and 2011 under treatments of no mulch, pine bark mulch, live Sedum
2
mulch, and 25, 50, and 100 g/m 14-14-14 slow release fertilizer. Grade and marketability based on USDA standards for snap beans
for processing (USDA, 1959).
Treatment
Pod Grade
Number of Pods
Total
Marketable Marketable
Biomass Fresh
Yield (g)
Yield (g)
Percent of
Weight (g)
Yield (%)
2010
2011
2010
2011
2010
2011
2.3 Aa
3.3 Ba
47.6 Aa
11.6 Ba
107.0 a
78.6 a
56.3 a
68.8 Aa
21.7 Bb
No mulch
2.4 Aa
3.3 Ba
37.2 Aa
21.3 Ba
88.4 a
61.8 a
60.4 a
52.9 Aa
42.4 Aa
Pine bark
2.4 Aa
3.5 Ba
38.8 Aa
7.4 Bb
93.8 a
66.9 a
54.2 a
59.2 Aa
13.3 Bb
Live
Sedum
2
2.4 Aa
3.4 Bab
34.5 Aa
8.5 Bb
78.8 b
53.9 b
55.7 a
49.7 Aa
14.9 Bb
25 g/m
2
50 g/m
2.3 Aa
3.6 Bb
45.2 Aa
14.5 Bab
96.9 ab
70.3 ab
60.7 a
65.1 Aa
28.9 Ba
2
2.4 Aa
3.2 Ba
44.0 Aa
17.3 Ba
113.5 a
83.1 a
54.4 a
66.2 Aa
33.7 Ba
100 g/m
N/A
N/A
N/A
N/A
168.0 a
127.2 a
72.9 a
N/A
N/A
2010
N/A
N/A
N/A
N/A
24.8 b
11.0 b
41.1 b
N/A
N/A
2011
Treatment means represent 24 observations, year means represent 48 observations. Means were separated using LSD with an alpha of
0.05. Lower case letters denote differences among treatments within each factor and capital letters denote differences between years
within treatment. Differences are for the transformed data for total yield and marketable yield.
101
Beans. Results for bean pod size are presented in table 3.5. Pod size was only affected
2
by mulching in combination with the 100 g/m fertilizer treatment in 2010, for which the no
mulch treatment produced larger fruit. Pod size was no affected by fertilizer treatment in
combination with pine bark mulch in 2010, and the no mulch and live Sedum treatment in 2011.
2
When fertilizer did affect pod size, there was only a difference between the 25 g/m and other
treatments in combination with the live Sedum in 2010, when the 100 g/m fertilizer treatment
produced the smallest fruit. Pod size of bean fruit was smaller in 2010 than 2011 for all
2
treatment combinations except the live Sedum mulch 25 g/m treatment combination.
The remaining results for bean harvest indices are presented in table 3.7. Mulching had
no effect on pod grade, number of pods in 2010, or total yield. Fertilizer treatments also had no
effect on pod grade in 2010, number of pods in 2010, and marketable percent of yield. In 2011,
2
pod grades in the 25 g/m fertilizer treatment did not differ from either other fertilizer treatments.
There were fewer pods in the live Sedum than the other mulch treatments in 2011 and there was
2
no difference between the number of pods from the 50 g/m treatment and the other fertilizer
treatments. The effect of fertilizer was similar on total yield. All harvest indices were higher in
2010 than in 2011.
Cucumbers. Results for cucumber fruit size are presented in table 3.5. Mulching had no
effect on fruit size in combination with any fertilizer treatment in 2010 and in combination with
2
the 50 g/m fertilizer treatment in 2011. Fertilizer treatments had no effect on fruit size in
combination with the no mulch and pine bark mulch treatments in 2010 and the live Sedum in
102
2
2011. In 2010, fruit growing in the live Sedum mulch were larger in the 100 g/m treatment than
2
the 25 g/m treatment. In 2011, fruit sizes in the no mulch and pine bark were smaller in the 50
2
g/m fertilizer treatments and smaller in the live Sedum than other mulches in combination with
2
the 25 and 50 g/m fertilizer treatments. Fruit were smaller in 2011 than in 2010 in all mulch
2
treatments within the 50 g/m fertilizer treatment and all fertilizer treatments within the live
Sedum mulch.
Table 3.8. Harvest indices of cucumbers (Cucumis sativus) in 2010 and 2011 under treatments of
2
no mulch, pine bark mulch, live Sedum mulch, and 25, 50, and 100 g/m 14-14-14 slow release
fertilizer. Grades and marketability based on the USDA standards for pickling cucumbers
(USDA, 1936).
Treatment
Fruit
Number Total
Marketable Yield Marketable Biomass
Grade of Fruit Yield
(g)
Percent of
Fresh
(g)
Yield (%)
Weight (g)
2010
2011
3.6 a
5.1 a
730.6 a
456.8 Ab
22.7 Ba 26.3 a
65.9 a
No mulch
3.6 a
5.4 a
753.8 a
693.1 Aa
18.5 Ba 31.7 a
73.0 a
Pine bark
5.7 a
782.1 a
569.3 Aab 26.5 Ba 30.8 a
53.8 b
Live Sedum 3.6 a
2
3.7 a
4.6 b
548.0 b 365.4 Ab
15.2 Ba 22.5 a
51.3 b
25 g/m
2
3.5 a
50 g/m
5.6 ab
703.5 b
661.9 Aa
16.6 Ba
32.0 a
64.9 ab
2
3.6 a
6.0 a
1015.1 a 691.9 Aa
35.9 Ba 34.2 a
76.4 a
100 g/m
3.3 a
8.1 a
1169.0 a N/A
N/A
49.0 a
93.4 a
2010
3.9 b
2.7 b
342.0 b N/A
N/A
10.1 b
35.0 b
2011
Treatment means represent 24 observations, year means represent 48 observations, except for
marketable yield from which treatment means represent 12 observations. Means were separated
using LSD with an alpha of 0.05. Lower case letters denote differences among treatments within
each factor and capital letters denote differences between years within treatment. Biomass fresh
weight differences are for the transformed data.
Results for the remaining harvest indices for cucumbers are presented in table 3.8.
Mulching had no effect on fruit grade, number of fruit, total yield, or marketable percent of yield
and fertilizer treatments had no effect on fruit grade and marketable percent of yield. There was
103
a positive dose response of fertilizer treatments on number of fruit and total yield. All harvest
indices were higher in 2010 than in 2011.
Table 3.9. Harvest indices of Budapest hot banana peppers (Capsicum annuum) for the 2011
growing season under treatments of no mulch, pine bark mulch, live Sedum mulch, and 25, 50,
2
and 100 g/m 14-14-14 slow release fertilizer. Grades and marketability based on the USDA
standards for sweet peppers (USDA, 2005).
Treatment Fruit
Fruit
Number Total
Marketable Marketable Biomass
Diameter Grade of Fruit Yield
Yield (g)
Percent of
Fresh
(cm)
(g)
Yield (%)
Weight
(g)
2.3 a
4.8 a
87.7 a
70.9 a
76.1 a
47.4 a
No mulch 2.4 a
2.7 ab
3.5 a
48.0 a
35.2 b
62.2 ab
38.6 a
Pine bark 2.2 a
1.9 b
3.0 b
3.9 a
29.1 b
20.2 b
44.8 b
37.5 a
Live
Sedum
2
1.9 b
3.2 b
2.0 c
12.8 c
7.8 c
41.7 b
24.1 c
25 g/m
2
2.2 ab
50 g/m
2.5 a
3.9 b
44.5 b
34.8 b
75.4 a
40.1 b
2
2.4 a
2.5 a
6.7 a
107.5 a 83.7 a
66.1 ab
71.2 a
100 g/m
Treatment means represent 12 observations. Means were separated using LSD with an alpha of
0.05. Letters denote differences among treatments; for total yield, marketable yield and biomass
fresh weight denote differences are for the transformed data.
®
Pepper. Yields from Sweetheart in 2010 were extremely limited (data not shown).
Statistical analysis was not performed because we believe that this lack of success was due to the
cultivar choice and not the experimental treatments. The Budapest hot banana peppers were
more successful in 2011. Results for all harvest indices are presented in table 3.9. Mulching had
no effect on number of fruit. Fruit growing in the live Sedum mulch had smaller diameter,
higher grades, lower total yields, and lower marketable percent of yield than fruit from the no
mulch treatments. Fertilizer treatment had a positive dose response on all harvest indices
measured.
104
Basil. Pine bark mulch produced plants with lower biomass fresh weight than the other
2
two mulch treatments and plants from the 100 g/m fertilizer treatment had the highest biomass
fresh weight of any fertilizer treatment (Table 3.10). There were no differences in the biomass
fresh weights of 2010 and 2011 or in marketable percent of biomass of any mulch treatment or
any fertilizer treatment (Table 3.10).
Table 3.10. Biomass fresh weight in grams of basil (Ocimum basilicum) for the 2010 and 2011
growing season and marketable fresh weight yield in grams and marketable percent of biomass
fresh weight for the 2010 growing seasons under treatments of no mulch, pine bark mulch, live
2
Sedum mulch, and 25, 50, and 100 g/m 14-14-14 slow release fertilizer.
Treatment
Biomass Fresh Weight
Marketable Yield (g)
Marketable Percent of
(g)
Biomass (%)
1098.1 a
146.2 a
11.3 a
No mulch
730.1 b
58.8 b
8.4 a
Pine bark
984.7 a
95.1 ab
10.2 a
Live Sedum
2
619.5 c
67.3 b
9.1 a
25 g/m
2
862.6 b
50 g/m
102.4 ab
11.2 a
2
1330.8 a
130.5 a
9.6 a
100 g/m
968.3 a
NA
NA
2010
906.9 a
NA
NA
2011
Treatment means represent 24 observations, year means represent 48 observations. Means were
separated using LSD with an alpha of 0.05.Letters denote differences among treatments within
each factor for the transformed data.
Table 3.11. Fresh weight yield in grams for chives (Allium schoenoprasum) in 2010 and 2011
2
under treatments of no mulch, pine bark mulch, live Sedum mulch, and 25, 50, and 100 g/m 1414-14 slow release fertilizer.
Treatment
2010
2011
27.7 Ba
649.5 Aa
No mulch
23.3 Ba
623.4 Aa
Pine bark
32.6 Ba
455.0 Ab
Live Sedum
2
21.5 Bb
434.2 Ab
25 g/m
2
29.0 Bab
50 g/m
449.8 Ab
2
33.1 Ba
843.8 Aa
100 g/m
Each mean represents 12 observations. Means were separated using LSD with an alpha of 0.05.
Capital letters in rows denote differences among years within treatments and lower case letters in
columns indicate differences among treatments within year for each factor.
105
Chives. In 2010, mulching had no effect on yield (Table 3.11). Fresh weight yield of the
live Sedum mulch treatment was lower than the other mulch treatments in 2011. Fertilizer
treatments had a positive dose response on yield in both 2010 and 2011 (Table 3.11). Fresh
weight yields were higher in 2011 than in 2010 (Table 3.11).
Substrate moisture. There was no difference between volumetric moisture content of the
3
3
no mulch and live Sedum mulch treatments (0.303 and 0.295 m /m , respectively). The pine
3
3
bark mulch treatment had the lowest volumetric moisture content at 0.270 m /m .
Discussion
Overall the vegetable species examined performed better in 2010 than 2011. Although
the overall temperature trends in 2010 and 2011 were similar, the 2011 growing season exhibited
more weather extremes. The growing season started cool and was followed by a warm, dry June.
Although July was the wettest month in 2011, precipitation came in a few very large storm
events and was followed by another dry period. 2010 had more consistent precipitation
throughout the growing season with more frequent moderate storm events. These weather
differences likely resulted in harsher growing conditions in 2011 despite irrigation.
Total yields from this study were improved over those previously found on green roof
platforms at MSU. High yields from this study were higher than those reported by Whittinghill
(2012) (Table 3.12). These yields were within the range of those expected based on
recommended planting density (Swiader and Ware, 2002) and reported harvest area and yields
(USDA, 2011) for tomato, bean and cucumber but not pepper (Table 3.12). This suggests that
more intensive management can improve yields on extensive green roofs to those comparable to
in-ground conventional agriculture. The persisting low yields of peppers suggests that further
106
research on pepper cultivars is necessary before their production on extensive green roofs will be
as productive as in-ground production.
Table 3.12. Estimated yield in grams per plant for the highest yields achieved in this study, from
chapter 2 (Whittinghill, 2012), and from reported growing area and yield data reported by the
USDA (2011) and recommended planting densities based on row and plant spacing (Swiader and
Ware, 2002).
Vegetable
Estimated from this study Yields from Ch 2
Estimated yield
(g/plant)
(g/plant) from USDA
Treatment or
Yield
(Whittinghill, 2012) (2011) and Swaider and
Year
(g/plant)
Ware (2002)
Mulch
916.9
Tomato
treatments
Green roof platform,
744.93-1347.08
2
1,204.1
2010: 285.5
100 g/m
2010
1,148.1
Mulch
53.5
Bean
treatments
Green roof platform,
14.85-22.90
2
56.7
2010: 40.6
100 g/m
2010
84.0
Mulching
391.0
Cucumber
treatments
Green roof platform,
281.36-1067.24
2
507.55
2010: 213.05
100 g/m
2010
584.5
No mulch and
43.8
Pepper
Green roof platform,
pine bark mulch
712.62-564.88
2011: 15.49
2
53.7
100 g/m
Estimated yields from this study derived by dividing the indicated mean by 2 plants. USDA
2
(2011) yield estimates derived by dividing the USDA reported yield/m by estimated plant
densities derived from Swiader and Ware (2002).
Mulch treatment. The most common outcome of the mulch treatments across fertilizer
treatments, growing seasons and vegetable or herb species was that there was no significant
difference between the productivity of crops for any of the mulch treatments. The second most
common outcome was that there was no difference between the no mulch and pine bark mulch
treatments. There are however some exceptions. For tomato, the live Sedum mulch treatment
produced the largest fruit (Table 3.5) and pine bark mulch produced the best quality fruit under
107
some fertilizer treatments and the higher marketable percent of yield (Tables 3.3 and 3.4).
Sedum species reduce surface and near surface temperatures on a green roof compared to bare
substrate (Butler and Orians, 2011; Durhman et al., 2006; Wolf and Lundholm, 2008), which
may have enabled fruit to grow larger before ripening. The pine bark mulch, however has a
similar color to the green roof substrate, and may not have reduced temperatures as much as the
Sedum mulch while still preventing evaporation from the substrate enabling the production of
better quality fruit. Pine bark mulch also showed promising results for bean plant biomass
production (Table 3.7), and cucumber size, marketable yield and plant biomass production
(Tables 3.3 and 3.7) although not clearly above the other mulch treatments. This could suggest
that the mulch’s ability to moderate moisture losses through the substrate were beneficial, but its
effect on surface and near surface temperatures as compared with the other mulch treatments
were not as beneficial. Further research including more extensive measurements of substrate
moisture levels and near surface, surface and substrate temperatures would determine more
specifically how pine bark mulch affected crop growth and production.
2
When combined with 100 g/m fertilizer applications, Sedum mulch also acted as a
competitor with tomato, bean, and cucumber, but only affected some aspects of production such
as color (Table 3.5), size of fruit (Tables 3.3), and biomass production (Tables 3.5 and 3.6).
Peppers were affected negatively by live Sedum mulch more than the other vegetables, with all
variables except number of fruit lower than the other mulch treatments (Table 3.8). It suggests
that the live Sedum mulch either acted as a competitor, decreasing crop production, or that the
resulting changes in surface and near surface temperatures reduced temperatures below those
favored by peppers. The perennial chives was not affected by mulch treatment during the first
year of production, but the Sedum mulch appeared to act as a competitor, reducing yields, during
108
the second year of production. Other research into the use of live cover crops between rows of
low-till systems have also shown mixed results in the performance of cabbage, snap beans
(Masiunas et al., 1997) tomato (Akintoye et al., 2004), broccoli (Chase and Mbuya, 2008), and
peppers (Chellemi and Russkopf, 2004). In these cases living mulches could act as competitors
with the desired crop under some management systems. In the case of living Sedum, it has been
shown to act as a competitor with herbaceous plants on a green roof under conditions when water
is not limiting (Butler and Orians, 2011). As irrigation was supplied to the vegetables and herbs
in this study, it is likely that enough irrigation was not a limiting factor. It is also possible that
Sedum was better able to make use of the supplied fertilizer than the vegetable and herb crops.
Sedum album has a very shallow, but dense root system and it is possible that the fertilizer was
taken up by the S. album as it was released before it reached the deeper, less dense roots of the
vegetable and herb species. It is likely that this form of living mulch should be avoided in chives
and Budapest hot banana peppers production.
The lack of difference between the volumetric moisture content of the no mulch and live
Sedum mulch treatments and lower volumetric moisture in the pine bark mulch treatment was an
unexpected result. It has been well established that the use of mulch can reduce moisture loss
through the growing substrate (Ham, et al., 1993; Masiunas et al., 2003; Ngouajio and Ernest,
2001, Tanner, 1974; Tarara, 2000). One explanation for the discrepancy between this and
previous research is the timing of volumetric moisture content measurements. Measurements
were taken in September of 2011, when temperatures had cooled down, become cloudy and there
were shorter periods of time between precipitation events. These weather conditions may have
resulted in conditions where the substrate under all mulch treatments could become and stay
saturated. Irrigation throughout the experiment may have also reduced differences between
109
mulch treatments because of greater water availability. Further experimentation with the use of
mulches in extensive green roof vegetable production where measurements are taken throughout
the growing season could reveal greater differences in the volumetric moisture content of
substrate under mulch treatments. Measurements throughout the growing season under irrigated
conditions may also show the extent to which mulch reduces water loss through evaporation
under conditions where water is less limiting.
Fertilizer treatment. The most frequent outcome of the fertilizer treatments was that they
had no effect on crop quality or production. The second most frequent outcome was that the 100
2
g/m treatment outperformed at least one of the other fertilizer treatments. Both basil and chives
are crops with yields dependent on vegetative growth. Nitrogen increases vegetative growth, so
2
higher yields in the 100 g/m fertilizer treatment would be expected.
There were, however some difference among crops and among measured variables within
2
crops. The 25 g/m fertilizer treatment either outperformed other fertilizer treatments or
performed second best for tomato color (Table 3.5) and marketable percent of yield (Table 3.6),
bean size and grade (Tables 3.3 and 3.5), and cucumber size (Table 3.5). There were also a
2
2
number of instances where the 50 g/m performed as well as the 100 g/m treatment, but better
2
than the 25 g/m treatment. Comparing recommended N needs of each vegetable and herb
species based on soil tests performed by the Michigan State University Soil and Plant Nutrient
Laboratory (East Lansing, MI) and recommended application rates from Warncke et al. (2004)
for mineral soils (Table 3.13), with the amounts nutrients provided by the fertilizer treatments
(Table 3.13) can explain some of these differences. The amount of phosphorus (P) and
2
potassium (K) applied by the 25 g/m fertilizer treatment were adequate for tomato, bean,
110
2
Table 3.13. The nutrient recommendations (g/m ) for each vegetable and herb crop based on
soil testing performed by the Michigan State University Soil and Plant Nutrient Laboratory
(MSUSPNL) (East Lansing, MI) and nutrient application recommendations from Warncke et al
2
2
(2004) and the nutrients supplied (g/m ) by the applications of 25, 50 and 100 g/m of a Lesco
Professional Landscape and Ornamental All-Purpose Fertilizer 14-14-14.
Nitrogen
Phosphorus
Potassium
2
2
2
(g/m )
(g/m )
(g/m )
13.45
0.0363
0.206
Crop
Tomatoes
4.483
0.138
0.0575
Beans
11.209
0.0312
0.06084
Cucumber
11.209
0.0454
0.1183
Peppers
4.88-9.76
10.74
6.835
Ground cover (from
MSUSPNL) as a
proxy for basil
and chives
2
3.5
3.5
3.5
Treatment
25 g/m
2
7
7
7
50 g/m
2
14
14
14
100 g/m
Tomatoes are Roma (VF) tomatoes (Lycopersicon esculentum), beans are early contender bush
beans (Phaseolus vulgaris), cucumbers are bush pickle hybrid cucumbers (Cucumis sativus),
®
peppers are Sweetheart hybrid sweet peppers in 2009 and 2010 and Budapest hot banana
peppers in 2011 (Capsicum annuum), basil is large-leaf Italian basil (Ocimum basilicum), and
chives (Allium schoenoprasum).
2
cucumber, and pepper. This may have been the influencing factor enabling the 25 g/m fertilizer
treatment to produce redder tomato and higher quality tomato, bean, and cucumber. The 50 g/m
treatment only supplied adequate N for bean, explaining why unlike the other vegetable species,
2
this treatment rather than the 100 g/m treatment provided the best yields. Only the 100 g/m
2
treatment provided adequate N for tomato, cucumber, and pepper. It is possible that fertilizer
with a higher N content, but similar P and K content could be applied in lower rates and yield
quality crops. Further research should be performed on fertilizer composition to improve the
efficiency of nutrient use by the crop plants. In a larger scale production system it may also be
111
2
possible to fertilize parts of the roof differently based on specific crop needs which would also
increase the efficiency of fertilizer use and minimize the impacts on runoff quality.
Conclusions
Use of more intensive management practices improved yields of tomato, bean and
cucumber plants enough that they fell within the range of those expected from in-ground
conventional agriculture. This supports the theory that extensive green roof agriculture is a
productive use of rooftop space. The use of pine bark mulch or live Sedum mulch in the
production of tomatoes, beans, and cucumbers did not have a clear impact on production in this
two-year study. It would appear that neither of these mulches would be necessary for successful
production of these crops on an extensive green roof in Michigan, provided that the roofs were
irrigated. There are other mulch alternatives not examined here, such as straw or paper pulp
mulch, which could improve production and warrant examination. The live Sedum album mulch
used in this study appeared to act as a competitor with Budapest hot banana peppers and chives
and should not be used in production of those crops. Although it did affect some aspects of
tomato, bean, and cucumber production, it need not be avoided in production of those crops in
Michigan. A lack of difference between the volumetric moisture content of the mulch treatments
examined in this study is more likely to be attributed to irrigation and weather patterns than
mulch treatments and measuring over a longer period of time during the growing season in future
research would better reveal the effects of various mulch treatments on substrate moisture
content in green roof vegetable production.
There were few differences between fertilizer treatments. This may be attributed to the
lowest fertilizer applications rates supplied adequate P and K for most crops in the study, but
112
inadequate N in all but the largest fertilizer applications. That adequate N was supplied by only
2
the 100 g/m fertilizer treatment explains why, in most cases, when there was a difference in
fertilizer treatments, this produced the best crops. This suggests that a fertilizer should be used
that will supply higher amounts of N per unit of P and K supplied to the plants. Further research
on fertilizers with different compositions including micronutrients would reveal the best mix of
N, P, and K to apply to vegetable production in extensive green roof agriculture. For the cases
2
where the 50 and 100 g/m fertilizer treatments were not different, examination of runoff water
nutrient levels could determine which is more appropriate to use from an environmental
standpoint. This would help prevent the occurrence of the negative impacts of fertilizer over
application.
Acknowledgements
Funding for this study was provided by Ford Motor Company, Dearborn, MI;
ChristenDETROIT Roofing Contractors, Detroit, MI; XeroFlor America, Durham, NC; Renewed
Earth, Kalamazoo, MI; and MSU AgBioResearch. Special thanks to the Michigan State
University Horticultural Teaching and Research Center for their support, and to Saloni Dagli and
Daniel Finks for their field assistance.
113
REFERENCES
114
REFERENCES
Abdul-Baki A.A., J.R. Stommel, A.E. Watada, J.R. Teasdale, and R.D. Morse. 1996. Hairy vetch
mulch favorably impacts yield of processing tomatoes. HortScience 31:338-340.
Abdul-Baki A.A. and J.R. Teasdale. 1993. A No-Tillage tomato production system using hairy
vetch and subterranean clover mulches. HortScience 28:106-108.
Akintoye, H.A., E.O. Agbeyi, and A.B. Olaniyan. 2004. The effects of live mulches on tomato
(Lycopersicon esculentum) yield under tropical conditions. Journal of Sustainable
Agriculture 26 (1):27-37.
Beck, D.A., G.R. Johnson, and G.A. Spolek. 2011. Amending greenroof soil with biochar to
affect runoff water quantity and quality. Environmental Pollution 159:2111-2118.
Butler, C. and C.M. Orians. 2011. Sedum cool soil and can improve neighboring plant
performance during water deficit on a green roof. Ecological Engineering 37:1796-1803.
Chase, C.A., and O.S. Mbuya. 2008. Greater interference from living mulches than weeds in
organic broccoli production. Weed Technology 22(2):280-285.
Chellemi, D.O., and E.N. Rosskopf. 2004 Yield potential and soil quality under alternative crop
production practices for fresh market pepper. Renewable Agriculture and Food Systems.
19(3):168-175.
Czemiel Berndtsson, J. 2010. Green roof performance towards management of runoff water
quantity and quality: a review. Ecological Engineering 36:351-360.
Czemiel Berndtsson, J., T. Emilsson, and L. Bengtsson. 2006. The influence of extensive
vegetated roofs on runoff water quality. Science of the Total Environment 355:48-63.
Durhman, A.K., D.B. Rowe and C.L. Rugh. 2006. Effect of watering regimen on chlorophyll
fluorescence and growth of selected green roof plant taxa. HortScience 41(7):1623-1628.
Emilsson, T. 2004. Impact of fertilization on vegetation development and water quality. In Proc.
of 2nd North American Green Roof Conference: Greening Rooftops for Sustainable
Communities, Portland, OR. 2 -4 June 2004. The Cardinal Group, Toronto.
Emilsson, T., J. Czemiel Berdtsson, J.E. Mattsson, and K. Rolf. 2007. Effect of using
conventional and controlled release fertilizer on nutrient runoff from various vegetated
roof systems. Ecological Engineering 29:260-271.
115
FLL [Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau (The Landscaping and
Landscape Developmant Research Society)]. 1995. Guideline for the Planning,
Execution, and Upkeep of Green-Roof Sites. Forschungsgesellschaft
Landschaftsentwicklung Landschaftsbau, Bonn, Germany.
Gruda, N. 2008. The effect of wood fiber mulch on water retention, soil temperature and growth
of vegetable plants. Journal of Sustainable Agriculture 32(4):629-643.
Ham J.M., G.J. Kluitenberg, and W.J. Lamont. 1993. Optical properties of plastic mulches affect
the field temperature regime. J. Amer. Soc. Hort. Sci. 118:188-193.
Hathaway, A.M., W.F. Hunt, and G.D. Jennings. 2008. A field study of green roof hydrologic
and water quality performance. American Society of Agricultural and Biological
Engineers 51(1):37-44.
Law, D.M., A.B. Rowell, J.C. Snyder, and M.A. Williams. 2006. Weed control efficacy of
organic mulches in two organically managed bell pepper production systems.
HortTechnology 16(2):225-232.
MacMillan, G. 2004. York university rooftop garden stormwater quantity and quality
performance monitoring report. In Proceedings of 2nd North American Green Roof
Conference: Greening rooftops for sustainable communities, Portland, OR, June 2-4,
2004. The Cardinal Group, Toronto, p. 8–24.
Masiunas, J.B., D.M. Eastburn, V.N. Mwaja, and C.E. Eastmans. 1997. The Impact of living and
cover crop mulch systems on pests and yields of snap beans and cabbage. Journal of
Sustainable Agriculture 9(2/3):61-89.
Masiunas J., E. Wahle, L. Barmore, and A. Morgan. 2003. A foam mulching system to control
weeds in tomatoes and sweet basil. HortTechnology 13(2):324-328.
Monks, C.D., D.W. Monks, T. Basden, A. Selders, S. Poland, and E. Rayburn. 1997. Soil
temperature, soil moisture, weed control, and tomato (Lycopersicon esculentum) response
to mulching. Weed Technology 11:561-566.
Monterusso, M.A., D.B. Rowe, and C.L. Rugh. 2005. Establishment and persistence of Sedum
spp. And native taxa for green roof applications. HortScience 40(2):391-396.
Ngouajio M., and J. Ernest. 2004. Light transmission through colored polyethylene mulches
affected weed population. HortScience 39(6):1302-1304.
Olsen, J.K. and R.K. Gounder. 2001. Alternatives to polyethylene mulch film- a field assessment
of transported materials in capsicum (Capsicum annuum L.). Australian Journal of
Experimental Agriculture 41:93-103.
116
Roberts, B.W. and J.A. Anderson. 1994. Canopy shade and soil mulch effect yield and solar
injury in pepper. HortScience 29:258-260.
Rowe, D.B. 2011. Green roofs as a means of pollution abatement. Environ. Pollution 159(89:2100-2110.
Tanner, C.B. 1974. Microclimate modification: Basic concepts. HortScience 9:555-560.
Tarara, J.M. 2000. Microclimate Modification with Plastic Mulch. HortScience. 35(2):169-180.
United States Department of Agriculture (USDA). 1991. United States standards for grades of
fresh tomatoes. Agricultural marketing service. Accessed 3 Oct 2009.
.
United States Department of Agriculture (USDA). 1959. United States standards for grades of
snap beans for processing. Agricultural marketing service. Accessed 3 Oct 2009.
.
United States Department of Agriculture (USDA). 1936. United States standards for grades of
pickling cucumbers. Agricultural marketing service. Accessed 3 Oct 2009.
.
United States Department of Agriculture (USDA). 2005. United States standards for grades of
sweet peppers. Agricultural marketing service. Accessed 3 Oct 2009.
.
VanWoert, N.D., D.B. Rowe, J.A. Andresen, C.L. Rugh, R.T. Fernandez, and L. Xiao. 2005.
Green roof stormwater retention: Effects of roof surface, slope, and substrate depth. J.
Environ. Qual. 34:1036-1044.
Warncke, D., J. Dahl, and B. Zandstra. 2004. Nutrient recommendations for vegetable crops in
Michigan. Michigan State University Extension Bulletin E2934.
Warnick, J.P., C.A. Chase, E.N. Rosskopf, E.H. Simonne, J.M. Scholberg, R.L. Koenig, and
N.E. Roe. 2006. Weed suppression with hydramulch a biodegradable liquid paper mulch
in development. Renewable Agriculture and Food Systems 21(4):216-223.
Whittinghill, L.J. 2012. Examination of vegetable production using green roof technology and
the potential impacts on the benefits provided by conventional green roofs. PhD
Dissertation, Michigan State University, East Lansing.
Whittinghill, L.J. and D.B. Rowe. 2011. The role of green roof technology in urban agriculture.
Renewable Agriculture and Food Systems. DOI 10.1017/S174217051100038X.
Wolf, D. and J.T. Lundholm. 2008. Water uptake in green roof microcosms: Effects of plant
species and water availability. Ecological Engineering 33:179-186.
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CHAPTER FOUR:
Stormwater runoff quantity and quality from two traditional and one vegetable producing
extensive green roof
118
Abstract
Stormwater retention is one of the well-studied benefits of green roofs. A roof’s ability
to retain stormwater depends on factors such as the intensity and duration of the rain event as
well as substrate depth, substrate moisture content, and vegetation type, health, density and water
use efficiency. Extensive green roofs used for crop production will differ from more traditional
Sedum and prairie covered extensive green roofs in plant density and water use efficiency, but
the impact on stormwater retention has not been well studied. Three vegetation types
(unfertilized Sedum and native prairie species mixes, and a fertilized vegetable and herb species
mix) were compared for stormwater runoff quantity over three growing seasons, and stormwater
runoff quality and substrate moisture content during one growing season. The prairie covered
green roofs had the greatest increase in runoff as precipitation increased, almost three times that
of Sedum or vegetable producing green roof treatments. Vegetation treatment had no effect on
-
-
runoff nitrate-nitrogen (NO3 ) concentrations, but NO3 concentrations decreased over the course
of the growing season. Runoff phosphorus (P) concentrations also decreased over time in the
Sedum and prairie treatments, which were lower than P concentration from the vegetable crop
-
system throughout the growing season. This is likely a result of the different amounts of NO3
and P applied to the vegetable treatment and the needs of the crop plants in that treatment.
Vegetation treatment had no effect on substrate moisture content. The similarities in water
retention and water quality between vegetable producing extensive green roofs and Sedum green
roofs suggest that vegetable production with careful nutrient management will not have a
negative impact on the ability of extensive green roofs to retain stormwater or manage
stormwater quality.
119
Introduction
Green roofs have been shown to have numerous benefits. These include reduced air and
noise pollution, increased habitat and biodiversity, increased roof lifespan, stormwater retention,
energy savings and mitigation of the urban heat island effect (Alexandri and Jones, 2008; Barrio,
1998; Carter and Jackson, 2007; Getter and Rowe, 2006; Getter et al., 2007; Loder and Peck,
2004; Saiz et al., 2006; VanWoert et al., 2005a; Wong et al., 2003). Stormwater retention and
energy savings are two benefits that have come to the forefront in light of growing concerns
about greenhouse gas emissions and difficulties in managing water flow off impervious surfaces
in urban areas. Both are impacting policy in the United States (Carter and Keeler, 2008, Lipton,
2005). Green roofs have been shown to retain between 52 and 100% of precipitation depending
on the intensity and duration of the rain event and depending on slope, substrate depth, substrate
moisture content, evapotranspiration rates, and vegetation (Czemiel Berndtsson, 2010; Getter et
al., 2007; Hathaway et al., 2008; Rowe, 2011; VanWoert et al., 2005a). This has been well
studied for roofs planted with a more typical selection of green roof plants, such as Sedum spp.
Agricultural production on green roofs is of growing interest and is in practice in many cities
around the world (Whittinghill and Rowe, 2011). There is however no literature on the impacts
of agricultural production on the ability of a green roof to retain stormwater or the impact on
runoff water quality, which could be of great concern due to nutrient supplementation necessary
for crop production (Whittinghill and Rowe, 2011).
Many other factors influence the ability of a green roof to provide benefits in the form of
storm water retention. These include water use efficiency of the vegetation and
evapotranspiration rates, microclimatic conditions including near-surface, and plant
temperatures, absorption of incoming radiation by the substrate surface and roof vegetation,
120
emission of sensible heat from the substrate and roof vegetation, plant health and survival, the
density of vegetation cover, shade cover provided by the vegetation, exposure and color of the
soil surface and soil moisture content (Durhman et al., 2006; Ham et al., 1991; Heilman et al.,
1996; Tanner, 1974; VanWoert et al., 2005a, 2005b). These variables can be explored by
examining vegetation covers that have varying water use efficiencies, growth habits, numbers of
plants and amounts of bare substrate. In this experiment two traditional extensive green roof
vegetation covers, a Sedum spp. mix and a native prairie species mix will be compared with a
vegetable and herb mix.
Vegetable and herb plants also have very different water needs than most species usually
found on extensive green roofs. Sedums, for example require very little water over the course of
a growing season (Durham et al., 2006; VanWoert et al., 2005b), while tomatoes are very
sensitive to changes in water availability. Dry spells can delay maturity of the tomato crop and
decrease yield and crop quality (Abdul-Baki and Teasdale, 2007). Other herbaceous plants
examined for growth on extensive green roofs were also found to be drought intolerant with
limited survival in the absence of irrigation (Durham et al., 2006; Monterusso et al., 2005). This
may include vegetable and herb crops. For standard in ground production, irrigation
recommendations are 3.05 to 12.19 mm/day (0.12-0.48 in/day) for tomatoes, 5.08 mm/day (0.2
in/day) for garden beans, and 3.62 to 7.26 mm/day (0.14 to 0.29 in/day) for cucumbers (Swiader
and Ware, 2002). The difference in the water needs of sedums and herbaceous plants can be
traced to their photosynthetic adaptations. Sedums exhibit crassulacean acid metabolism (CAM)
under drought conditions and reduce transpiration water loss during the day by opening their
stomata at night (Getter and Rowe, 2006). Prairie species are often a mix species exhibiting C3
and C4 photosynthesis and many vegetable and herb species exhibit C3 photosynthesis and
121
possess no mechanism for reducing daytime transpiration water loss. On green roofs that are not
irrigated Sedum spp. and many prairie species also have the ability to go dormant during periods
of drought, during which time transpiration is reduced to near zero.
These three vegetation covers also exhibit very different growth habits. Sedum spp. are
succulent ground covers and tend to remain close to the ground without large leaf structures,
while prairie species exhibit a wide variety of growth forms from broad-leafed rosettes to tall
grasses. Vegetable and herb species also exhibit wide variety in growth forms, although many
are broad leafed and many grow with either a bush or vine shape. The taller and broad leafed
growth forms of prairie and vegetable and herb species could enable denser canopy cover, than
the lower growing Sedum spp.
It is also expected that sedums and prairie species will completely cover the rooftop area
on a mature green roof, shading the substrate and minimizing evaporation (Durhman et al., 2006;
VanWoert et al., 2005b; Wolf and Lundholm, 2008). Vegetables and herbs however, are often
planted with empty space between them. The substrate may remain exposed for the entire
growing season if the crop canopy does not close. These differences in coverage of the substrate
also translate into differences in number of plants. With complete coverage the Sedum spp. and
prairie roofs with have many plants per unit area, while vegetable crops typically have densities
2
of between 2 and 35 plants/m , depending on the variety (Swiader and Ware, 2002). These
differences could lead to very different energy balances on green roofs created using the two
different types of vegetation, specifically where substrate moisture content, and therefore storm
water retention are concerned.
The main objective of this experiment was to determine what, if any, differences exist in
stormwater retention and substrate moisture content of extensive green roofs planted with
122
vegetables and herbs for food production, and those planted with more typical mixes of either
Sedum or prairie species. This was achieved by monitoring runoff from three green roof types
over three growing seasons. Substrate moisture measurements and runoff water samples were
taken during one growing season to evaluate differences in substrate moisture content and runoff
water quality. As the capacity of a green roof to retain storm water is related to the substrate
moisture content at the onset of a precipitation event and the water holding capacity of the
substrate (VanWoert et al., 2005a) it was expected that green roofs planted with vegetables and
herbs would retain more storm water than the green roofs planted with either the sedum or prairie
mixes, but have lower substrate moisture content. It is also expected that runoff water from the
green roof planted with vegetables and herbs would have higher concentrations of nutrients
because fertilizer must be used to provide adequate nutrients for vegetable and herb production.
Methods
Experimental design. Three landscape systems were examined; (1) a Sedum mix,
hereafter referred to as sedum green roof, containing Sedum acre ‘Golden Carpet’, S. album, S.
kamtschaticum, and S. spurium ‘Summer Glory’; (2) a native prairie mix, hereafter referred to as
prairie green roof, containing by percentage of seed weight: 0.5% Achillea millefolium (yarrow),
2% Asclepias tuberosa (butterflyweed), 1% Asclepias syriaca (common milkweed), 4% Aster
novae-angliae (New England aster), 0.5 % Aster pilosus (hairy aster), 3% Coreopsis lanceolata
(sand tickseed), 1% Desmodium canadense (showy tick trefoil), 2% Echinacea purpurea (purple
coneflower), 0.5% Kuhnia eupatoroides (false boneset), 2% Lupinus perennis (wild lupine), 4%
Monarda fistulosa (burgamot), 0.5% Oenothera biennis (common evening primrose), 2%
Penstemon digitalis (foxglove beardtongue), 4% Ratibida pinnata (grayheaded coneflower), 6%
123
Rudbeckia hirta (blackeyed Susan), 1% Rudbeckia triloba (three-lobed coneflower), 1%
Silphium integrifolium (rosinweed), 1% Silphium laciniatum (compass plant), 4% Solidago
rigida (stiff goldenrod), 3% Solidago speciosa (showy goldenrod), 2% Verbena stricta (hoary
vervain), 12% Andropogon gerardii (big bluestem), 6% Elymus canadensis (Canada wild rye),
5% Panicum virgatum (switch grass), 20% Schizachyrium scoparium (little bluestem), 12%
Sorghastrum nutans (Indian grass); and (3) vegetable and herb mix, hereafter referred to as
vegetable green roof, of Lycopersicon esculentum (Roma (VF) tomatoes), Phaseolus vulgaris
(early contender bush beans), Cucumis sativus (bush pickle hybrid cucumbers), Capsicum
®
annuum (Sweetheart hybrid sweet peppers) in 2009 and 2010, Ocimum basilicum (large-leaf
Italian basil), Allium schoenoprasum (chives) (Gurney’s Seed and Nursery Co., Scarlet Tanager
LLC., Greendale, IN), and C. annuum (Budapest hot banana peppers) (Seedway, Hall, NY) in
2011. These vegetable and herb species were selected because of their availability, growth habit
(determinate or bush variety), and common use in home gardens.
The experiment was conducted on green roof platforms at the Michigan State University
Horticultural Teaching and Research Center (HTRC) in East Lansing, MI, during three growing
seasons from 2009 to 2011. Green roof platforms were constructed as per VanWoert et al.
(2005a). Green roof substrate (Renewed Earth, Kalamazoo, MI) (Table 4.1) was installed on
June 10, 2009 to a depth of 10.5 cm over a XeroFlor XF-105 drainage layer (XeroFlor America,
Durham, NC). These platforms contained three 0.8 x 2.4 m sections. In order to control for edge
effect, the green roof types were alternated between the three sections in the four replicate
platforms.
124
Table 4.1. Initial physical and chemical properties of substrate
Component
Unit
86
Total Sand (%)
52.02
Extremely coarse sand (>2 mm) (%)
11.33
Very coarse sand (1-2 mm) (%)*
8.94
Coarse sand (0.5 -1 mm) (%)*
0
Medium sand (0.25-0.5 mm) (%)*
8.32
Fine sand (0.10-0.25 mm) (%)*
0
Very find sand (0.07-0.10 mm) (%)*
Extremely fine sand ( < 0.07 mm) (%)* 5.37
Silt (%)
Clay (%)
11
3
Bulk density (g/cm3)
Pore space (%)
Air filled porosity (%)
Water holding capacity at 0.01 MPa (%)
1.20
18.97
21.33
15.86
pH
Conductivity (EC) (m mho/cm)
7.2
1.59
65
Nitrate (mg/kg)
4.5
Phosphorus (mg/kg)
71
Potassium (mg/kg)
793
Calcium (mg/kg)
95
Magnesium (mg/kg)
65
Sodium (mg/kg)
94
Sulfur (mg/kg)
0.7
Boron (mg/kg)
24
Iron (mg/kg)
5.2
Magnesium (mg/kg)
4.3
Zinc (mg/kg)
0.6
Copper (mg/kg)
Analysis per A&L Great Lakes Laboratories, Inc., Ft. Wayne, IN.
* Analysis per Renewed Earth, Inc., Kalamazoo, MI.
The sedum and prairie treatments were sown from seed on May 16, 2009. The sedum
was planted by mixing 1.2 g of seed with 400 mL of sand and broadcasting the mixture over
each plot by hand. Similarly, the prairie mix was also hand broadcast at a rate of 2.83 g of seed
per plot. Plots were covered with shade cloth until seeds germinated.
125
Table 4.2. Planting and maintenance dates of the vegetable green roofs for the growing seasons
of 2009, 2010 and 2011.
Action
2009
2010
2011
May 9
May 3 and 14
May 6
Seeds planted
May 29
May 15 and17
June 16
Plugs planted
May 29
May 29
June 30
Pepper plugs planted
May 29
June 30
June 7
Irrigation started
October 2
October 3
October 1
Irrigation ended
June 13
July 1
July 22
Fertilizer application 1
N/A
August 9
August 26
Fertilizer application 2
Due to the limited space available two plants of each vegetable and herb species were
evenly spaced in two rows of six with the specific location of each plant randomly selected at the
beginning of the 2009 growing season. Vegetables were sown into 48-cell plug trays and grown
outside, in East Lansing, MI (Table 4.2) and then transplanted in the platforms as plugs (Table
4.2). Capsicum annuum (Budapest hot banana peppers) were sown from seed into 98-cell plug
trays on April 6, 2011 and grown in a greenhouse until May 24, 2011, then transferred to a lath
Table 4.3 Composition of Lesco Professional Landscape and Ornamental All-Purpose Fertilizer
14-14-14 (Lesco, Inc., Cleveland, OH).
Nutrient
Percent Content by Weight (%)
14.00
Total Nitrogen
5.48
Ammoniacal Nitrogen
8.55
Urea Nitrogen
14.00
Phosphate (P2O5)
14.00
Soluble Potash (K O)
2
19.40
14.40
5.00
0.45
0.005
0.45
0.05
2.00
Total Sulfur
Free Sulfur
Combined Sulfur
Total Iron
Water soluble Iron
Total Manganese
Water soluble manganese
Chlorine Max
126
house at the HTRC, until they were planted in the platforms (Table 4.2). Platforms were watered
three times daily for 15 min using overhead sprinklers throughout the 2009 growing season and
three times daily for 5 min using micro-emitters throughout the 2010 and 2011 growing seasons
(Table 4.2). Lesco Professional Landscape and Ornamental All-Purpose Fertilizer 14-14-14
(Table 4.3) (Lesco, INC, Cleveland, OH) was applied to the green roof platforms at a rate of 25
2
g/m once in 2009 and twice during 2010 and 2011 (Table 4.2).
Data collection and analysis. Stormwater runoff was measured using TE525WS tipping
bucket rain gauges (Campbell’s Scientific, Inc., Logan, UT) that were connected to a CR10X
datalogger (Campbell’s Scientific, Inc., Logan, UT) using 3 AM16T multiplexers (Campbell’s
Scientific, Inc., Logan, UT). A guard constructed from 25.4 cm round plastic plant saucers was
placed around the outlet of the aluminum troughs to prevent precipitation falling outside each
platform area from entering the tipping buckets. Rainfall was measured using an additional
tipping bucket adjacent to the platforms. The datalogger was programmed to collect rainfall and
runoff amounts from tipping buckets every minute and to summarize totals at 15 min intervals
during the growing seasons of 2009, 2010 and 2011. Rain events were determined to be
independent from each other if they were separated by more than 6 hrs and runoff had stopped
before that time had passed. Rain events were categorized into three intensity groupings; light (<
2 mm), medium (2 to 10 mm) and heavy (> 10 mm). Data on ambient weather conditions were
taken from the Michigan Automated Weather Network (MAWN) station East
Lansing/MSUHORT located at the HTRC adjacent to the platforms.
Samples of the first 125 mL (63.66 mm) of runoff were collected five times during the
2011 growing season. One sample was taken before the first fertilizer application on July 11,
2011. Two samples were then taken after the first fertilizer application on August 16, and
127
August 26, 2011. Two samples were also taken after the second fertilizer application on
-
September 17, and October 2, 2011. All samples were analyzed for nitrate-nitrogen (NO3 ) and
phosphorus (P). Analysis was performed by the Michigan State University Soil and Plant
Nutrient Laboratory in East Lansing, MI.
Soil moisture was measured using a theta meter HH1 (Delta-T Devices, Cambridge,
England). Three measurements were taken from each plot, at the center of the north end, middle
and south end, on five afternoons between September 15 and September 20, 2011. Platforms
were set at a 2% slope with the top edge of the high end 0.9 m above ground and oriented with
the low end of the slope facing south to maximize sun exposure.
Data were analyzed using SAS (Version 9.1, SAS Institute, Cary, NC). Non-normal data
-
were analyzed after applying a square root transformation for NO3 and logarithmic
transformations for P and substrate moisture content. All values are presented as backtransformed data. Influence diagnostics were used to identify outliers, which were removed if
they were deemed unrepresentative. Multiple linear regression was performed on precipitation
events comparing runoff to precipitation for reach vegetation treatment (PROC GLM, Version
-
9.1, SAS Institute, Cary, NC). Mean NO3 and P content were analyzed using an ANOVA
model with landscape system and time as fixed effects. Mean volumetric moisture content of the
substrate was analyzed using an ANOVA model with landscape system and measurement
location within the plot as fixed effects. Significant differences were determined using multiple
comparisons by LSD with an alpha of 0.05 (PROC MIXED, Version 9.1, SAS Institute, Cary,
NC).
128
Total precipitation = 307.34 mm
45.00
40.00
35.00
30.00
25.00
20.00
15.00
10.00
5.00
0.00
25.00
20.00
15.00
10.00
5.00
0.00
Total precipitation = 303.26 mm
B
Temperature (°C)
35.00
30.00
25.00
20.00
15.00
10.00
5.00
0.00
Temeprature (°C)
C
35.00
Total precipitation = 367.50 mm
30.00
25.00
20.00
15.00
10.00
5.00
0.00
180.00
160.00
140.00
120.00
100.00
80.00
60.00
40.00
20.00
0.00
Precipitation (mm)
Temperature (°C)
30.00
90.00
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
Precipitation (mm)
35.00
air temperature
Precipitation (mm)
precipitation
A
1 2 3 4 5 6 7 8 9 101112131415161718
Time (weeks)
Figure 4.1. Weekly average maximum air temperature in Celsius and weekly total precipitation
in mm for the growing seasons of (A) 2009 (May 31-Oct 2) (B) 2010 (May 25-Oct 2) and (C)
2011 (May 29-Oct 1).
129
Results
Weather. Maximum and minimum ambient air temperatures and measured precipitation
for each month during the three growing seasons (2009, 2010, and 2011) are shown in Figure
4.1. During 2009, maximum temperatures in May, July and August were lower than the
following two growing seasons and similar maximum temperatures were recorded during 2010
and 2011. Minimum temperatures were also lower in May, September, and October in 2009,
than those recorded in 2010 and 2011. Total precipitation was greatest during 2011, however,
2009 experienced more precipitation during June and August than 2010 and 2011 but lower in
September and July, respectively (Figure 4.1). Incoming solar radiation was similar for all three
growing seasons with totals for June, July, August and September of 2.24, 2.16, and 2.21 million
2
kJ/m for 2009, 2010, and 2011, respectively.
130
A
0.35
Water Quantity (mm)
0.3
0.25
precipitation
0.2
sedum
0.15
vegetables
prairie
0.1
0.05
0
15 30 45 60 75 90 105 120 135 150 165 180
Water Quantity (mm)
B 1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
15 30 45 60 75 90 105 120 135 150 165 180
Water Quantity (mm)
C 2.5
2
1.5
1
0.5
0
15 30 45 60 75 90 105 120 135 150 165 180
Time From Start of Precipitation (min)
Figure 4.2. Runoff hydrographs of precipitation events of (A) light, (B) medium, and (C) heavy
rainfall. Rain events recorded at 15 min intervals. Values are averages of measurements taken
from all tipping buckets for each treatment within each precipitation event size category over the
growing seasons of 2009, 2010, and 2011.
131
Water retention. Initial runoff from the platforms was lower than the amount of
precipitation (Figure 4.2). There was, however, no clear delay in runoff or difference between
vegetation types. There was a significant interaction between precipitation and vegetation type
(F = 13.857, p <0.0001, α = 0.05). Regression lines for sedum and vegetable green roofs were
very similar with much higher amounts of runoff per mm precipitation than the regression line
predicted for the prairie green roof (Figure 4.3). This indicates that as rainfall increased, the
amount of runoff from the sedum and vegetable green roofs increased at similar rates, while the
amount of runoff from the prairie green roofs increased at a slower rate. No relationship was
found between reference potential evapotransporation or treatment and the amount of runoff
from irrigation events (data not shown).
50
45
y = 0.6581x, R² = 0.796, p <0.0001
40
Runoff (mm)
35
y = 0.6076x, R² = 0.72, p <0.0001
30
25
Sedum
20
Prairie
Vegetable
15
Sedum
10
Prairie
y = 0.3220x, R² = 0.494, p <0.0001
5
Vegetable
0
0
10
20
30
40
Precipitation (mm)
50
60
70
Figure 4.3. Multiple linear regressions of precipitation and runoff for the growing seasons of
2009, 2010 and 2011. Each marker represents a single observation.
132
Table 4.4. Nitrate concentrations (mg/L) in the first 125 mL of runoff water taken in 2011 from
extensive green roofs vegetated with a mix of Sedum species, a native prairie mix, and a
fertilized vegetable and herb garden before the first fertilizer application and 3 and 5 weeks after
the first and second fertilizer applications.
Nitrate content
(mg/L)
0.14 a
Treatment Sedum
0.15 a
Prairie
0.22 a
Vegetable
0.30 a
Time
Initial
0.13 b
3 weeks after first fertilizer application
0.29 a
5 weeks after first fertilizer application
3 weeks after second fertilizer application 0.04 c
5 weeks after second fertilizer application 0.04 c
Means represent 20 observations for treatments and 12 observations for time. Means were
separated using LSD with an α of 0.05. Letters indicate significant differences for each factor.
-
Water quality. Initial NO3 and P concentrations can be attributed to the organic matter
present in the green roof substrate (Table 4.1). The treatment-time interaction had no effect on
-
NO3 concentration in runoff (F = 0.37, p > 0.5, α = 0.05). Vegetation treatment had no effect on
-
runoff water NO3 concentrations (Table 4.4). Nitrate concentration decreased over time with
higher initial values and values after the first fertilizer application than after the second fertilizer
-
application (Table 4.4). NO3 concentrations for the sedum and vegetable green roof treatments
and three weeks after the first fertilizer application were lower (Table 4.4). This did not affect
-
significant differences among green roof treatments, but NO3 concentrations three weeks after
the first fertilizer application were lower than those for the initial and five weeks after the first
fertilizer application sample values, but still higher than both samples taken after the first
fertilizer application (Table 4.4).
133
Phosphorus concentration in the first 125 mL runoff water was higher earlier in the
growing season for the sedum and prairie green roofs (Table 4.5). The vegetable green roof
exhibited the highest P concentrations three weeks after the second fertilizer applications, but the
P concentration of the initial sample time was not different from any other sample time (Table
4.5). Five weeks after the first fertilizer application P levels in runoff from the vegetable plots
were higher than those of the other two roof types which did not differ from each other (Table
4.5). This remained so for the rest of the sample times (Table 4.5).
Table 4.5. Phosphorus concentrations (mg/L) in the first 125 mL of runoff water taken in 2011
from extensive green roofs vegetated with a mix of Sedum species, a native prairie mix, and a
fertilized vegetable and herb garden before the first fertilizer application and 3 and 5 weeks after
the first and second fertilizer applications.
Initial
3 weeks after 5 weeks after 3 weeks after 5 weeks after
first fertilizer first fertilizer second
second
application
application
fertilizer
fertilizer
application
application
0.15 Aa
0.08 Aa
0.04 Bb
0.03 Bb
0.02 Bb
Sedum
0.12 Aa
0.07 ABa
0.04 BCb
0.02 Cb
0.03 Cb
Prairie
0.23 ABa
0.14 Ba
0.15 Ba
0.44 Aa
0.21 Ba
Vegetable
Means represent 4 observations. Means were separated using LSD with an α of 0.05. Capital
letters in rows indicate significant differences among sample times within green roof treatments
and lower case letters in columns indicate differences among green roof treatments within
sample times.
Substrate Moisture. The treatment-location of measurement interaction had no effect on
volumetric moisture content (F = 1.18, p > 0.5, α = 0.05). There was no difference between the
volumetric moisture content of any of the green roof treatments (Table 4.6). Volumetric
moisture content was different in each of the locations measured and was highest at the south
(low) end of the plots and lowest and the north (high) end of the plots (Table 4.6).
134
3
3
Table 4.6. Volumetric moisture content (m /m ) of extensive green roofs vegetated with a mix of
Sedum species, a native prairie mix, and a fertilized vegetable and herb garden.
3 3
Treatment/Location
Volumetric Moisture Content (m /m )
0.298 a
Sedum
0.287 a
Prairie
0.303 a
Vegetable
0.271 c
Top
0.296 b
Middle
0.321 a
Bottom
Means represent 60 observations. Means were separated using LSD with an α of 0.05. Letters
denote significant differences.
Discussion
Water retention. The lack of clear delay in runoff in the hydrographs was unexpected.
VanWoert et al (2005a) found delays of up to 55 min depending on the size of the rain event.
Getter et al. (2007) however found negligible delays in runoff similar to those in Figure 4.3.
Getter et al. (2007) speculated that this discrepancy was due their use of a steeper slope than
some used by VanWoert et al. (2005a). This study used the same 2 % slope use by VanWoert et
al., (2005a), so this explanation does not apply here. Another explanation was the moisture
conditions of the substrate before each rain event (Getter et al., 2007). This explanation fits as
plots were irrigated throughout the summer, so substrate moisture content before each rain event
could have been higher than in either of those two studies.
The lower amount of runoff per mm of precipitation for the prairie green roof (Figure
4.4) could be explained by the different microclimates on each of the three green roof types. The
prairie green roof did not have complete coverage of the substrate like the sedum green roof,
enabling greater evaporation from the substrate surface and more plants than the vegetable green
roof, implying greater amounts of transpiration than from vegetables. Both of these factors have
been shown to enable the substrate to dry out to a greater extent between rain events and
therefore hold more water when the next rain event occurs, thus reducing runoff (Dunnett et al.,
135
2008; Nagase and Dunnett, 2012; VanWoert et al., 2005a; Voyde et al., 2010). Although greater
evapotranspiration, and therefore greater water retention capacity, was expected on the vegetable
green roof than the sedum green roof, it appears that the vegetable green roof did not retain as
much runoff as the sedum green roof (Figure 4.4). Irrigation was supplied to the green roof and
may have effected how stormwater was retained by the three vegetation types. Other research
has shown that irrigation can reduce the amount of stormwater retained for larger precipitation
events (Scholl et al., 2011).
Another explanation for the differences in stormwater retention is plant morphologies.
Studies have shown that taller and wider plants and plants with broader leaves intercept more
rainfall, adding water storage not included in the physical properties of the substrate (Dunnet et
al., 2008; Nagase and Dunnet, 2012). The prairie green roof, not only has more plants than the
vegetable green roof, but a greater variety of growth forms including tall grasses and lower lying
rosette forms which will have a greater chance of intercepting water before it reaches the
substrate. Dunnett et al. (2008) and Nagase and Dunnet (2012) also found that denser root
morphologies hold more water. Both the prairie green roof and the sedum green roof had more
plants and greater coverage, suggesting a more extensive root system, which would enable both
to capture more water than the vegetable green roof. This may account for the difference
between the regression lines of the sedum green roof and the vegetable green roof. The
differences between the regression lines for the sedum and vegetable green roofs were not
however very large (Figure 4.4), suggesting that production of vegetables on extensive green
roofs will not negatively affect stormwater retention as compared with sedum green roofs.
Runoff comparisons from rooftop scale agricultural and more traditional extensive green roofs
will determine if these results are applicable on that scale.
136
Table 4.7. Nitrogen and Phosphorus supplied to the vegetable green roof treatment from the 25
2
g/m Lesco Professional Landscape and Ornamental All-Purpose Fertilizer 14-14-14 applied
twice during the growing season as compared with nutrient recommendations based on substrate
tests and plant nutrient use (Warncke et al, 2004).
Nitrogen from Recommended Phosphorus
Recommended
fertilizer
Nitrogen
from fertilizer Phosphorus
2
2
2
2
(g/m )
(g/m )
(g/m )
(g/m )
3.5
13.45
3.5
0.036
Lycopersicon
esculentum (Roma
(VF) tomatoes)
3.5
4.48
3.5
0.019
Phaseolus vulgaris
(early contender
bush beans)
3.5
11.21
3.5
0.031
Cucumis sativus (bush
pickle hybrid
cucumbers)
3.5
11.21
3.5
0.045
Capsicum annuum
®
(Sweetheart
hybrid sweet
peppers)
Water quality. Differences in runoff quality among treatments and over time can be
explained by comparing plant nutrient needs with the amount of fertilizer applied to the
vegetable plots, and previously researched patterns in nutrient leaching as green roofs age. The
2
25 g/m of fertilizer applied to the vegetable green roof did not provide adequate N as compared
with application recommendations based on plant nutrient uptake (Warncke et al., 2004) (Table
4.7). It is then possible the N added by the fertilizer was taken up by the vegetable and herb
plants, not allowing extra runoff. The P provided by the fertilizer was however more than
necessary for the vegetable plants (Table 4.7). This would explain why P content of the
vegetable green roof runoff was higher than the other treatments (Table 4.6). These runoff P
concentrations remained high throughout the growing season, a known phenomenon when
-
controlled release fertilizers are used (Emilsson et al., 2007). The pattern in declining NO3 and
137
P content of the prairie and sedum green roofs can be explained through the aging process of
green roofs. Nutrients leach out of green roofs in runoff, but nutrient leaching often declines
over time as the amount of organic matter in the roof substrate decreases and the readily soluble
nutrients have already been leached out of the roof substrate (Czemiel Berndtsson, 2010).
-
All NO3 concentrations observed were well below the EPA 10 mg/L guideline for
drinking water (USEPA, 2012a). All P concentrations also fell below the Michigan 1 mg/L
guideline for point sources (MDEQ, 2012). Other Midwestern states have more stringent P
guidelines ranging from 0.012 mg/L for lakes with trout in Minnesota to 0.09 mg/L for shallow
lakes and reservoirs in Minnesota (USEPA, 2012b). When compared with grey water
-
concentrations of NO3 , concentrations in this study are lower than those found in grey water
form bathrooms (0.28-6.3 mg/L), laundries (0.4-2 mg/L), or kitchens (0.3-5.8 mg/L), but within
the low end of concentrations found in grey water form mixed sources (0-4.9 mg/L)(Eriksson et
al., 2002). Concentrations of P from the sedum and prairie roofs are lower than those found in
any grey water source (0.06-74 mg/L) (Eriksson et al., 2002) starting the fifth week after the first
fertilizer application. Runoff from the vegetable roofs contains P concentrations in the low end
of reported grey water concentrations (Eriksson et al., 2002). When compared with runoff
-
concentrations of NO3 (0.07-0.8 mg/L) and P (0.01-3 mg/L) from other runoff quality studies
performed on green roofs (Czemiel Berndtsson, 2010), the highest concentrations found in this
-
-
study (0.29 mg/L of NO3 and 0.44 mg/L of P) are moderate. When compared to NO3 and P
losses in container nursery production, which can range be as high as 7.55 and 1.4 mg/L,
respectively (Warsaw et al., 2009), the values found in this experiment are quite low.
138
Further experimentation on different types of fertilizers with different nutrient
concentrations may reveal a fertilizer nutrient combination where vegetable crops are supplied
enough of the essential nutrients without allowing excess to be leached off the roof. Adding
amendments to the substrate may be another method of reducing nutrient leaching from the roof
that was not explored in this study. For example, amending green roof substrates with biochar,
-
has been shown to increase retention of many nutrients including NO3 (Beck et al., 2011). Our
study also only examined nutrient concentrations in the first 125 mL (63.66 mm) of runoff, but
-
-
total NO3 and P quantities can be made. NO3 runoff for all three growing seasons was 1.91 mg,
1.16 mg of which came from the vegetable green roofs (Table 4.8) and the estimated total P
runoff for all three growing seasons was 1.39 mg, 1.02 mg of which came from the vegetable
green roofs (Table 4.9). When compared to total losses of up to 133 mg/pot (Fare et al., 1994)
2
-
2
and 39.4 mg/m ∙ day (Warsaw et al., 2009) of NO3 and 9 mg/m ∙ day (Warsaw et al., 2009) of
P, the total nutrient loads estimated for this study are quite small.
Table 4.8. Estimated total nitrate loads in milligrams for runoff from extensive green roofs
vegetated with a mix of Sedum species, a native prairie mix, and a fertilized vegetable and herb
garden during the growing seasons of 2009-11.
Treatment/Location
2009
2010
2011
Total
0.27
0.15
0.00
0.42
Sedum
0.01
0.30
0.02
0.33
Prairie
0.56
0.59
0.01
1.16
Vegetable
0.84
1.04
0.03
1.91
Total
139
Table 4.9. Estimated total phosphorus loads in milligrams for runoff from extensive green roofs
vegetated with a mix of Sedum species, a native prairie mix, and a fertilized vegetable and herb
garden during the growing seasons of 2009-11.
Treatment/Location
2009
2010
2011
Total
0.08
0.13
0.00
0.21
Sedum
0.00
0.15
0.01
0.16
Prairie
0.38
0.63
0.01
1.02
Vegetable
0.46
0.91
0.02
1.39
Total
Substrate Moisture. The lack of difference between volumetric moisture content of the
treatments may, in part, be explained by the fact that these roofs were irrigated and by the
weather conditions at the time measurements were taken. Irrigation events never really allowed
the substrate to become completely dry. Also, substrate moisture measurements were taken in
September 2011, when the air temperature lows were lower than earlier in the season (Figure
4.1C) and there were more frequent small rain events. It is possible that the substrate in all three
treatments remained saturated during the time measurements were taken. Results may have been
much different in July when temperatures were higher and precipitation less frequent. Season
long substrate moisture measurements may reveal differences during such times of year and
patterns in volumetric moisture content over the course of a growing season. Analysis of
patterns over the course of a growing season could enable more informed efficient use of
irrigation by reducing applied irrigation during cool wet periods were precipitation provides
vegetable plants with adequate water. The difference in volumetric moisture content between
locations measured within a plot is consistent the expected drainage of water down the slope of
the roof.
140
Conclusions
Results of this study show that there are no adverse effects of cultivating vegetable and
herb crops and herb crops on extensive green roof in Michigan on stormwater retention. This is
encouraging as it could mean that agricultural rooftops will be just as beneficial to reducing the
stormwater runoff load of an urban area as other green roofs, but measurements taken on full
scale roofs will be needed to confirm that these results are applicable on that scale. There was no
difference between the volumetric moisture content of the three green roof types. This was
unexpected because of assumed differences in plant coverage and differences in plant
physiologies, but may be a result of applied irrigation. Some of it may be due to the irrigation
and weather at the time measurements were taken. Further examination of substrate moisture
over the course of the growing season may reveal subtle differences between the green roof
types.
Effects on runoff quality are less clear. Under the fertilization regimen used for this
-
study, there was no increase in runoff NO3 concentrations in the vegetable green roof over the
other, more traditional green roof types, but there was an increase in P. This is likely due to the
mismatch in crop plant nutrient needs and the amounts of those nutrients supplied with the
fertilizer used. Better understanding of this impact of crop production on green roofs could be
achieved with a wider look at fertilizers with different nutrient concentrations, both conventional
and organic, and analysis of more N types in green roof runoff. Further examination of
concentrations of all N types, paired with the types of N found in fertilizers used could also
create a broader picture of how fertilizer use efficiency impacts runoff quality. Finally, further
examination of total nutrient loads form the roofs could reveal differences between vegetation
141
types not found in this study that may be important to the process of minimizing the impact of
fertilizers on runoff water quality.
Acknowledgements
Funding for this study was provided by Ford Motor Company, Dearborn, MI;
ChristenDETROIT Roofing Contractors, Detroit, MI; XeroFlor America, Durham, NC; Renewed
Earth, Kalamazoo, MI; and MSU AgBioResearch. Special thanks to the Michigan State
University Horticultural Teaching and Research Center for their support, and to Saloni Dagli for
her field assistance.
142
REFERENCES
143
REFERENCES
Abdul-Baki, A.A. and J.R. Teasdale. 2007. Sustainable production of fresh-market tomatoes and
other vegetables with cover crop mulches: Farmers’ Bulletin No. 2279. United States
Department of Agriculture, Agricultural Research Service.
Alexandri, E. and P. Jones. 2008. Temperature decreases in an urban canyon due to green walls
and green roofs in diverse climates. Building and Environment 43:480-493.
Barrio, E.P.D. 1998. Analysis of green roofs cooling potential in buildings. Energy and
Buildings (27):179-193.
Beck, D.A., G.R. Johnson, and G.A. Spolek. 2011. Amending greenroof siol with biochar to
affect runoff water quantity and quality. Environmental Pollution 159:2111-2118.
Carter, T. and C.R. Jackson. 2007. Vegetated roofs for stormwater management at multiple
spatial scales. Landscape and Urban Planning 80:84-94.
Carter, T. and A. Keeler. 2008. Life-cycle cost-benefit analysis of extensive vegetated roof
systems. Journal of Environmental Management 87:350-363.
Czemiel Berndtsson, J. 2010. Green roof performance towards management of runoff water
quantity and quality: A review. Ecological Engineering 36:351-360.
Czemiel Berndtsson, J., T. Emilsson, and L. Bengtsson. 2006. The influence of extensive
vegetated roofs on runoff water quality. Science of the Total Environment 355:48-63.
Dunnett, N., A. Nagase, R. Booth and P. Grime. 2008. Influence of vegetation composition on
runoff in two simulated green roof experiments. Urban Ecosystems 11:385-398.
Durhman, A.K., D.B. Rowe, and C.L. Rugh. 2006. Effect of watering regimen on chlorophyll
fluorescence and growth of selected green roof plant taxa. HortScience 41(7):1623-1628.
Emilsson, T., J. Czemiel Berndtsson, J.E, Mattsson, and K. Rolf. 2007. Effect of using
conventional and controlled release fertilizer on nutrient runoff from various vegetated
roof systems. Ecological Engineering 29:260-271.
Eriksson, E., K. Auffarth, M. Henze, and A. Ledin. 2002. Characteristics of grey wastewater.
Urabn Water 4:85-104.
Fare, D.C., C>H. Gilliam, and G.J. Keever. 1994. Cyclic irrigation reduces container leachate
nitrate-nitrogen concentration. HortScience 29(12):1514-1517.
144
Getter, K.L. and D.B. Rowe. 2006. The role of extensive green roofs in sustainable development.
HortScience 41(5):1276-1285.
Getter, K.L, D.B. Rowe, and J.A. Andresen. 2007. Quantifying the effect of slope on extensive
green roof stormwater retention. Ecological Engineering 31:225-231.
Ham, J.M., J.L. Heilman, and R.J. Lascano. 1991. Soil and canopy energy balances of a row crop
at partial cover. Agronomy journal. 83(4):744-753.
Hathaway, A.M., W.F. Hunt, and G.D. Jennings. 2008. A field study of green roof hydrologic
and water quality performance. American Society of Agricultural and Biological
Engineers 51(1):37-44.
Heilman, J.L., K.J. McInnes, R.W. Gesh, R.J. Lanscano, and M.J. Savage. 1996. Effects of
trellising on the energy balance of a vineyard. Agricultural and Forest Meteorology
81:79-93.
Liptan, T. 2005. Portland: A new kind of stormwater management, p. 121-123. In Earthpledge.
Green roofs: Ecological design and construction. Schiffer Books, Atglen, Pa.
Loder, A. and S.W. Peck. 2004. Green roofs’ contribution to smart growth implementation, p 824. In Proc. of 2nd North American Green Roof Conference: Greening rooftops for
sustainable communities, Portland, OR. 2-4 June 2004. The Cardinal Group, Toronto.
Michigan Department of Environmental Quality (MDEQ). 2012. Water quality parameters.
Accessed 1 Aug 2012. .
Monterusso, M.A., D.B. Rowe, and C.L. Rugh. 2005. Establishment and persistence of Sedum
spp. and native taxa for green roof applications. HortScience 40(2):391-396.
Nagase, A. and N. Dunnett. 2012. Amount of water runoff from different vegetation types on
extensive green roofs: Effects of plant species, diversity and plant structure. Landscape
and Urban Planning 104(3-4):356-363.
Rowe, D.B. 2011. Green roofs as a means of pollution abatement. Environmental Pollution
159(8–9):2100–2110.
Saiz, S., C. Kennedy, B. Bass, and K. Pressnail. 2006. Comparative life cycle assessment of
standard and green roofs. Environ. Sci. Technol., 40(13):4312-4316.
Schroll, R, J. Lambrinos, T. Righetti, and D. Sandrock. 2011. The role of vegetation in regulating
stormwater runoff from green roofs in a winter rainfall climate. Ecological Engineering.
37(4):595-600.
145
Swiader, J.M. and G.W. Ware. 2002. Producing Vegetable Crops, Fifth Edition. Interstate
Publishers, Inc. Danville, Illinois.
Tanner, C.B. 1974. Microclimate modification: Basic concepts. HortScience 9:555-560.
U.S. Environmental Protection Agency (USEPA). 2012a. Drinking water contamination.
Accessed 1 Aug 2012. .
U.S. Environmental Protection Agency (USEPA). 2012b. Water quality standards repository:
State, tribal & territorial standards. Accessed 3 Aug 2012.
.
VanWoert, N.D., D.B. Rowe, J.A. Andresen, C.L. Rugh, R.T. Fernandez, and L. Xiao. 2005a.
Green roof stormwater retention: Effects of roof surface, slope, and media depth. J.
Environ. Qual. 34:1036-1044.
VanWoert, N.D., D.B. Rowe, J.A. Andresen, C.L. Rugh, and L. Xiao. 2005b. Watering regime
and green roof substrate design affect Sedum plant growth. HortScience 40(3):659-664.
Voyde, E., E. Fassman, R. Simcock, and J. Wells. 2010. Quantifying evapotraspiration rates for
New Zealand green roofs. Journal of Hydrologic Engineering 15(6):395-403.
Warncke, D., J. Dahl, and B. Zandstra. 2004. Nutrient recommendations for vegetable crops in
Michigan. Michigan State University Extension Bulletin E2934.
Warsaw, A.L., R.T. Fernandez, B.M. Cregg, and J.A. Andresen. 2009. Container-grown
ornamental plant growth and water runoff nutrient content and volume under four
irrigation treatments. HortScience 44(6):1573-1580.
Whittinghill, L. J. and D.B. Rowe. 2011. The role of green roof technology in urban agriculture.
Renewable Agriculture and Food Systems. DOI 10.1017/S174217051100038X.
Wolf, D. and J.T. Lundholm. 2008. Water uptake in green roof microcosms: Effects of plant
species and water availability. Ecological Engineering 33:179-186.
Wong N.H., S.F. Tay, R. Wong, C.L. Ong, and A. Sai. 2003. Life cycle cost analysis of rooftop
gardens in Singapore. Building and Environment 38:499-509.
146
CHAPTER FIVE:
Quantifying carbon sequestration of various green roof and landscape systems
147
Abstract
Interest in reducing carbon emissions and credit trading programs has been increasing,
but in order to enforce regulations or participate in credit trading programs organizations must
first understand their landscapes and how they sequester carbon and how long they will store that
carbon. A great deal of research has been done on natural and agricultural landscapes, and some
on urban forests and street trees. Little work has been done on ornamental landscapes, which
could potentially contain a large quantity of carbon. This study compared the carbon
concentrations of nine in ground and three green roof landscape systems of varying complexity
to determine their potential to sequester carbon. Soil or substrate samples were analyzed prior to
planting in 2009 and soil/substrate, below- and above-ground biomass were analyzed at the end
of the 2010 and 2011 growing seasons. Landscape systems containing more woody structures,
such as the three shrub systems and the herbaceous perennials and grasses had higher carbon
content than other landscape systems. The native prairie mix also had high carbon content,
because of the high volume of plant biomass in that landscape system. The vegetable and herb
garden and vegetable green roof sequestered a moderate amount of carbon. The Sedum and
prairie green roofs contained less carbon than their counterpart in-ground landscape systems,
suggesting that although green roofs do sequester a small amount of carbon, greater benefit can
be achieved in ground level landscape systems. Ornamental landscapes have good potential for
carbon sequestration but management practices can affect their net carbon sequestration and the
permanence of the carbon sequestered, which could make their inclusion in carbon credit
programs difficult.
148
Introduction
Growing concerns about global climate change and atmospheric carbon levels have led to
an increase in legislation and programs designed to reduce carbon emissions and remove carbon
from the atmosphere. Some such initiatives include the Kyoto Protocol, which required
participating countries to reduce carbon emissions to 5% below 1990 levels by 2012 (UNFCCC,
1997), and the Chicago Climate Exchange (CCX), which also calls for emissions reductions, but
enables those unable to meet the goals of the program to purchase credits from other participants
(CCX, 2009). There have also been other trading programs focused on greenhouse gasses
including nitrogen oxides (Clark et al., 2005; Clark et al., 2006) and sulfur dioxide (Clark et al.,
2006). Individuals or groups wishing to participate in such programs must understand how their
landscapes sequester carbon.
There are large differences in the ability of landscapes to sequester carbon. These
differences are functions of species diversity, plant physiological characteristics, species
abundance, and climate (Kucharik et al., 2003; Matamala et al., 2008; Sandermann and
Amundson, 2009; Tilman et al., 2006). Landscapes with greater species diversity have been
shown to sequester larger amounts of carbon (Tilman et al., 2006) and landscape age can impact
the amount of carbon sequestered. As landscapes age increases in litter, microbial and root
biomass have been recorded (Matamala et al., 2008) as well as increases in woody biomass
(Fang et al. 2007) depending on the type of vegetation. Management can also play a large role in
not only how much carbon can be sequestered (Balesdent et al., 2000; Fang et al., 2007; Wu, et
al., 2008), but also in their ability to offset carbon emissions. Cultivation in an agricultural
setting decreases the amount of carbon contained in a landscape by reducing soil organic matter
(Balesdent et al., 2000), microbial and root biomass (Matamala et al., 2008), and often large
149
woody plants such as forests were converted to smaller herbaceous crop plants (Rhodes et al.,
2000). More intensive management, which often entails greater carbon emissions, of ornamental
landscapes and forests changes the balance between carbon emitted and carbon sequestered.
This leads to a point in time when more carbon is emitted due to management practices than can
be taken up by the landscape during the course of a year (Nowak et al., 2002).
Much of the research conducted on carbon sequestration has been in natural landscapes
and agricultural lands, but recently the focus has been shifted to include urban landscapes such as
forests and urban street trees. Frequently these settings are more managed and are exposed to
different conditions than natural landscapes. Due to higher temperatures in urban areas than
surrounding rural areas, urban forests tend to be greater sinks for carbon during the growing
season, but a greater source of carbon emissions during the winter than their rural counterparts
(Awal et al., 2010). Management highlights the issues of quality and permanence that are
important when considering a landscape as potential carbon credit. Different land owners tend to
manage differently (Bigsby, 2009) and it has been suggested that when an urban tree dies and is
replaced no new carbon is sequestered, but the tree merely offsets the carbon being released by
the decaying dead tree (Nowak et al., 2002). This may also be true of ornamental landscapes
composed of shrubs and herbaceous plants. The ornamental horticulture industry is comprised of
land in the United States for both production and planting and the planting substrate typically
used has greater carbon content than field soils (Marble et al., 2011). However, the amount of
carbon contained in the plants, and what happens after they are transplanted from the nursery to
the landscape is not well understood.
This study will build on the work of Miller (2012) and Getter et al. (2009). The work
done by Miller (2012) focuses on determining the amount of carbon contained in shrubs,
150
herbaceous perennials, grasses and ground covers grown in the nursery industry. Getter et al.
(2009) found that several extensive green roofs located in Michigan and Maryland stored an
2
average of 162 g C/m in above-ground biomass with variation due to roof age and substrate
2
depth. In a second study, a 6 cm deep sedum based roof contained 375 g C/m in above- and
below-ground biomass and substrate organic matter. They speculated that differences between
their findings and those of others examining the issue of carbon sequestration may be due to the
age of the plants in the study, and differences between the physiology of succulents used on
green roofs and the plants used in previous research on forests and agriculture.
The objectives of this study are twofold and attempt to determine how plants examined
by Miller (2012) sequester carbon after planting and address some gaps in knowledge found by
Getter et al (2009). The first objective is to quantify the amount of carbon sequestered by
ornamental and green roof landscapes of varying complexity. The second objective is to then
determine what if any differences in carbon sequestration exist between the landscape systems
and differences in carbon sequestration between green roof landscapes and similar landscape
systems at ground level.
Methods
This study examined the carbon content of thirteen ornamental landscape systems of
varying complexity over the course of three years at the Michigan State University Horticulture
Teaching and Research Center (HTRC) in East Lansing, MI. Of these, nine landscape systems
were grown at ground level and four of these were repeated in elevated roof platforms (hereafter
referred to as green roof platforms). A randomized complete block design was used with four
replicates per treatment.
151
Landscape Systems at Ground Level. The landscape systems examined at ground level
included (1) Kentucky bluegrass lawn, (2) native prairie mix, (3) succulent rock garden
consisting of Sedum, (4) woody ground covers, (5) herbaceous perennials and grasses, (6)
deciduous shrubs, (7) broad-leaf evergreen shrubs, (8) narrow-leaf evergreen shrubs, and (9)
vegetable and herb garden. The ground level plots were prepared by first treating the area to be
®
used with glyphosate (Roundup , Monsanto, St. Louis, MO). Once the existing vegetation was
dead the area was deep tilled by spading on May 25, 2009. The ground area was then divided
into 2.4 x 2.4 m (8 x 8 ft) plots separated by 1.5 m (5 ft) of turf. Two of these plots were divided
into three 0.8 x 2.4 m sections for the vegetable plots, but only the four edge sections were used.
These plot sizes were chosen because they matched the sizes of the existing green roof platforms
used for the corresponding landscape systems in the study. Overhead irrigation was supplied for
the 2009 growing season while plants were establishing. No irrigation was provided during the
Table 5.1 Composition of Lesco Professional Landscape and Ornamental All-Purpose Fertilizer
14-14-14 (Lesco, Inc., Cleveland, OH).
Nutrient
Percent Content by Weight (%)
14.00
Total Nitrogen
5.48
Ammoniacal Nitrogen
8.55
Urea Nitrogen
14.00
Phosphate (P2O5)
14.00
Soluble Potash (K O)
2
19.40
14.40
5.00
0.45
0.005
0.45
0.05
2.00
Total Sulfur
Free Sulfur
Combined Sulfur
Total Iron
Water soluble Iron
Total Manganese
Water soluble manganese
Chlorine Max
152
2010 and 2011 growing seasons. The vegetable plots were fertilized with Lesco Professional
Landscape and Ornamental All-Purpose Fertilizer 14-14-14 (Table 5.1) (Lesco, INC, Cleveland,
OH) at a rate of 10 g/m2 once in 2009 and twice during 2010 and 2011 (Table 5.2).
Table 5.2. Planting and maintenance dates of vegetable and herb garden and vegetable green roof
plots for the 2009, 2010 and 2011 growing seasons.
Activity
2009
2010
2011
May 9
May 3 and 14
May 6
Seeds planted
Plugs planted:
May 29
May 15
June 16
Green roof platforms
May 29
May 27
June 19
At grade
Pepper plugs planted:
May 29
May 29
June 30
Green roof platforms
May 29
May 29
July 6
At grade
May 29
June 30
June 7
Irrigation On
October 2
October 3
October 1
Irrigation Off
Fertilizer application 1
June 13
July 1
July 22
Green roof platforms
June 13
July 1
July 22
At grade
Fertilizer application 2
N/A
August 9
August 26
Green roof platforms
N/A
August 12
August 26
At grade
Biomass harvest:
August 22October 1 and 2
Ocimum basilicum October 1 and 2
October 2
October 2
October 1 and 2
Allium schoenoprasum October 1 and 2
September 20
August 29
Phaseolus vulgaris October 1 and 2
August 24
August 29
Cucumis sativus October 1 and 2
October 2
October 1 and 2
Capsicum annuum October 1 and 2
October 1 and 2
Lycopersicon esculentum October 1 and 2 October 2 and 3
Landscape Systems on Green Roof Platforms.
Four of the landscape systems, (10)
extensive green roof consisting of Sedum (hereafter referred to as Sedum green roof), (11)
extensive green roof consisting of a native prairie mix (hereafter referred to as prairie green
roof), (12) extensive green roof consisting of herbaceous perennials and grasses (hereafter
referred to as ornamental green roof) and (13) extensive green roof consisting of vegetable and
153
Table 5.3. Initial physical and chemical properties of green roof substrate.
Component
Unit
86
Total Sand (%)
52.02
Extremely coarse sand (>2 mm) (%)
11.33
Very coarse sand (1-2 mm) (%)*
8.94
Coarse sand (0.5 -1 mm) (%)*
0
Medium sand (0.25-0.5 mm) (%)*
8.32
Fine sand (0.10-0.25 mm) (%)*
0
Very find sand (0.07-0.10 mm) (%)*
Extremely fine sand ( < 0.07 mm) (%)* 5.37
Silt (%)
Clay (%)
11
3
Bulk density (g/cm3)
Pore space (%)
Air filled porosity (%)
Water holding capacity at 0.01 MPa (%)
1.20
18.97
21.33
15.86
pH
Conductivity (EC) (m mho/cm)
7.2
1.59
65
Nitrate (mg/kg)
4.5
Phosphorus (mg/kg)
71
Potassium (mg/kg)
793
Calcium (mg/kg)
95
Magnesium (mg/kg)
65
Sodium (mg/kg)
94
Sulfur (mg/kg)
0.7
Boron (mg/kg)
24
Iron (mg/kg)
5.2
Magnesium (mg/kg)
4.3
Zinc (mg/kg)
0.6
Copper (mg/kg)
Analysis per A&L Great Lakes Laboratories, Inc., Ft. Wayne, IN.
* Analysis per Renewed Earth, Inc., Kalamazoo, MI.
herb plants (hereafter referred to as vegetable green roof), were also replicated on green roof
platforms. The green roof platforms were constructed as per VanWoert et al. (2005). The green
roof platform plots consist of a previously installed XeroFlor XF-105 drainage layer (XeroFlor
154
America, Durham, NC) and 10.5 cm of Green roof substrate (Renewed Earth, Kalamazoo, MI)
(Table 5.3) installed on June 10, 2009.
The extensive green roofs, prairie green roofs, and ornamental green roofs were watered
three times daily for 15 min using overhead sprinklers throughout the 2009 growing season and
once daily for 15 min using micro-emitters throughout the 2010 and 2011 growing seasons
(Table 5.2). The vegetable green roofs were fertilized in the same manner as the ground plots
(Table 5.2) and were irrigated three times daily for 15 min using overhead sprinklers throughout
the 2009 growing season and three times daily for 5 min using micro-emitters throughout the
2010 and 2011 growing seasons (Table 5.2). Lesco Professional Landscape and Ornamental AllPurpose Fertilizer 14-14-14 (Table 5.1)(Lesco, INC, Cleveland, OH) was applied to all vegetable
2
green roof plots at a rate of 25 g/m once in 2009, and twice in 2010 and 2011 (Table 5.2).
Plant Selection. Kentucky bluegrass was installed as sod on July 30, 2009. The native
prairie mix ground plots and prairie green roofs were sown from seed on May 30, 2009 and May
16, 2009 respectively. Each plot or platform was hand broadcast with 9.1 g of seed containing
by weight 0.5% Achillea millefolium (yarrow), 2% Asclepias tuberosa (butterflyweed), 1%
Asclepias syriaca (common milkweed), 4% Aster novae-angliae (New England aster), 0.5 %
Aster pilosus (hairy aster), 3% Coreopsis lanceolata (sand tickseed), 1% Desmodium canadense
(showy tick trefoil), 2% Echinacea purpurea (purple coneflower), 0.5% Kuhnia eupatoroides
(false boneset), 2% Lunpinus perennis (wild lupine), 4% Monarda fistulosa (burgamot), 0.5%
Oenothera biennis (common evening primrose), 2% Penstemon digitalis (foxglove beardtongue),
4% Ratibida pinnata (grayheaded coneflower), 6% Rudbeckia hirta (blackeyed Susan), 1%
Rudbeckia triloba (three-lobed coneflower), 1% Silphium integrifolium (rosinweed), 1%
Silphium laciniatum (compass plant), 4% Solidago rigida (stiff goldenrod), 3% Solidago
155
speciosa (showy goldenrod), 2% Verbena stricta (hoary vervain), 12% Andropogon gerardii (big
bluestem), 6% Elymus canadensis (Canada wild rye), 5% Panicum virgatum (switch grass), 20%
Schizachyrium scoparium (little bluestem), and 12% Sorghastrum nutans (Indian grass). The
succulent rock garden and prairie mix ground plots were covered with straw after seeding to
provide shade through establishment and the extensive green roof platforms and prairie green
roof platforms were covered with shade cloth until germination.
The succulent rock gardens and Sedum green roofs were sown from seed with four Sedum
species: S. acre, S. album, S. kamschaticum, and S. spurium on May 30, 2009 and May 16, 2009
respectively. These species were planted by mixing 1.2 g of seed with 400 mL of sand and
broadcast the mix over each plot or platform by hand. The Sedum species selected are common
green roof species which have previously been examined for carbon content and carbon
sequestration potential on an individual basis by Getter et al. (2009).
Woody ground cover plots were planted from 50 cell plugs (110.90 mL (3.75 fl oz) per
cell) of Vinca minor on August 7, 2009. Herbaceous perennials and grasses ground plots and
ornamental green roofs were each planted with one Miscanthus sinensis ‘Silver Arrow’, two
Perovskia atriplicifolia ‘Little Spire’ (‘Little Spire’ Russian sage), and five Echinacea puprpurea
‘Magnus’ (‘Magnus’ purple cone flower), five Hemerocallis ‘Mary’s Gold’ (‘ Mary’s gold
daylily), and five Rudbeckia speciosa ‘Viette’s Little Suzy’ (‘Little Suzy’ dwarf orange
coneflower) from 2.5 L (2.6 qt) pots in a uniform pattern on August 7, 2009. Due to poor
overwintering of M. sinensis that area of the plots was planted with eight Allium cernuum and
eight A. senescens on May 16, 2010.
Deciduous shrub ground plots were each planted with three Spirea media ‘Snow Storm’
(Snow Storm
TM
spirea), one Physocarpus opulifolius ‘Summer Wine’ (summer wine common
156
®
ninebark), and one Weigela florida ‘Wine and Roses’ (Wine and Roses weigela) on August 25,
2009. The broad-leaf evergreen ground plots were each planted from with six Buxus
sempervirens ‘Green Velvet’ (green velvet boxwood) and the narrow-leaf evergreen ground plots
were each planted with two Pinus mugho (Mugo Pine), two Juniperus chinensis ‘ Sea Green’
(sea green Chinese juniper) and two Taxus x media ‘Densiformis’ (dense spreading yew) on
October 26, 2009. All three of the shrub types were planted from 11.35-L (3-gal) containers.
Pine bark mulch was spread on the deciduous shrub, broad-leaf evergreen, and narrow leaf
evergreen ground plots to a depth of 7.62 cm (3 in) and the herbaceous perennial and grasses
ground plots to a depth of 3.81 cm (1.5 in) on May 28, 2010 and the ornamental green roofs to a
depth of 2.54 cm (1 in) on June 29, 2010. Mulch was included in the study as many ornamental
landscapes incorporate mulch for aesthetics and weed control.
The vegetable ground plots and vegetable green roofs were each planted with two plants
each of Lycopersicon esculentum (Roma (VF) tomatoes), Phaseolus vulgaris (early contender
bush beans), Cucumis sativus (bush pickle hybrid cucumbers), Capsicum annuum (Sweetheart
®
hybrid sweet peppers), Ocimum basilicum (large-leaf Italian basil) , and Allium schoenoprasum
(chives) (Gurney’s Seed and Nursery Co., Scarlet Tanager LLC., Greendale, IN). These
vegetable and herb species were selected because of their availability, growth habit (determinate
or bush variety), and common use in home gardens. Plugs were grown from seed in 48-cell plug
trays (120 mL (4.06 fl oz) per cell) and grown outside, in East Lansing, MI, until transplanting
into the ground and green roof plots (Table 5.2). Capsicum annuum either did not fruit or did not
germinate in all three years, and were replaced in 2011 with C. annuum (Budapest hot banana
peppers) (Seedway, Hall, NY). These were sown from seed into 98-cell plug trays on April 6,
157
2011 and grown in a greenhouse until May 24, 2011, then transferred to a lath house at the
HTRC until planting in the ground and platforms (Table 5.2).
Data collection and analysis. Soil and substrate samples collected prior to planting
between July 7 and 22, 2009 were analyzed to determine initial carbon content of the soil and
green roof substrate. Initial soil samples for the vegetable and herb gardens were collected prior
to planting in May 2010. Carbon content analysis was performed on above-ground biomass,
below-ground biomass (roots), and soil and substrate collected at the end of the growing seasons
in 2010 and 2011. Samples were collected on August 31 and September 21, 2010, and August
30, September 12, and September 20, 2011. Plots were divided quarters and each quarter was
subdivided into 16 squares. One each sampling date, four subsamples were taken from each plot,
one from each quarter, and the subsamples were combined. Only two subsamples were taken
from each vegetable and herb garden and vegetable green roof plot because of their smaller size.
Sampling locations were randomly selected and recorded so that no square was sampled more
than once.
Initial soil samples were collected at depths of 0 to 10.2 cm and 10.2 to 20.4 cm using a
7.6 cm diameter soil corer. Subsequent sampling was based on the methods described in Getter
et al. (2009). Above-ground biomass inside a 7.6 cm (3 in) ring, excluding shrubs, was cropped
even with the soil or substrate, placed in paper bags, weighed, dried 60˚C for 1 wk, weighed
again, and ground in a Wiley mill using a 60-mesh stainless steel screen. Samples were then
pulverized on a roller mill, placed in glass vials and stored in a desiccator until analysis of total
carbon concentration using a Carlo Erba NA1500 Series 2 N/C/S analyzer (CE Instruments,
Milan, Italy). Above-ground biomass in the herbaceous perennial and grasses, deciduous, broadleaf evergreen, and narrow-leaf evergreen shrubs, and ornamental green roofs was estimated
158
based on plant volume and a model (Miller, 2012). Plant volume measurements were taken on
October 21 and November 4, 2010, and October 24 and 30 and November 1, 2011. Aboveground biomass of the vegetable plots was measured after the time of the last harvest (Table 5.2).
Plants were weighed, dried at 60°C, and weighed again. Carbon accumulation was calculated by
multiplying dry matter weight by total carbon concentration found in the literature.
After the removal of above-ground biomass, all below-ground biomass and soil carbon
content were collected from a 7.62 cm diameter, 10.2 cm (4 in) deep soil core, which was
removed and stored in plastic bags, weighed in the bag, and separated using a 4.0 mm sieve.
Gravel remaining on the sieve was weighed. All root material was removed from the retained
and sieved matter using forceps. Root material was rinsed with deionized water, cleaned with a
phosphate-free dilute detergent, rinsed with deionized water, and soaked in a 0.01 mol/L
NaEDTA solution for 5 min. Cleaned roots were dried at 60˚C for 2 d in paper bags, weighed,
ground and analyzed for total carbon content as previously described.
Remaining sieved soil was mixed, placed in a small paper bag, weighed, dried at 60˚C for
2 d, and weighed again. Soil moisture content was calculated by subtracting the dried soil and
bag weight from the original weight. Bulk density was also calculated by dividing the original
soil sample weight, corrected for moisture content and root and gravel weight, by the volume of
3
the collected soil (ring surface area times the depth of the sample, 463 cm ). A 25 g portion of
each sample was then pulverized and analyzed for total carbon content as previously described.
Data on ambient weather conditions were taken from the Michigan Automated Weather Network
(MAWN) station East Lansing/MSUHORT located at the HTRC adjacent to the platforms and
ground plots.
159
Mean carbon content was analyzed using ANOVA model in which landscape type and
year were fixed effects. Normality issues were resolved using a logarithmic transformation for
above-ground biomass, below-ground biomass, soil and substrate, and total carbon content.
Influence diagnostics were used to identify outliers, which were removed if they were deemed
unrepresentative. The least significant differences (LSD) method of multiple comparisons was
used to determine significant differences between treatment means (PROC MIXED, Version 9.1,
SAS Institute, Cary, NC).
Results
Weather. During 2009, maximum ambient air temperatures in May, July and August
were lower than the following two growing seasons and similar maximum temperatures were
recorded during 2010 and 2011 (Figure 5.1). Minimum ambient air temperatures were also
lower in May, September, and October in 2009, than those recorded in 2010 and 2011. Total
precipitation was greatest during 2011, however, 2009 experienced more precipitation during
June and August than 2010 and 2011 but less in September and July (Figure 5.1). Incoming
solar radiation was similar for all three growing seasons with totals for June, July, August and
2
September of 2.24, 2.16, and 2.21 million kJ/m for 2009, 2010, and 2011, respectively.
160
precipitation
A
air temperature
25.00
20.00
15.00
10.00
5.00
0.00
B
Temperature (°C)
35.00
Total precipitation = 303.26 mm
30.00
25.00
20.00
15.00
10.00
5.00
0.00
Temeprature (°C)
C
35.00
Total precipitation = 367.50 mm
30.00
25.00
20.00
15.00
10.00
5.00
0.00
Precipitation (mm)
Temperature (°C)
30.00
45.00
40.00
35.00
30.00
25.00
20.00
15.00
10.00
5.00
0.00
180.00
160.00
140.00
120.00
100.00
80.00
60.00
40.00
20.00
0.00
Precipitation (mm)
90.00
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
Precipitation (mm)
Total precipitation = 307.34 mm
35.00
1 2 3 4 5 6 7 8 9 101112131415161718
Time (weeks)
Figure 5.1. Weekly average maximum air temperature in Celsius and weekly total precipitation
in mm for the growing seasons of (A) 2009 (May 31-Oct 2) (B) 2010 (May 25-Oct 2) and (C)
2011 (May 29-Oct 1).
161
Above ground biomass. All results for above-ground carbon content are presented in
Table 5.4. The three shrub landscape systems, the herbaceous perennials and grasses and
ornamental green roof had the highest amount of carbon in above ground biomass in both years
followed by the native prairie mix. The vegetable and herb garden and vegetable green roofs had
the lowest amount of carbon in above-ground biomass. For the remaining landscape systems, the
prairie green roof was more like the succulent rock garden and Sedum green roof than the inground native prairie mix. The prairie green roof was the only green roof landscape system to
have lower above-ground carbon content than its in-ground counterpart. The only two landscape
systems to show an increase in above ground biomass between 2010 and 2011 were the
succulent rock garden and Sedum green roofs.
2
Table 5.4. Carbon content (kg/m ) of above ground biomass of all landscape systems at the end
of the 2010 and 2011 growing seasons.
Landscape system
2010
2011
In Ground
58.61* Aa
65.27 Aa
Broad leaf evergreen shrubs
52.91 Aa
52.89 Aa
Deciduous shrubs
61.33* Aa
55.59 Aa
Herbaceous perennials and grasses
1.33 Acde
2.69 Acd
Kentucky blue grass lawn
15.30 Ab
15.96 Ab
Native prairie mix
40.42 Aa
66.53 Aa
Needle leaf evergreen shrubs
1.94 Bcd
3.91Ac
Succulent rock garden
0.03 Af
0.05 Ae
Vegetable and herb garden
1.12 Ae
1.73 Ad
Woody ground covers
Green Roof
33.61 Aa
64.42 Aa
Herbaceous perennials and grasses
1.27 Ade
4.16 Ac
Native prairie mix
2.22 Bc
3.49 Acd
Succulent rock garden
0.05 Af
0.05 Ae
Vegetable and herb garden
Means represent 4 observations. *Means represent 3 observations. Means were separated using
LSD with an alpha of 0.05. Capital letters denote differences in rows between years within each
treatment and lower case letters denote differences in columns among treatments for the
transformed data.
162
Below ground biomass. All the results for below-ground carbon content of the root
systems are presented in Table 5.5. Below-ground carbon of the Kentucky bluegrass lawn was
higher than any other landscape system in 2010 and higher than all but the vegetable and herb
garden in 2011. Broad-leaf evergreen shrubs, deciduous shrubs and the ornamental green roof
had the lowest three below-ground carbon amounts during both growing seasons. For the
landscape systems in between, herbaceous perennials and grassed and vegetable and herb garden
fell in the upper half of the middle during both years. The ornamental green roof was the only
green roof landscape system to have lower below-ground carbon content than its in-ground
counterpart. The deciduous shrub landscape system was the only landscape system to exhibit
lower below-ground carbon in 2010 than in 2011.
2
Table 5.5. Carbon content (kg/m ) of below ground biomass of all landscape systems at the end
of the 2010 and 2011 growing seasons.
Landscape system
2010
2011
In Ground
0.08 Af
0.10 Ag
Broad leaf evergreen shrubs
0.04 Bf
0.14 Afg
Deciduous shrubs
0.96 Ab
0.58 Abcd
Herbaceous perennials and grasses
3.52 Aa
3.25 Aa
Kentucky blue grass lawn
0.96 Abc
0.32** Adefg
Native prairie mix
0.27 Ade
0.55 Abcd
Needle leaf evergreen shrubs
0.59 Abcde
0.44 Acde
Succulent rock garden
0.83 Abcd
1.42* Aab
Vegetable and herb garden
0.96* Abcd
0.32 Adef
Woody ground covers
Green Roof
0.18 Aefg
Herbaceous perennials and grasses 0.19 Ae
0.34 Acde
0.72 Abcd
Native prairie mix
0.30 Acde
0.47 Abcd
Succulent rock garden
N/A
0.79 bc
Vegetable and herb garden
Means represent 4 observations. *Means represent 3 observations. ** Means represent 2
observations. Means were separated using LSD with an alpha of 0.05. Capital letters denote
differences in rows between years within each treatment and lower case letters denote
differences in columns among treatments for the transformed data.
163
Soil/substrate. The treatment-year interaction and year had no effect on soil or substrate
carbon content (F = 1.30, p = 0.130, α = 0.05 and F = 1.77, p = 0.174, α = 0.05, respectively).
All results for soil carbon content are presented in Table 5.6. The vegetable and herb garden had
the highest soil carbon content of any landscape system. This was followed by the three shrub
landscape systems, herbaceous perennials and grasses and native prairie mix which did not differ
from each other. The Sedum, prairie, and ornamental green roofs had the lowest carbon substrate
carbon contents of any landscape system. All green roof landscape systems had lower substrate
carbon content than their in-ground counterpart landscapes.
Table 5.6. Carbon content of soil or substrate of all landscape systems at the beginning of the
2009 growing season and the end of the 2010 and 2011 growing seasons.
2
Landscape system
Carbon content (kg/m )
In Ground
13.41 b
Broad leaf evergreen shrubs
12.92 bc
Deciduous shrubs
12.24 bc
Herbaceous perennials and grasses
10.11 e
Kentucky blue grass lawn
12.94 bc
Native prairie mix
12.41 bc
Needle leaf evergreen shrubs
10.28 de
Succulent rock garden
43.47 a
Vegetable and herb garden
11.69 cd
Woody ground covers
Green Roof
3.27 f
Herbaceous perennials and grasses
3.13 f
Native prairie mix
3.22 f
Succulent rock garden
9.82 e
Vegetable and herb garden
Means represent 12 observations. Means were separated using LSD with an alpha of 0.05.
Letters denote differences among treatments for the transformed data.
Total carbon content. All results for total carbon content are presented in Table 5.7. The
three shrub landscape systems and herbaceous perennial and grasses have the highest total
carbon contents in 2010. In 2011 they are among the highest, but the ornamental landscape
164
system has similar total carbon content. The Sedum green roof and prairie green roof have the
lowest total carbon content during both years, much lower than the corresponding succulent rock
garden and native prairie mix in-ground landscape systems. The prairie green roof and
ornamental green roof were the only two landscape systems to exhibit an increase in total carbon
content between 2010 and 2011.
2
Table 5.7. Total carbon content (kg/m ) of all landscape systems at the end of the 2010 and 2011
growing seasons.
Landscape system
2010
2011
In Ground
67.89 Aa
78.75 Aa
Broad leaf evergreen shrubs
66.38 Aa
65.67 Aab
Deciduous shrubs
75.10 Aa
68.75 Aab
Herbaceous perennials and grasses
14.99 Ac
15.82 Ad
Kentucky blue grass lawn
30.63 Ab
28.57 Ac
Native prairie mix
60.73* Aa
62.91 Aab
Needle leaf evergreen shrubs
13.33 Ac
12.30 Ad
Succulent rock garden
39.29 Ab
54.18 Ab
Vegetable and herb garden
15.40 Ac
13.43 Ad
Woody ground covers
Green Roof
37.38 Bb
67.70 Aab
Herbaceous perennials and grasses
4.63 Bd
8.09 Ae
Native prairie mix
5.87 Ad
7.12 Ae
Succulent rock garden
N/A
11.03 d
Vegetable and herb garden
Means represent 4 observations. *Means represent 3 observations. Means were separated using
LSD with an alpha of 0.05. Capital letters denote differences in rows between years within each
treatment and lower case letters denote differences in columns among treatments for the
transformed data.
Discussion
Carbon content. Overall the three shrub landscape systems, the herbaceous perennial and
grasses, ornamental green roof, and native prairie mix landscape systems contained more carbon
than other landscape systems examined in this study (Tables 5.4, 5.6 and 5.7). The three shrub
landscapes were made up of more woody structures than other landscape systems and woody
165
structures have been shown to contain more carbon than other plant structures (Fang et al.,
2007). The herbaceous perennials and grasses, ornamental green roof, and native prairie mix
landscape systems contained higher volumes of plant biomass than many of the other landscape
systems, which could account for their high carbon contents. The three shrub landscape systems
and herbaceous perennial and grasses were also mulched, which reduced weed pressure, reduced
the need to weed the plots and disturb the soil, and the breakdown of mulch over time may have
contributed to soil carbon content.
Broad-leaf evergreen shrubs, deciduous shrubs, and the ornamental green roof landscape
systems did, however, have the lowest below-ground carbon contents during both growing
seasons (Table 5.5), contrasting with their high above-ground biomass, soil/substrate, and total
carbon contents. Below-ground carbon contents for these and the narrow-leaf evergreen
landscape systems were estimated based on above-ground biomass volume based on information
from Miller (2002), which may have resulted in underestimation of below-ground biomass.
Interestingly, the Kentucky bluegrass lawn had the highest below-ground carbon content and was
likely caused by the very dense root systems formed by this landscape system.
In many cases the green roof landscape systems contained less carbon than their
corresponding in-ground landscape systems. The lower above-ground biomass of the prairie
green roof may have had a number of causes. Full coverage of the plots was not achieved, unlike
the in-ground native prairie mix plots and less colonization by weed species in the green roof
platforms than the in-ground plots may have increased the amount of time it would take to
achieve full coverage. Shallower substrate may have inhibited root growth, which would have
reduced the size of plant the plots could support, limiting plant above-ground biomass volume.
The nature of the green roof substrate, which has good drainage, may have enabled more
166
substrate carbon to leave landscape systems than what leached out of the soil of the in-ground
landscape systems.
Only six landscape systems showed an increase in carbon content between 2010 and
2011. These were succulent rock garden and Sedum green roof for above-ground biomass
carbon content (Table 5.4), deciduous shrubs and vegetable and herb garden for below-ground
biomass carbon content (Table 5.5), and prairie green roof and ornamental green roof for total
carbon content (Table 5.7). The succulent rock garden and Sedum green roof and prairie green
roof took longer than other landscape systems to achieve the desired coverage of plot area, which
may explain the differences between the two years for those landscape systems. Many of the
other landscape systems contained either slow growing plants (the two evergreen shrub plots) or
plants that experienced winter dieback followed by regrowth, resulting in little difference in
biomass between the two growing seasons. The vegetable and herb garden experienced more
weed pressure in 2011 and inclusion of weed roots in the sampling may have inflated the below
ground carbon content of this landscape system.
Implications. All of the green roof platform landscape systems exhibited greater carbon
sequestration than that reported by Getter et al. (2009). After adjusting for initial substrate
2
carbon content (3.15 kg C/m ) the Sedum, prairie and ornamental green roofs contained 4.67,
2
5.64, and 65.25 kg C/m , respectively after the third growing season compared to .37 kg C/m
2
(Getter et al., 2009). For the prairie and ornamental green roofs this is not surprising as the
species represented in those landscape systems have much greater above-ground biomass and
more woody structures than the Sedum spp. examined by Getter et al. (2009). Getter et al.
(2009) also only examined above- and below-ground biomass and substrate carbon content for
single species of Sedum in a substrate only 6 cm deep. This suggests that green roofs with
167
deeper substrates are capable of greater carbon sequestration. In other landscape systems,
greater species diversity can lead to greater carbon sequestration (Rhodes, et al., 2000; Tilman et
al., 2006). This may be the case in green roofs as well, but when Getter et al. (2009) examined
existing green roofs with mixes of Sedum species, their results were consistent between studies.
Another possible explanation for the discrepancy between the Getter et al. (2009) studies and this
one is that all of the roofs that they examined were not irrigated. The irrigation supplied by this
study may have enabled greater growth of the Sedums.
Getter et al. (2009) also examined the carbon budget of a green roof and reported that the
materials needed to install the type of green roof used in that study had an embodied energy of
2
2
6.5 kg C/m with a payback period due to energy savings of about .70 kg C/m of 9 yrs. When
the carbon sequestered by the green roof vegetation was included it reduced that payback period
by 2 yrs (Getter et al., 2009). Based on those calculations, assuming a similar embodied carbon
2
for the substrate used, roofs with a 10.2 cm depth would contain 10.5 kg C/m with a payback
period of 15 yrs assuming similar energy savings. For this study the Sedum and prairie green
roofs examined in this study reduce the carbon payback period to 2.2 and 1.9 yrs, respectively.
The vegetable and ornamental green roofs reduced the payback period even further to 1.2 and 0.2
yrs, respectively.
The carbon contents of the in-ground landscape systems indicate that there may be a great
deal of potential for carbon sequestration in ornamental landscapes. Marble et al. (2011) stated
that approximately 200,000 ha of land in the U.S. are devoted to nursery production. Assuming
that a similar amount of land is in ornamental landscapes, excluding urban forests, that land
could store between 0.25 and 1.59 Pg C based on landscapes examined in this study with the
lowest and highest total carbon content (Table 5.7). The total carbon content of landscape
168
systems examined in this study also fall within the range of some reported values for forests in
2
the United States, which range between 15.18 and 72.12 kg C/m (Smith et al., 2004).
There are however some important questions that need to be asked about the quality and
permanence of that carbon storage. Nowak et al. (2002) reported that management practices had
a large impact on the ability of urban forests and trees to sequester carbon and that more
intensive management with powered machinery and tools reduced the net amount of carbon
sequestered. The same would be the case in ornamental landscapes. The Kentucky bluegrass
lawn in this study was mowed as needed at least three times a growing season with a gasoline
powered mower, resulting in carbon emissions. Many lawns are mowed much more frequently
than that, some as often as once a week, which would result in even more carbon emissions,
which would erode away at the already small amount of total carbon sequestered by that
landscape type (Table 5.5). Many landscapers also manicure shrubs, which was not done for this
study, and would also result in emissions that would reduce the net carbon sequestered.
Management practices, or rather the change of management practices as land changes hands has
been a barrier to small forests entering the carbon exchange (Bigsby, 2009). This may also be
the case of ornamental landscapes, for which management may differ depending on not only
ownership, but the landscapers in charge of managing the land.
Management and ownership changes in ornamental landscapes can also mean the
complete removal of plant material in the landscape, which raises another issue to consider: the
fate of removed material. In many landscape systems dead material is removed and either
chipped, burned or put in a landfill. Nowak et al. (2002) has suggested that in the case of urban
trees, when a tree is replaced and mulched its replacement will not sequester any more carbon
than is emitted by the decomposing mulch, generating a cap on how much carbon a landscape
169
will contain before it reaches equilibrium. The same is likely true of ornamental landscapes.
Herbaceous perennials and grasses are often cut back on an annual basis to remove unsightly
dead materials. We performed this maintenance for the herbaceous perennial and grasses,
vegetable and herb garden, ornamental green roof and vegetable green roof landscape systems,
but were unable to monitor the removed carbon. Nor could we account for carbon released when
the native prairie mix and prairie green roof landscape systems died back during the winter.
Once this is taken into account it could be that net carbon sequestration for these landscape
systems is lower than this study would suggest. More research should be done to determine how
changes in ownership and management of ornamental landscape systems and the fate of removed
materials affect both the quantity of carbon sequestered and its permanence. Without this
information it will be difficult to move forward in including ornamental landscapes in carbon
trading programs.
Conclusions
Results of this study suggest that ornamental landscapes both in-ground and on green
roofs have the ability to sequester carbon. The landscape systems that were able to sequester the
most carbon contained higher amounts of woody plant structures and higher plant biomass
volumes, such as the three shrub landscape systems and the herbaceous perennial and grasses,
native prairie mix, and ornamental green roof landscape systems. Two of the green roof
landscape systems examined, Sedum and prairie green roofs, did not sequester as much carbon as
their counterpart in-ground landscape systems. This was likely due to differences in soil and
substrate properties and the ability the landscape systems to reach 100% surface coverage. Even
so the green roof landscape systems sequestered more carbon than shown by previous research
170
and their use may reduce the payback period of carbon embodied in the green roof materials
from 15 yrs to less than 3 yrs. Although this may be promising for the green roof industry,
greater carbon sequestration can still be achieved on the ground and carbon sequestration will
likely only be a secondary benefit of green roofs. The in-ground landscape systems also show
promise for contributions to carbon sequestration and possibly carbon trading, with total carbon
values similar to some reported in the literature for forests in the United States.
The types of landscape systems examined also raised some questions about how
maintenance would affect net carbon sequestration. Research could be done to address the
questions of how management practices and the tools used in landscaping affect carbon
emissions and therefore net carbon sequestration and how the removal of materials from the
landscape changes the permanence of the carbon sequestered and the time frame in which the
landscape will reach carbon equilibrium.
Acknowledgements
Funding for this study was provided by the Michigan Nursery and Landscape
Association; Project GREEEN (Generating Research and Extension to meet Economic and
Environmental Needs); Ford Motor Company, Dearborn, MI; ChristenDETROIT Roofing
Contractors, Detroit, MI; XeroFlor America, Durham, NC; Renewed Earth, Kalamazoo, MI; and
MSU AgBioResearch. Special thanks to the Michigan State University Horticultural Teaching
and Research Center for their support; to paid and volunteer field and lab assistants Saloni Dagli,
Daniel Finks, Craig Miller, Sarah Greer, and Jane Whittinghill; and to Mike Cook for performing
the carbon analysis.
171
REFERENCES
172
REFERENCES
Awal, M.A., T. Ohta, J. Matsumoto, T. Toba, K. Daikoku, S. Hattori, T. Hiyama, and H. Park.
Comparing the carbon sequestration capacity of temperate deciduous forests between
urban and rural landscapes in central Japan. Urban Forestry & Urban Greening 9:261270.
Balesdent, J., C. Chenu, and M. Balabane. 2000. Relationship of soil organic matter dynamics to
physical protection and tillage. Soil & Tillage Research 53:215-230.
Bigsby, H. 2009. Carbon banking: creating flexibility for forest owners. Forest Ecology and
Management 257:378-383.
Brown, S. 2002. Measuring carbon in forests: current status and future challenges.
Environmental Pollution 116:363-372.
CCX. 2009. Chicago Climate Exchange. Accessed 1 May 2009.
.
Clark, C., P. Adriaens, and B. Talbot. 2006. Probabalistiv economiv analysis of green roof
benefits for policy design. In: Proc. of 4th North American Green Roof Conference:
Greening Rooftops for Sustainable Communities, Boston, MA. 11-12 May 2006. The
Cardinal Group, Toronto.
Clark, C., B. Talbot, J. Bulkley, and P. Adriaens. 2005. Optimization of green roofs for air
pollution mitigation. In: Proc. of 3rd North American Green Roof Conference: Greening
Rooftops for Sustainable Communities, Washington, DC. 4-6 May 2005. The Cardinal
Group, Toronto.
Fang, S. J. Xue, and L. Tang. 2007. Biomass production and carbon sequestration potential in
poplar plantations with different management patterns. Journal of Environmental
Management 85:672-679.
Getter, K.L., D.B. Rowe, G.P. Robertson, B.M. Cregg, and J.A. Andresen. 2009. Carbon
sequestration potential of extensive green roofs. Environmental Science and Technology
43(19):7564-7570.
Kucharik, C.J., J.A. Roth, and R.T. Nabielski. 2003. Statistical assessment of a paired-site
approach for verification of carbon and nitrogen sequestration on Wisconsin
Conservation Reserve Program land. Journal of Soil and Water Conservation 58(1):5867.
173
Marble, S.C., S.A. Prior, G.B. Runion, H.A. Torbert, C.H. Gilliam, and G.B. Fain. 2011. The
importance of determining carbon sequestration and greenhouse gas mitigation potential
in ornamental horticulture. HortScience 46 (2):240-244.
Matamala, R. J.D. Jastrow, R.M. Miller, and C.T. Garten. 2008. Temporal changes in C and N
stocks of restored prairie: implications for C sequestration strategies. Ecological
Applications 18(6):1470-1488.
Miller, T. 2012. Quantifying carbon sequestration in woody and herbaceous nursery production
systems. Masters Thesis Michigan State University, East Lansing, MI.
Nowak, D.J., J.C. Stevens, S.M. Sisinni, and C.J. Luley. 2002. Effects of urban tree management
and species selection on atmospheric carbon dioxide. Journal of Arboriculture 28(3):113122.
Rhodes, C.C., G.E. Eckert, and D.C. Coleman. 2000. Soil carbon differences among forest,
agriculture and secondary vegetation in lower montane Ecuador. Ecological Applications
10(2):497-505.
Sanderman, J. and R. Amundson. 2009. A comparative study of dissolved organic carbon
transport and stabilization in California forest and grassland soils. Biogeochemistry
92:41-59.
Smith, J.E., L.S. Heath, and P.B. Woodbury. 2004. How to estimate forest carbon for large areas
form inventory data. Journal of Forestry 102(5):25-31. Accessed 18 March 2012.
.
Tilman, D., J. Hill, and C. Lehman. 2006. Carbon-negative biofuels from low-input high
diversity grassland biomass. Science 314:1598-1600.
United Nations Framework Convention on Climate Change (UNFCCC). 1997. Kyoto Protocol.
Accessed 31 March 2009. .
VanWoert, N.D., D.B. Rowe, J.A. Andresen, C.L. Rugh, R.T. Fernandez, and L. Xiao. 2005.
Green roof stormwater retention: Effects of roof surface, slope, and media depth. J.
Environ. Qual. 34:1036-1044.
Wu, L., Y. Wood, P. Jiang, L. Li, G. Pan, J. Lu, A.C. Chang, and H.A. Enloe. 2008. Carbon
sequestration and dynamics of two irrigated agricultural soils in California. SSSAJ
72(3):808-814.
174