SUB-SURFACE DRIP AND OVERHEAD IRRIGATION EFFECTS ON ASPARAGUS
PRODUCTION UNDER MICHIGAN GROWING CONDITIONS
By
Benjamin Byl

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
Horticulture - Master of Science
2013

ABSTRACT
SUB-SURFACE DRIP AND OVERHEAD IRRIGATION EFFECTS ON ASPARAGUS
PRODUCTION UNDER MICHIGAN GROWING CONDITIONS
By
Benjamin Byl
Asparagus is a perennial crop historically grown without irrigation in western Michigan for the
processing market. Shifts to fresh market production, new hybrids, increased incidence of
summer drought, and increased disease pressure, justify evaluation of more intensive production
practices including irrigation. Field and greenhouse trials evaluating the impact of drought stress
and irrigation delivery system (overhead vs. sub-surface drip) on two varieties (Guelph
Millennium [GM] vs. Jersey Supreme [JS]) were initiated to guide Michigan asparagus grower’s
irrigation decisions. Short-term results from these studies indicate a variety of positive plant
responses to irrigation treatments. Asparagus yields increased from 6 to 21% with trickle and
overhead irrigation treatments in GM and JS during the 2012 field season. With supplemental
irrigation, increases in stem number, light interception, fern height, root carbohydrates,
cladophyll weight, and dry fern weight occurred. Cultivar responses to irrigation treatments
differed depending on drought stress severity and plant growth stage. Increased yields for GM
were attributable largely to increased weight per spear, rather than increased spear number as
seen in JS. Results from multi-season greenhouse trials with GM demonstrated that prolonged
low-level drought stress reduced fern growth and root weight, while short-duration intense
drought stress had greater impact on root carbohydrate concentration and short-term yield.
Overall, these results suggest that under weather conditions similar to those of 2011-2012
irrigation increases yield and plant health enough to justify the added costs of irrigation for
Michigan asparagus growers.

ACKNOWLEDGEMENTS

I would first like to thank my advisor, Dan Brainard for all of the guidance and support
during the last several years at MSU. He’s a great man to work for and with both on campus and
in the field. To Mathieu Ngouajio and Zsofia Szendrei for guidance as my committee members
and to all my lab mates for support over the past several years. To the USDA-MDA Specialty
Crop Block Grant program and the Michigan Asparagus Research Board that provided funding
for this research. To John Bakker and Norm Myers for help in Oceana county as invaluable
sources of advice and information. To Sam Peck and Tim Vinke for helping get the data
collection work finished. Finally, to my family members and Amber for supporting me. I look
forward to spending more time around you all now that this project is wrapping up.

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TABLE OF CONTENTS

LIST OF TABLES.……………………………………………………………………….…..vi
LIST OF FIGURES.…………………………………………………………………………viii
CHAPTER 1: Introduction.……………………….………………………….………................1
Asparagus growth and development…………………………………………………....1
The US asparagus industry…………..………………....………………………….........3
Irrigation for asparagus production...…………………….……………………………..5
Previous irrigation studies in asparagus...……………………………………………....8
Goals and scope of this work………….………...……………………………………...9
CHAPTER 2: Effects of sub-surface and overhead irrigation on Michigan asparagus………..11
Introduction……………………………………………………………...……………..11
Materials and Methods…………………………………………………………………14
Site characteristics and experimental design…………...……………………....14
Trial establishment…………………………………………………...…………15
Moisture monitoring and irrigation………………….…..……………………...16
Light interception……………………………………………………..………...17
Root sampling………………………………………..……………....................18
Yield and quality assessment………………………..…………….....................19
Fern evaluation……………………………………………………………….....19
Pest evaluation…………………………………………………………………..20
Statistical analysis…………………………………...………………………......20
Results…………………………………………………………………………………...21
Rainfall, irrigation and soil moisture……………………..……………………...21
Fern development ...……………………………………………………………..22
Root brix………………….…………………………………………...…………23
Pest evaluation………………………………………………………...…………23
Yield and quality….………………………………………..……………………25
Soil chemical properties……………………………………………....…............26
Summary and Discussion………………………………………………………………..26
CHAPTER 3: Effects of long- and short duration drought stress on asparagus….………..........29
Introduction……………………………………………………………………………...29
Materials and Methods………….……………………………………………………….32
Site characteristics and experimental design……………………………….........32
Trial establishment………………………………………………………………32
Moisture monitoring and irrigation……………………………………………...34
Root sampling……………………………………..………………..…………...35
Fern evaluation.…………………………………………………………….........36
Yield and quality assessment.…………………….……………………………..37
Second year fern growth and root sampling.……………………………………37
Statistical analysis.…………………………………………………….………...38

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Results….………………………………………………………………………………..39
Stem number during the first growing season…………………………...………40
Fern dry weight……….………………………………………………………….40
Yield, spear number and spear size after one season…………………………….41
Stem number and height in second growing season……………………………..41
Root weight and soluble solids….…………………………………….…………42
Summary and Discussion………………………………………………………………...43
Dormancy………………………………………………………………..……….44
Temperature regulation……..…………………………………...……………….45
Unusually tall ferns in second season……………………………………………47
Future work……………………………………………………………………………....47
CHAPTER 4: Discussion………………………………………………………...………………49
Field and greenhouse trial………………………………………………………..49
Future work……………..………………………………………………………..50
Industry implications…………………………………………………………….51
APPENDIX………………………………………………………………………………………53
LITERATURE CITIED…..……………………………………………………………………...78

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LIST OF TABLES

Table 2-1. Schedule of major events, 2010-2012…………………………….…........54

Table 2-2. Stem number by category, 8 August 2011 and 2 August 2012…….……..55

Table 2-3. Light interception by asparagus fern, 2011 and 2012………..……...........56

Table 2-4. Fern stem and cladophyll dry weight, 2011 and 2012…………................57

Table 2-5. Root soluble solid concentration, 2011-2012....………….........................58

Table 2-6. Weed and disease severity, 2011..…………………………...…………..59

Table 2-7. Density and fern damage from key insect pests 2012………………........60

Table 2-8. Asparagus yield and quality, 201………………………………………..61

Table 2-9. Soil chemicals properties, 2012…….……………………………………62

Table 3-1. Calendar of activities…………………………………………………….71

Table 3-2. Total stem number during first season.…………………………………..72

Table 3-3. Fern dry weight………….……………………………………………….73

Table 3-4. Spear weight and number………………………………………………...74

Table 3-5. Total stem number following harvest……………………………………75

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Table 3-6. Total stem height after harvest………………………………………..…76

Table 3-7.Final root weight and soluble solids, 2011-12……………………………77

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LIST OF FIGURES

Figure 2-1. Estimated evapotranspiration minus rainfall 1997-2012………….63

Figure 2-2. Rainfall and irrigation during 2011………………………………..64

Figure 2-3. Rainfall and irrigation during 2012…………….………………….65

Figure 2-4 Mean volumetric water content by irrigation treatment 2011….…..66

Figure 2-5. Mean volumetric water content by irrigation treatment 2012.…….67

Figure 2-6. Mean volumetric water content by irrigation treatment 8/8/11……68

Figure 2-7. Mean daily water loss July 2011……….………………………….69

Figure 2-8. Mean daily water loss June 2012……………………….……….…70

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Sub-surface Drip and Overhead Irrigation Effects on Asparagus Production under
Michigan Growing Conditions

CHAPTER 1
Introduction
Asparagus growth and development
Asparagus officinalis is a perennial vegetable crop that is hand-harvested every spring.
Root carbohydrates are replenished following harvest, during the summer and fall from
photosynthesis occurring in a two-meter tall fern canopy. Herbicides are typically used twice a
year to control weed populations and very little tillage occurs after the year of planting. Most
Michigan fields are established using crowns—year old root systems grown from seed offsite
during the previous year—instead of greenhouse grown seedling transplants.
During the spring of field establishment 70 cm trenches, or furrows, are plowed into the
sandy soil. Crowns are set out on the bottom of the furrow with granular fertilizer and covered
with soil by eroding the trench walls with a soil blade. As the asparagus develops over the
growing season furrows are filled gradually in by grading soil from the berms into the trenches
(Zandstra et al. 1992).
Asparagus plants consist of two main sections: the above ground fern, and the
subterranean crown consisting of rhizomes and roots. The crown is the heart and central
interchange of the plant, producing growth buds for future spears and storing resources for future
growth. Asparagus roots can be separated into two categories: the numerous thin, short-term
feeder roots that secure nutrients and water and the fleshy storage roots that ensure overwinter
plant survival by amassing soluble carbohydrate reserves (Drost 1997).
The fern consists of fibrous stalks, which develop from edible immature spears. As stems
mature, branches develop off of growth scales, and plants “fern out” producing many

11

photosynthetic leaflets as they mature (Drost 1997). The small leaflets that develop off the
branches and stems of asparagus are called cladophylls; they are the most critical photosynthetic
area of the plant.
The simplest method of gauging asparagus size and health is by observing the fern
canopy that develops after the spring harvest season has concluded. Several studies (Guo 2001;
Read 2009) have shown that fern weight is correlated with yield the following season. Although
in general this rule is true, overall fern size should not be the sole means of predicting asparagus
yield (Schaller and Paschold 2009). Survival of the crown is the most critical aspect of
producing asparagus because replacing plants in an established stand is generally not feasible
and the early loss of plants results in compounding yield losses over multiple years (Sinton et al.
2008; Sterrett et al. 1990).
Photosynthesis levels over the preceding season dictate the yield for the following year’s
harvest. Most of the above-ground portions of the asparagus plant rapidly green up with
chlorophyll from their initial white or yellow coloration, as they are exposed to light. Stem
tissue has some photosynthetic capability and stomata, but the cladophylls are the center of the
plant’s photosynthesis production (Suzuki et al. 2002). Elevated photosynthetic rates were found
in higher yielding cultivars, with high ratios of cladophyll to stem (Faville et al. 1999). Guo
(2001) illustrated that asparagus partitioning to cladophylls is highly variable between cultivars
of asparagus, and that higher yielding types also having more cladophyll weight than lower
yielding varieties. Because of their wavy micro-surface, varied width, and small size, physical
measurements of cladophyll surface areas are difficult, so using dry weight is considered good
scientific practice (Guo 2001). Levels of water use efficiency and total photosynthesis varied
with stomatal size and density between asparagus cultivars (Schaller and Paschold 2009).

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There have been a number of root studies that have investigated both the size and
carbohydrate levels of the asparagus plant throughout the calendar year either in the field or in
potted specimens (Haynes 1987; Drost and Wilcox-Lee 1997; Drost and Wilson 2003; Schaller
and Paschold 2009; Paschold et al. 2004). The rooting zone of a mature asparagus plant is
substantial, typically reaching at least a meter below the surface with the majority of root weight
in the top 50 cm near the crowns (Drost 1997). In New Zealand, Wilson et al. (2002) developed
a field-based carbohydrate evaluation protocol using refractometers to measure simple sugar
levels in root tissue throughout the year. Although this model has not been widely adopted by
Michigan growers to schedule harvest length, it remains a practical tool for research into root
status.

The U.S. asparagus industry
Michigan has been one of the nation’s three largest asparagus producers for the past fifty
years. In 2010, there were 146 independent asparagus farms with 4,309 ha in production with a
cash value of over $16 million (USDA-ERS 2010). Michigan harvest occurs from May to June
in sandy soils near the Lake Michigan shoreline. Historically asparagus production in the
Midwest has focused on making canned and frozen products but there has been a recent shift
towards fresh markets. Many mid-sized family fruit and vegetable operations cultivate between
twenty to one hundred acres of asparagus to diversify their cropping systems; asparagus extends
the working season for harvest laborers and helps growers by generating cash-flow to fund other
operations before revenue from fruit production arrives.
Although the past fifteen years have seen increased U.S. asparagus consumption, overall
domestic production has decreased. This is primarily due to increased competition from Peru,

33

which supplied 81% of the asparagus consumed in the U.S. in 2009 (Dartt et al. 2009).
Increasing asparagus imports from Latin America decreased market prices significantly for the
first decade of the 2000s. This price drop led to a significant decrease in U.S. production
acreage (Read 2009) with the number of new asparagus plantings decreasing and many older
fields being plowed out. Established asparagus fields historically lasted 25 years or more, but
with the onset of fungal diseases, as well as shifting markets, the duration of profitable
production declined to about twelve years. Five or six years are always required for plants to
reach full production potential, meaning there are far fewer productive years given declining
stand longevity (Dartt et al. 2009).
In traditional production areas prevalence of root pathogens has been a leading factor in
asparagus production decline (Hausbeck 2012). Much of the prime land in western Michigan
has been cropped in asparagus for decades; growers are forced to replant over previous plantings
into soils with elevated populations of fungal pathogens, including Phytophthora and Fusarium
species (Rodriguez 2010). Starting in the 1990’s Jersey Giant was the dominant variety planted
in Michigan; it produces well in early life, but experiences high plant mortality in later years,
probably because of intolerance to these pathogenic fungi.
Growers are now faced with a narrow production window to recuperate the large sunk
costs from the first several years of establishment and cultivation before the stand declines and
production erodes. Asparagus decline and low prices have led to a decrease of Michigan
production acreage from 6,880 hectares in 1990 to under 4,451 hectares currently, and fields are
being replanted at much higher cost than the $5,000 ha establishment costs of even ten years ago
(Ball et al. 2001).

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Asparagus establishment costs have risen dramatically in the past several years, as new
hybrids planted on fumigated soils at higher densities have become the norm. In the nineties
Michigan State University Extension recommendations were for 29,640 crowns ha. without
fumigation (Zandstra et al. 1992). Now growers are typically planting at least 37,050 crowns ha
to raise long-term harvest labor efficiency. Seed costs for modern hybrid varieties have doubled
to $2,250 ha from several years ago and seed costs of these hybrids are twenty times greater than
the traditional Mary Washington varieties. At current planting densities, the cost of hybrid
asparagus crowns reached $7,500 ha in 2013 (Nourse 2013). In addition, soil fumigation
increases startup input costs today. With the EPA phase-out of several fumigants, fumigation
costs have risen from approximately $750-900 ha to at least $1,250 ha while treatment efficacy
has declined (Hausbeck 2012). Given these rising input costs, growers have increased incentive
to intensify production systems through additional practices including irrigation.

Irrigation for asparagus production
Although irrigation has been a mainstay in many significant asparagus production regions
around the world, unlike Michigan they tend to be naturally arid. While the asparagus plant is
drought hardy (Krug 1998), increases in productivity are necessary to ensure adequate return on
the large financial investment of establishing asparagus fields. In Michigan, an annual yield
increase of approximately 9% from a baseline yield of 3,359 kg ha has been estimated to justify
the installation of irrigation in the first seven years (Harsh 2010). However, such estimates are
based on a wide range of assumptions, and very few studies have been conducted to evaluate the
potential biological and economic benefits of irrigation in asparagus production for Michigan.

55

Worldwide, irrigation is typically applied to asparagus fields by one of three methods: 1)
furrow irrigation—the traditional practice in the American west; 2) center pivot sprinkler
systems; and 3) trickle irrigation, either through tubing buried at crown level or placed on the
soil surface. Each system has its own pros and cons. Furrow irrigation—the oldest system—
floods production areas periodically using networks of ditches to disperse river water. Due to
low water use efficiency, hilly-topography, coarse soil types, and water withdrawal restrictions,
furrow irrigation isn’t used in Michigan.
Center pivot systems are relatively common in Michigan but have traditionally been
installed only on annual vegetable crops with high value and drought-susceptibility like carrots
or cucurbits. Given Michigan’s high relative humidity and modern spray nozzles these systems
can be relatively water efficient, reaching levels of 80% (Kelley 2013; Nelson 2013). Center
pivot has the additional benefit of being able to activate residual herbicides, move mobile N
fertilizers into soils, and establish cover crops in the fall without reliance on rainfall. Center
pivot systems have a working life far longer than asparagus; once sunk costs are recouped with
asparagus production, the equipment can continue to be used for another twenty years. On the
other hand, center pivot systems may induce higher weed germination and growth by moistening
the soil surface. Overhead systems like center pivot can also exacerbate foliar diseases by
increasing periods of leaf wetness. Center pivot systems don’t achieve reasonable economies of
scale on fields less than 15 hectares or in irregularly shaped fields, and they require large water
sources and fuel costs for delivery (3,000 L per minute for a 35 ha system) (Neibergs and Waters
2009; Kelley 2013).
Trickle irrigation is common in many intensively produced vegetables in Michigan, but is
rarely used in asparagus. In arid asparagus production regions, trickle tubing is placed on the

66

soil surface and rolled up annually (e.g. Peru), or experimentally, buried below the soil surface
and left in place for the lifetime of the asparagus stand (Sinton and Wilson 2008; Ley and
Agenbroad 1989). In such sub-surface drip systems, thick-walled tubing is usually buried in
every asparagus row. Tubing with emitters every 46 cm are typically placed at crown level at the
time of crown transplant (Netafim 2010). With sub-surface drip, a significant amount of soil
surface moisture evaporation is avoided facilitating lower water use requirements. Buried trickle
line has the additional potential benefit of facilitating fertilizer or pesticide application directly to
the root zone where they can be most effective. Since drip systems can be broken into zones,
water pressure and pump requirements are low, reducing costs. Trickle tubing can also be placed
on the surface annually but tube malfunctions and total combined installation and removal costs
are greater long-term compared to buried tubing.
Irrigation could prove valuable in Michigan production for several reasons. Asparagus
planted with irrigation is at a lower risk of establishment failure, and yield may be increased
significantly through higher per-picking yield and a more aggressive harvest schedule (Schaller
and Paschold 2008; Bussell et al. 1984; Sterrett et al. 1990). In irrigated systems, it may be
possible to increase planting densities, making harvest, fumigation, spraying and cultivation
more efficient. The need for fumigation at planting could be reduced with alternate chemical
applications through use of irrigation throughout the production cycle and increased nutrient use
efficiency could provide value in addition to replacing transpired water (Netafim 2010).

77

Previous irrigation studies in asparagus
Depending on the plant’s age or variety, soil conditions, and average rainfall, irrigation
trials have illustrated drastically increased yields in many parts of the world, including rain-fed
climates similar to Michigan (Hartmann 1981; Rolbiecki and Rolbiecki 2007).
Several irrigation studies conducted in rain-fed asparagus production regions of Europe
suggest that the potential benefits are large. For example, Rolbiecki and Rolbiecki (2007) found
gains of over thirty percent with many cultivars in Polish variety trials. Hartmann (1981,1996)
found number of spears increased 30% in sandy soils in Germany. He advocated use of trickle
irrigation in his climate because of water savings and noticeable benefit with higher producing
cultivars; European varieties showed particularly significant production increases with Grolim
yields more than doubling with the addition of irrigation.
The method of irrigation can also have a dramatic effect on asparagus crop response,
profitability, and environmental impact. (Sterrett et al., 1990) found that buried trickle irrigation
greatly improved establishment survival rate in young asparagus, particularly with transplants.
Lifespan and maintenance cost of buried tubing also were noted to be better. Netafim, one of the
world’s larger trickle manufacturers, actively promotes subsurface installation of its products in
asparagus (2010). Overhead sprinkler systems had asparagus survival rates below un-irrigated
control with noted wear to fragile ferns (Sterrett et al. 1990).
The effects of irrigation depend not only on climate and method, but also on the specific
variety being evaluated. For example, Schaller and Paschold (2009a) showed significant
differences in physiological drought response between Backlim and Grolim varieties. Likewise,
Rolbiecki and Rolbiecki (2007) and Sinton and Wilson (2008) demonstrated dramatic yield
differences between cultivars when subjected to drought stress. Several trials (Wilson and

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Sinton 1996; Rolbiecki and Rolbiecki 2007) have found that the Jersey Giant variety—the most
prevalent Michigan variety—is also one of the least impacted by supplemental irrigation. One of
the core purposes of these trials is to evaluate the economic cost and benefit of overhead and
sub-surface irrigation in Michigan asparagus. To our knowledge, no studies have compared the
effects of irrigation on yield with varieties such as Guelph Millennium and Jersey Supreme,
currently being planted in Michigan.

Goals and scope of this work
With this background in mind, experimental trials were planned to collect information on
the potential costs and benefits of irrigation applicable to Michigan’s field conditions and
climate. In the first study (described in detail in Chapter 2), the effects of two irrigation systems
(subsurface drip versus overhead) on two asparagus varieties (Guelph Millennium and Jersey
Supreme), representing the most commonly planted MI asparagus genetics were evaluated in a
large field experiment. Yield and multiple aspects of plant health were evaluated over three
successive seasons. The goal of this trial was to provide Michigan growers with applicable data
that would assist them in deciding whether to invest in irrigation, and how to optimize irrigation
systems to improve profits. We hypothesized that: 1) if typical historic drought stress periods
occurred during the study, irrigation would increase fern growth, root carbohydrate storage and
ultimately yield; 2) the yield benefits of irrigation would depend on asparagus variety, with the
newer Millennium variety responding more favorably to irrigation than Jersey Supreme; 3) yield
benefits would also depend on delivery systems, with sub-surface drip improving yields and
reducing costs relative to overhead irrigation; and 4) sub-surface drip irrigation would result in

99

fewer weed and disease problems relative to overhead irrigation due to reduced leaf and soilsurface wetting.
This large-scale irrigation trial was supplemented with a greenhouse irrigation trial
(described in detail in Chapter 3) aimed at better understanding the response of Millennium to
baseline irrigation and drought treatments. We hypothesized that drought periods of two weeks
or more in summer would have a negative impact on fern growth, carbohydrate storage, and
yield the following spring. An important secondary objective of this trial was to better
understand how root growth and carbohydrate storage responded to different forms of drought
stress.

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CHAPTER 2
Effects of Sub-surface and Overhead Irrigation on Michigan Asparagus
Introduction
As the weather patterns in the Great lakes region shift, intense summer heat and more
sporadic rainfall have occurred with increased regularity. For example, since 1980 Michigan’s
average temperature has warmed by two degrees, and although precipitation has also increased in
the last 50 years, this increase is concentrated during the cooler seasons (MSU Enviro-weather,
2013). During the critical period of fern growth and carbohydrate replenishment (July-August),
rainfall is often well below evapo-transpirational demand (Figure 2-1), resulting in drought stress
in un-irrigated fields. Evapotranspiration (ET) is the amount of soil moisture lost to the air from
all surfaces in a cropping area. In water requirement forecasting, each crop is given a coefficient
that represents it’s water requirement relative to a reference crop at various crop growth stages.
Relatively little is known about the impact of alternative irrigation systems on commonly
grown asparagus varieties under Michigan weather and soil conditions. Drought stress may be
an important factor contributing to the decline in asparagus fern health and yield. Although
asparagus is deep rooted and relatively drought tolerant, soil water content during fern growth is
an important determinant of crop yields (Drost and Wilcox-Lee 1997; Hartman 1981). Drought
stress during fern growth can limit the capacity of plants to produce the soluble carbohydrates in
roots necessary for high yields in subsequent seasons (Drost and Wilcox-Lee 1997). Stressed
plants may also be more susceptible to fungal diseases that increasingly plague the asparagus
industry, including Fusarium (Morrison et al. 2012) and Phytophthora (Saude et al. 2008).
Warmer temperatures and more variable rainfall patterns observed in MI in recent years make
irrigation an increasingly important tool for reducing risks of yield loss in asparagus production.

1111

Irrigation may also create opportunities for valuable complementary practices including
cover-cropping and fertigation. In irrigated systems, cover crops growing below the fern canopy
may be established with reduced risk of competition for water with the asparagus crop.
However, care must be taken to avoid competition for water with the crop. Living mulches in
un-irrigated asparagus in Wisconsin suppressed weeds but in some cases reduced yields (Paine et
al. 1995). In Michigan, winter rye sown as a living mulch immediately following asparagus
harvest was found to be beneficial for suppressing certain weeds including Conyza canadensis,
but also reduced volumetric water content to dangerously low levels during fern growth in the
absence of irrigation (Brainard et al. 2012). Under irrigated systems, cover crops may be more
widely adopted without adversely impacting asparagus.
Irrigation may also facilitate delivery of pesticides and fertilizers to improve the
efficiency and efficacy of agrichemicals, and reduce their adverse environmental impacts.
Fertigation and chemigation through drip tape is common in many vegetable crops, but has not
been extensively explored in asparagus. By targeting agrichemicals directly to the root zone
through sub-surface drip tubing, losses through leaching and volatilization are minimized.
Asparagus growers adopting overhead irrigation report improvements in the efficacy of soilapplied herbicides requiring moisture for activation. This is particularly important following
asparagus harvest when the optimal window for herbicide application is very short. Dry
conditions during this period result in reduced herbicide efficacy. Some growers are forced to
delay herbicide applications and extend harvest while waiting for rain to activate herbicides
(Brainard, personal communication); this practice stresses asparagus by both limiting the period
of fern growth for carbohydrate replenishment and drawing energy for more production.

1212

With these potential benefits in mind, MI asparagus growers have begun to adopt
irrigation into their systems, but little information is available to guide them in their choices
regarding delivery system (overhead vs. drip). Almost all investment in asparagus irrigation so
far has been in center-pivot systems. Theoretically, drip irrigation systems will save water and
fuel costs associated with irrigation. Drip systems also minimize the risk of foliar disease by
avoiding leaf-wetting. By avoiding moistening the soil surface, drip systems may also be less
likely to stimulate weed and foliar fungal growth (e.g. rust). However, overhead irrigation
systems are viewed by growers as less costly to maintain and provide several potential
advantages including activation of soil-applied pesticides, spear cooling during harvest, and
uniform establishment of cover crops. Before growers will consider less familiar alternatives
like drip, they need data on its potential economic and biological advantages.
The impacts of irrigation on asparagus are likely to vary considerably with variety, but
little information is available regarding response of typical Michigan varieties to irrigation.
University of Guelph’s Millennium has not been included in many previous international cultivar
trails because of the relative recent entry into world markets. In Ontario and Michigan
production of Millennium in soils nearing loam classification have been very good, suggesting
having higher available water in sand would be beneficial. New Zealand irrigation trials found
that Jersey Giant asparagus was not responsive to irrigation but those trials were in soils with
high moisture retention ability and adequate natural rainfall; Jersey Supreme will probably share
some of those responses because of shared paternity. In North Carolina variety trials,
Millennium and Supreme both performed in the top quartile of available varieties over a fiveyear period (Cantaluppi 2011).

1313

To address grower interests, and fill existing knowledge gaps, a large-scale field
experiment was initiated in 2010 to evaluate the long-term impact of irrigation system (none,
sub-surface drip, or overhead) on fern growth, yield and profitability of both Guelph Millennium
and Jersey Supreme asparagus varieties. Secondary objectives included assessment of irrigation
systems impact on the incidence of insect, disease, and weed pests. The long-term objective of
the trial is to evaluate the development and subsequent decline of asparagus stand in each of the
six treatments, to assess whether irrigation can delay the decline of productive asparagus fields.
We hypothesized that 1) irrigation during fern growth would increase yield and quality of
asparagus; 2) these benefits would be greater for Guelph Millennium compared to Jersey
Supreme; and 3) sub-surface drip irrigation would provide equivalent yield improvements to
overhead, while reducing delivery costs as well as disease and weed problems.

Materials and Methods
Site characteristics and experimental design
A field experiment was established in Oceana county, western Michigan in 2010. Six
experimental treatments were examined consisting of three irrigation systems (none, overhead,
and sub-surface drip) and two varieties (“Jersey Supreme” and “Guelph Millennium”). Four
replicates of each treatment were arranged in a split plot design with irrigation as the main plot
factor, and variety as the subplot factor. Each of the 24 subplots measured 6 x 18.2 m with four
rows of asparagus spaced 1.5 m apart. Data were collected from the inside two rows of each plot
to minimize edge effects.
Soils were representative of those found locally in asparagus production; a well-drained
glacial moraine mixture of Spinks loamy fine sands and Perrinton loams, on slopes of up to six

1414

degrees with southern exposure. Initial soil tests indicated a soil composition of 86% sand, 6%
silt, and 8% clay averaged through the usable soil profiles, a surface pH slightly above neutral
and slightly acidic subsoil. The experimental area had no reported history of previous asparagus
production and had been either fallowed or cover-cropped for several years prior to planting.
Soil chemical characteristics immediately prior to planting fell into the following range: (pH 6.37.0 with levels soil organic matter at 1.1 to 1.3%)

Trial establishment
Asparagus was established from one-year-old crowns grown locally from seed on soil
that had been fumigated to reduce risk of fungal pathogens. Two asparagus varieties
representing the most common breeding programs were used for the trial: “Jersey Supreme” (JS)
from Vilmorin Seed, and University of Guelph’s “Millennium” (GM). The growth physiology of
these two varieties differs noticeably but both have been top performers in MI variety trials for
spear yield and quality.
On 17 May 2010, crowns were planted at a density of 36,889 ha. in crested furrows with
alternating placement on 150 cm row spacing. The weight of GM crowns was less than JS but
both varieties appeared un-desiccated and disease free. Liquid 10-34-0 fertilizer was applied as
directed by MSU soil test in furrow bottom of all plots. Netafim Uniram tubing with 45 cm
emitter spacing and 1.59 liter per hour output was placed below the crowns in-furrow in subsurface drip irrigation plots. Irrigation tubes were connected to buried 2.5 cm poly-pipe headers
but irrigation was not initiated during the first season because of limited evapotranspiration with
young plants and ample rainfall. Furrows were filled in slowly to hill over crowns with several
cultivations during June and July as plants became established.

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During the summer of 2010, asparagus spear emergence was relatively uniform between
plots and treatments after planting. Fungicides, insecticides and herbicides were applied in
accordance with standard grower practice for newly planted fields. In accordance with grower
practice in first year after establishment, two harvests were taken in 2011 with combined yield of
approximately 225 kg. Detailed yield data was not recorded since irrigation had not been
initiated the season before and significant frosting had occurred in early May.

Moisture monitoring and irrigation
Soil volumetric water content (VWC) was monitored with Sentek’s Diviner 2000 system
(Stepney, AU); one meter long specialized PVC tubes were hand-augured into each plot to
record moisture at ten-cm intervals to a depth of 100 cm. Readings were conducted at least on a
weekly basis throughout the fern growth period in both 2011 and 2012, with additional readings
taken before and after each irrigation event. In addition, VWC at crown depth was data-logged
hourly using two Decagon (Pullman, WA) EC-5 moisture sensors installed in each plot.
Irrigation water was supplied from a bored well through a steel reservoir tank with
supplemental electric pressure pump. Turbine waterflow meters (GPI, Wichita, KS) were
installed at the manifold to record water use in each irrigation zone. From the edge of each
overhead plot, irrigation was applied through Nelson Orbitors (Model R-10, Walla Walla, WA)
with road guard blinders with an overlapping, alternating 5.8 m radius semi-circle pattern spray
on top of 1.5 meter PVC risers. Irrigation events occurred as dictated by Diviner readings and
required from 24 to 30 hours to complete. Moisture levels were allowed to depreciate to
approximately 50% available water in the rooting zone before irrigation events were initiated.
Irrigation was applied to replenish water in the rooting zone to field capacity. In both 2011 and

1616

2012, six and seven irrigation events occurred, with application rates ranging from 1.2 to 3 cm
(Figure 2-2). Initial calculations for necessary irrigation time for the sprinklers did not
adequately raise soil moisture levels so additional time was added in those treatments. Due to
water pressure limitations, only a portion of the sprinkler plots could be irrigated simultaneously
so a zone system was established.
Climate data was first gathered from MSU’s Enviroweather station four miles to the west
and later by a Decagon weather station (EM50) on site for the 2011 and 2012 growing seasons.
Data gathered by the station included: ET, rainfall, PAR, and soil temperature. Additional data
was gathered during the growing season from repeated visual evaluation of ferns, stems, and
spears. These data parameters included: spear number, height, physiological stage, disease, and
insect prevalence. Uniformity in data collection standards with concurrent greenhouse trials was
attempted whenever possible. When visually evaluating fern canopy years of field scouting
experience allowed us to recognize and quantify unexpected, observable trends including
differences in foliar damage from herbicides, fern canopy closure rates, pest prevalence, and
autumn fern senescence rates.

Light interception
Canopy density was evaluated by measuring light interception with a portable PARsensing bar (Lightscout 6 quantum bar, Spectrum Technologies, Aurora, IL). Photosynthetically
active radiation, PAR, is the portion of the light spectrum usable to plants. In each plot, one
reading was taken above the canopy and five readings taken below the canopy at approximately
30 cm above ground level. Light interception (LI) was calculated according to the following
equation:

1717

(1) LI = 100 – (PARb/PARa)*100
Where PARb is PAR at 30 cm below the canopy and PARa is PAR above the canopy.

Root sampling
Soil cores were taken with 7.6 cm diameter hand augers (Sentek 47 mm auger)
throughout each season to monitor root development in multiple soil profiles both in the
senescence period and post-harvest. Soils were kept in coolers and processed on campus within
several weeks of collection. Roots were separated from soils with sieves, weighed, and analyzed
for carbohydrate levels according to the method developed for the New Zeeland Aspire root
carbohydrate system (Wilson et al. 2002). Root samples were collected, washed, and placed into
the freezer overnight to separate fructose from cell tissue. Just prior to analysis, root samples
were thawed, dab dried with paper, cut into 0.5 cm sections and at least 2g were crushed in a
garlic press to collect root sap. The refractive index (RI) of extracted sap was evaluated using an
Atago PAL-1 digital handheld refractometer. This device reports RI readings in degrees Brix
based on the relationship between RI and the sucrose concentration (% w/w) of a pure sucrose
solution at 20 °C. Samples were taken at both the crest and trough of carbohydrate stages at
senescence and early canopy establishment stages to evaluate usable carbohydrate levels for the
crown. These samples could be referenced against each other and with existing databases of
asparagus field root carbohydrate levels. Root fresh weights were taken initially but the auger
method was deemed too inaccurate for usable biomass distribution projections using the soil core
method (Drost and Wilson 2003).

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Yield and quality assessment
Yields were determined from the center two rows of each sub-plot on fifteen different
harvest dates from 19 April to 29 May 2012. Harvest was done by hand, using the snapping
method common in Michigan. Spears were harvested at 15-25 cm length. Those spears with
diameters below 1 cm, as well as prematurely ferning or damaged spears were discarded.
Cumulative fresh weight of marketable yield was determined and used for analysis. In addition,
spear quality was assessed from all spears on two harvest dates (14 May and 24 May),
classifying spears into the following size grades by diameter at the butt: Small (between 7.9 and
12.7 mm), Regular (12.7 to 17.5 mm) and Jumbo (greater than 17.5 mm). The total number and
fresh weight of spears in each size grade was evaluated, and mean weight per spear calculated by
dividing the total number of spears by their total weight for each size category.

Fern evaluation
Fern samples were collected twice per season in 2011 and 2012. Once before cladophyll
(photosynthetic fernlet) loss in early fall (4 Oct, 2011 and 28 Sept 2012) and again after winter
dormancy had fully occurred after first frost in late November (15 Nov 2011 and 9 Nov 2012).
At the first sampling date five ferns were randomly selected from each subplot, cut at the soil
surface and weighed fresh. A subsample of five plants from each subplot was then dried,
separated into cladophyll and stem tissue, and weighed. At the November sampling date, all fern
from four 1 m subsamples per subplot were cut off at the soil surface and weighed in the field.
Field weights were taken for each plot with an accuracy of 5 g. In addition a 25 fern subsample
from each plot was weighed fresh, dried, and weighed again to obtain an estimate of fern
moisture content at the main research greenhouse in Lansing, where it was air-dried for a week

1919

in open paper grocery bags. At this point fern samples were separated into stem and cladophyll
proportions with hand shaking in paper bags and with manual separation. Dried stems and
branches were weighed as were the dried cladophylls taken from them. By compiling this data
differences in the amount of leaf surface potential between treatments could be analyzed.

Pest evaluation
Generally pest pressure was monitored and controlled using IPM standards set by MSUE
and local vegetable cropping consultants to minimize interference with treatment effects. When
scouting resulted in abnormal trends with entomological or pathological pests, relevant
specialized MSU labs were consulted for appropriate reactions. Pest pressure and populations
were evaluated on either per plant or per m of row basis from data rows depending on the
situation.

Statistical analysis
The fixed effects of irrigation (none, overhead or drip) and variety (Millennium or Jersey
Supreme) on asparagus stem number, stem dry weight, yield, root soluble carbohydrate content
(Brix), weed and insect density, disease ratings and volumetric water content were analyzed
using PROC MIXED procedures in SAS (SAS Institute, 2009) with replicate (block) treated as a
random effect. To improve assumptions of normality and homogeneity, stem number data was
either log- or square root-transformed; cladophyll data was log transformed; and weed and insect
density data square root transformed. All other responses did not require transformation. P-value
0.10 was used as the statistical benchmark of significant differences in these trials due to the
variability inherent with asparagus field data; P-values below 0.10 are included in all tables to

2020

allow the reader to see when greater confidence levels were observed in trends. Volumetric water
content (VWC) data was aggregated by depth to analyze season long changes in water
availability. To evaluate differences in water use by depth (10 cm increments to 1 m in depth)
between varieties, the change in VWC during extended dry periods (7 July – 24 July, 2011 and
17 June – 14 July, 2012) was evaluated in un-irrigated treatments. For each dry period, daily
water loss from each soil depth was calculated by dividing total water loss from that depth by the
number of days. For this analysis, only the fixed effect of variety was evaluated, with replicate
(block) treated as a random effect. Differences between treatments were evaluated using the
pdiff LSMEANS option in the PROC MIXED procedure.

Results
Rainfall, irrigation and soil moisture
Rainfall was below long-term averages in both 2011 and 2012, with significant periods of
drought occurring in both years (Figures 2-2 and 2-3). At our experimental site, dry periods
occurred mostly during July and August in 2011 (Figure 2-2), and mostly in June during 2012
(Figure 2-3). Approximately 15 cm of overhead irrigation was applied in six (2011) and seven
(2012) separate application events each year.
Volumetric water content (VWC) to 1 m depth revealed that in 2011, irrigated treatments
had significantly greater VWC for an extended period ranging from late July until late August
(Figure 2.4). In contrast, differences in VWC in 2012 were limited to the first two weeks in July,
and the last week in August (Figure 2-5). These patterns were as expected given the timing of
rainfall and irrigation in each year (Figures 2-2 and 2-3).

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Not surprisingly, overhead irrigation resulted in higher soil moisture at the soil surface
during dry periods in both years. For example, on 28 August 2011, VWC in the top 20 cm of
soil was approximately twice as high in overhead compared to sub-surface drip treatments
(Figure 2-6). In contrast, VWC between 20 and 60 cm was higher in sub-surface drip treatments.
Although effects of variety on soil VWC on specific dates were generally not detected
(data not shown), changes in VWC during dry periods following rainfall events revealed
significant differences in both total moisture removal by variety, as well as the depth of water
removal (Figures 2-7 and 2-8). In both years, Jersey Supreme removed more total water during
dry periods than Guelph Millennium, and also removed water from deeper in the soil profile. In
2011, total water loss from the top 1 m of soil between 11 July and 24 July was 1.46 mm/day in
Jersey Supreme treatments, compared to 1.03 mm/day for Millennium (Figure 2-7). Similarly, in
2012, total water loss was 2.46 mm/day for Jersey Supreme compared to 1.83 mm/day for
Millennium (Figure 2-8). In Millennium treatments, approximately 58% of this water loss came
from the top 30 cm of soil, whereas for Jersey Supreme, only 45% came from the top 30 cm of
soil.

Fern development
In both 2011 and 2012, fern stem number in August was affected by variety, with Jersey
Supreme having more total stems, and more mature stems in both years (Table 2-2). Irrigation
had little effect on stem number in either year. In 2012, more new stems were detected in unirrigated treatments, presumably due to delayed fern development prior to August counts.
Light interception from developing fern—an indirect measurement of fern leaf area—was
significantly higher in both irrigated treatments compared to the un-irrigated control 2011, and

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significantly higher for Jersey Supreme compared to Millennium in both years (Table 2-3).
During the 2012 season a significant impact of irrigation on light interception existed with the
New Jersey variety at the 6 August sampling date, but with Millennium asparagus differences
declined below significant levels.
During the 2011 season Jersey Supreme fern dry weight was more than 25% greater than
that of Guelph Millennium at both early and late fall sampling dates (Table 2-4). Sub-surface
drip irrigation increased cladophyll dry weight by 55% in 2011 over control plots.

Root brix
Analysis of root carbohydrates followed the seasonal trends we generally expected as laid
out in the Aspire carbohydrate system (Wilson et al. 2002). Root brix levels decreased slightly
with harvest, troughed as the fern canopy developed, and moved up to peak levels by the time of
fall senescence as carbohydrates were added due to fern photosynthesis (Table 2-5). The effects
of irrigation on brix levels were not significant at most sampling dates. However, in fall 2011,
drip irrigation increased brix levels in New Jersey Supreme but reduced brix levels in
Millennium.

Pest evaluation
Contrary to expectations, irrigation treatments had no detectable effect on purple spot
severity or on Marestail density in 2011 (Table 2-6). We had anticipated that overhead irrigation
might increase purple spot severity by increasing leaf wetness relative to sub-surface drip and
non-irrigated treatments. Rust and purple spot fungi were present in trials but remained under
threshold level of control with foliar sprays as dictated by the IPM program. Rust levels

2323

remained under control both seasons through preventative applications of fungicide and also
were not particularly severe in nearby production areas.
Herbicide applications in the 2011 season were limited due to asparagus’s susceptibility
to chemical damage before it becomes fully established. Summer annuals were not a major
problem in this trial due to good chemical control after 2010; hand weeding and selective
herbicides in the initial season maintained field cleanliness. However, numerous Marestail
seedlings were evident in fall 2010, and large Marestail weeds were present in all plots by the
end of 2011. We had hypothesized that overhead irrigation would promote weed growth by
increasing moisture availability at the soil surface. However, no irrigation effect on weeds was
detected (Table 2-6) in this trial. Marestail density was higher in Guelph Millennium treatments
relative to Jersey Supreme treatments, presumably due to greater light penetration under the
smaller Millennium fern during the fall of 2010. In 2012, greater residual herbicide use was
possible because of increased tolerance with older asparagus so weed populations were
diminished. In addition, canopy closure for both varieties was greater in 2012, leading to very
few weeds.
Certain insect populations in the field were at times allowed to rise above IPM threshold
levels due to close proximity with ongoing entomological trials. Invasive Japanese beetles
populations in the region were unusually high in 2011 and 2012 and caused some topical foliar
damage in this trial. Insect monitoring conducted in early July 2012 revealed that asparagus
beetle densities were much greater in overhead irrigation plots (Table 2-7). This effect was
particularly noticeable in New Jersey Supreme where beetle densities in overhead-irrigated plots
were nearly thirty times more prevalent than in un-irrigated controls. By August of both years
defoliation of ferns was noticeable in some plots as a result of beetle feeding. In contrast, no

2424

effects of irrigation on either Japanese beetles, or asparagus miner damage were detected (Table
2-7). Common asparagus beetle and asparagus miner populations were in the normal population
range during the years of the trial. Asparagus aphid, cutworm, yellow striped armyworm were
problematic in nearby production fields during this trial but were not noticeable in our plots.

Yield and quality
Asparagus yield in 2012 was influenced both by variety and by irrigation (Table 2-8).
The average yield difference with both varieties together in the abbreviated harvest season was
an additional 11 to 12%. Precipitation and degree-day accumulation in 2010 were not extreme,
allowing for good initial plant set for all plots. But due to high temperatures and spotty
precipitation in the 2011 and 2012 there were numerous stressful drought periods in Western
Michigan. Yield increases in weight of marketable asparagus ranged from 6-9% with the New
Jersey Supreme to 13-21% with Millennium. Local evaluation trials have established that
baseline un-irrigated yields with both these cultivars exceed the existing local production
average, so these production gains are substantial in real terms. Millennium spear weight
increased 7-12% for irrigated plots; a production shift which lowers per unit harvest cost.
Overhead irrigation also increased Millennium spear number in 2012 but did not increase spear
number in drip plots. New Jersey yields showed an alternate trend with a 15% increase in spear
number for drip irrigation but no significant increase in numbers with overhead irrigation
treatment.

2525

Soil chemical properties
At this stage of the trial major changes in soil conditions between treatments are not yet
evident. Although annual soil tests have indicated several gradual changes are occurring: In
overhead irrigation plots both the pH and Mg levels increased, possibly as a result of deposition
of minerals from hard-water from the well. Potassium levels appear to be low in soils of the
New Jersey overhead irrigation plots so further available nutrient testing at crown level in future
seasons may be justified. Inclusion of cover crops into the system would play a positive role in
increasing soil organic matter and buffering nutrient levels but thus far the practice has been
avoided to avoid confounding data from ongoing experimental treatments.

Summary and Discussion
Overall, short-term results from this study confirmed most of our original hypotheses.
Most notably, irrigation during in the 2011 growing season resulted in 2012 crop yield increases
of 6 to 21% depending on variety and delivery system (Table 2-8). Given prices received by
growers in 2012, these yield increases corresponded to an increase in gross revenue of
approximately $250 to $1500 per hectare. These yield improvements are particularly impressive
given that yields in 2012 were reduced substantially by a late frost. Given estimated cost of
approximately $2,500 per hectare to install irrigation, yield improvements similar to what we
observed in 2011 need to occur 2 to 10 times during the anticipated 15-year life of the asparagus
planting to justify installation of irrigation.
Although there was no statistical difference between yield responses of Jersey Supreme
and Millennium to irrigation in 2012, our study revealed some interesting differences in varietal
response. First, fern growth of Jersey Supreme was consistently more responsive to irrigation

2626

than that of Millennium. This was reflected both in fern dry weight at the end of the season
(Table 2-4), and in light interception rates during fern growth (Table 2-3). Second, Millennium
yield improvements were equivalent or greater than those of Jersey; these results suggest that
fern growth is a poor indicator of irrigation response in Millennium. Trends in both fern dry
weight and light penetration suggest that Jersey Supreme fern growth may be more responsive to
sub-surface drip irrigation compared to overhead irrigation, and vice-versa for Guelph
Millennium. Third, in 2012 the effect of irrigation on cladophyll dry weight differed
significantly by variety; overhead irrigation significantly increased cladophyll dry weight for
Jersey Supreme, but had not detectable effect on cladophylls of Millennium. Fourth, the root
carbohydrate concentrations of Jersey and Millennium varieties also responded very differently
to irrigation (Table 2-5). As expected, Jersey Supreme root soluble solids concentration (brix)
increased with irrigation. In contrast, soluble solids in Millennium were either unaffected or
declined with irrigation. These results suggest that previous attempts among researchers in New
Zealand (Wilson et al. 2002) to monitor the status of total root carbohydrate storage by
examining brix from root samples are likely highly misleading and counterproductive for making
management decisions for the variety Millennium. As noted by the original proponents of this
system, brix sampling must be accompanied by sampling of total storage root biomass to get a
more accurate measurement of total carbohydrate storage (Drost and Wilson 2002).
Unfortunately, sampling of total root biomass is a very labor-intensive process given storage
roots heterogeneous distribution in the soil. Potential inaccuracies due to the inclusion of
inactive storage roots in older plants could further complicate carbohydrate testing’s validity
(Read 2009).

2727

Our results suggest that had intensive root sampling been conducted in this trial,
Millennium may have been shown to respond to irrigation by increasing total storage root
biomass rather than simply increasing carbohydrate concentration. Future research elucidating
the irrigation response mechanisms which increased Millennium yields would be helpful for
developing and adapting applied analysis methods, which in turn would lead to improved field
management tools. The greenhouse trial described in Chapter 3 is a first step towards achieving
that goal.

2828

CHAPTER 3
Effects of long- and short-duration drought stress on asparagus
Introduction
There has been little grower interest in irrigating asparagus in Michigan until recently
since it was relatively drought tolerant and lacked high profit margins during the past several
decades. In recent years the cost of producing asparagus has risen sharply, along with the farm
gate price, so risk is more intrinsic in production of this uninsurable crop. Irrigation is
increasingly seen as an insurance policy to protect grower’s substantial investments from direct
drought loss as well as a means to maximize yield potential. There were several years in the
2000’s when Western Michigan experienced notable drought conditions. Yields and asparagus
stand health were noted to be generally lower in the years following stressful drought conditions.
It was uncertain what percentage of the yield decrease could be attributed to drought induced
stunting and how much was the result of other related factors, including increased incidence of
root disease from abiotic stress. Local growers became more interested in getting information
about the drought tolerance of their asparagus and potential plant responses to abiotic stress so
they could make better production decisions.
Although field irrigation experiments provide the most relevant information to growers,
greenhouse experiments have the distinct advantage of providing controlled conditions to better
understand the response of asparagus to specific drought conditions. Greenhouse experiments in
confined containers also allow more thorough sampling of root tissue than is feasible under field
conditions.
Numerous previous greenhouse studies of asparagus drought have been conducted since
the 1990’s (Schaller and Paschold 2009; Liddycoat and Wolyn 2009; Hebner et al. 2006;

2929

Bhowmik et al. 2001; Drost and Wilcox-Lee 1989; Drost and Wilcox-Lee 1997; Nicola and
Basoccu 2000; Elmer 1995), but few have direct relevance to Michigan soils, varieties and
current growing practices. Most greenhouse irrigation research has been too short in duration to
gain perspective on the cumulative impact of drought stress on asparagus development (Drost
and Wilcox-Lee 1989; Elmer 1995). Likewise, most greenhouse experiments have been too
limited in physical scale—using small greenhouse pots—to adequately assess impacts of drought
stress without root restriction. In some cases, potting soil was used instead of the droughty sands
found in most asparagus production areas. Additionally, in many trials asparagus was started
from seedlings whose drought tolerance differs dramatically from the crowns typically utilized in
Michigan. Finally, in many studies, archaic non-hybrids or foreign white spear cultivars were
used that don’t share enough traits with the varieties currently being planted in Michigan for
meaningful extrapolation of results.
Nonetheless, several greenhouse trials provide insight into asparagus response to drought
stress that may be relevant for Michigan. Drost and Wilcox-Lee (1989, 1997), using relatively
small pots and high organic potting soil demonstrated that yield the following season was
impacted by drought pressure at fern stage; growth of rhizome buds was negatively impacted by
limited water supply and asparagus showed a linear decrease in crown mass accumulation when
subjected to high water deficits. Paschold and Schaller (2003, 2009) have done extensive work
evaluating effects of drought stress on asparagus using sandy soils in large-volume containers
over several seasons. Although this work focused primarily on varieties relevant in Europe, it
demonstrated the value of large container evaluations, and that stomatal activity and hydraulic
xylem activity can already be inhibited before asparagus is visibly drought stressed.

3030

For Michigan asparagus growers, several important information gaps remain. Historic
climate data from Michigan’s primary asparagus growing region (Hart, MI), demonstrate that
there are often extensive periods during asparagus fern growth in which evapotranspiration
exceeds rainfall (Figure 2-1 and data not shown). For example, during the years 2005-7 three to
four weeks of drought stress occurred. Over the past 15 years, drought periods of 3-4 weeks in
July and August occurred 7 times. The extent to which commercially grown asparagus varieties
in Michigan (e.g. Guelph Millennium) may be able to tolerate such acute drought conditions is
unknown. Nor is it well understood how longer-duration, lower level drought stress may
influence asparagus growth. Better understanding of the conditions contributing to yield loss
from drought stress should help growers make more informed decisions about whether to invest
in irrigation, and how to best to utilize irrigation systems already in place.
The project goal was to evaluate the effects of both long- and short-duration drought
stress under controlled conditions on: asparagus fern establishment, yield, as well as root
biomass and carbohydrate accumulation. This trial’s relevance to local production was ensured
by using sandy soils in a large soil volume container with a popular newer variety: Guelph
Millennium. Millennium was selected because of its widespread commercial availability, and its
noted absence in many previous irrigation trials. Additionally, surprisingly high yield gains with
many European varieties that share Millennium parentage (Panka and Rolbiecki 2009; Rolbiecki
and Rolbiecki 2008; Lamparski and Rolbiecki 2006) indicate that Millennium might be more
sensitive to drought than New Jersey Supreme, which shares lineage with the relatively drought
tolerant New Jersey Giant cultivar. Our hypothesis was that drought stress would lead to reduced
fern and root growth rates and reduced spear yields. We further anticipated that the nature of

3131

drought stress (prolonged low level stress vs. short-duration acute stress) would differentially
impact fern growth, root growth, carbohydrate concentration and yields.

Materials and Methods
Site characteristics and experimental design
This trial was established in the spring of 2011 in the Michigan State University
Horticulture Farm head house. This is a dual layer plastic, year-round structure that is six meters
tall with thermostatically controlled forced air heaters and limited cooling capability from a
venting roof. With greenhouse trials water inputs could be finely controlled, allowing for testing
the cumulative impact over several seasons of drought conditions typical of Oceana County.
Four treatments were examined in a complete factorial design with four replications. Factors
included baseline water level (high or low) and drought stress (none or 2-3 weeks of water
withholding) imposed over two growing seasons, with an imposed dormancy period separating
the two seasons. The entire experiment was repeated in time, with the first cycle occurring over
two asparagus seasons during 2011-12, and the second cycle occurring over two asparagus
seasons during 2012-13. The two cycles were conducted in MACX (Decade Products, Grand
Rapids, MI). The same soils were used during both cycles since there was no evidence of soil
pathogens and since root tissue had been separated so it would not have deleterious effect on new
crowns. The timing of key activities for this trial is summarized in Table 3-1.

Trial establishment
Eight MACX produce bins were divided in half with impermeable 3 cm thick foam board
partitions to create sixteen equal 115 x 50 x 78 cm plots. Plot soil volume was 391 L with a 10

3232

cm lip above soil level to ensure plot separation. This substantially larger soil volume compared
with most previous trials allowed for reduced edge effects in general, interplant root interaction,
and longer periods of growth before root restriction became severe. MACX boxes are also
useful, because they are designed for easy lifting and transport using a forklift and since transport
to cold environments was necessary to induce dormancy. Boxes were placed with approximately
20 cm separating each box to facilitate airflow and access for data collection.
Glacial sand of similar texture to West Michigan’s was collected near an existing
asparagus field trial at the “Sand Hill” farm in East Lansing. Soil was collected from the A, B,
O, and C profiles with a backhoe from two nearby excavation holes at the same site; soil
composition was 83% sand, 12% silt, 5% clay. Half of the A-B soil was sterilized with steam for
12 hours to kill weed seeds and put aside to be used as a topsoil layer over the asparagus crowns.
The remaining soil was hand shoveled into 10 cm soil layers from two trailers to simulate real
soil profiles and to equalize pH imbalances between the soils from the two holes, which ranged
from moderately acidic to alkaline. Between each profile a mixture zone was worked up with a
spade to avoid potential root limiting layers.
Year old Millennium asparagus crowns were donated by Michigan commercial growers
(Ron Richter and Oomen Farms). Disease-free crowns with several viable growth buds and
storage roots were weighed and selected for uniformity; 16 sets of five crowns were selected
with combined weights for each plot within 5 g of each other.
Fertilizer was added based on soil tests and MSU recommendations for asparagus.
Fertilizers were applied following typical grower practice, with an initial application of monoammonium phosphate fertilizer (12-52-0) applied below the crowns at planting. Additional urea

3333

and 17% N-P-K fertilizer applications were applied to the soil surface and raked in after each
successive harvest period as dictated by soil tests.
Supplemental lighting was provided with timer-controlled 400W metal halide lights
suspended 3 m directly above each MACX box. Light levels were logged hourly with a Hobo U12 data logger (Onset, Bourne, MA), along with temperature and relative humidity to calculate
growing degree-day (GDD) accumulation.
Growing season length in the greenhouse was calibrated to roughly match the five-year
GDD base 40°F average of 2500 in Hart, MI in the center of the asparagus-producing region.
After achieving 2500 GDD each growing season, plants were removed via forklift from the
greenhouse and placed either outside if temperatures were near freezing point or in a walk-in
cooler kept dark at 3°C to simulate winter dormancy (Table 3-1).

Moisture monitoring and irrigation
Baseline irrigation consisted of either a “high” level of irrigation slightly in excess
evapotranspiration value of asparagus. “Low” irrigation treatments received one half the amount
of water as high treatments, and were established to assess the impact of a prolonged period of
continuous sub-optimal soil moisture levels. In acute drought stressed treatments irrigation was
withheld for 2-3 weeks at the end of the fern canopy development stage (Table 3-1).
Irrigation was achieved primarily through buried Netafim Uniram tape with pressure
compensating emitters on 30 cm spacing; water input was regulated within ten percent of 0.19 L
per minute. Each plot had five emitters; four were buried at crown depth and one was slightly
above soil level. During the initial watering of each growing season a watering wand was used
to bring soil in all plots to field capacity.

3434

Moisture was monitored with the same equipment as in the larger field trial described in
Chapter 2. A Sentek Diviner tube was cut to 75 cm and used to track moisture throughout the
soil at depths from 10-60 cm in each plot. The top 10 cm values were discarded because of lack
of good soil contact. Tubes were installed 20 cm from both the front and sidewalls to ensure
accurate readings without grossly infringing on the asparagus crown’s growth area. Soil
moisture levels were taken with the Diviner on at least a weekly basis throughout the growing
season with additional readings taken prior to or following irrigation events. One Decagon
moisture sensor (EM10) per plot was also used to data-log (EM5) soil moisture hourly at crown
level, and to serve as a backup data gathering system in the event that Diviner-2000
measurements failed. Due to the reliability of the Diviner probe Decagon sensor use was
discontinued after the first two growing seasons.

Root sampling
Root sampling protocols followed Drost (2002) and the Aspire soluble carbohydrate
monitoring system developed in New Zealand (Wilson et al. 2002). Fleshy root samples were
taken at time of planting from surplus crowns to establish a baseline soluble root carbohydrate
level. Additional fleshy root samples were taken at the end of the first growing season, postdrought the second season, and at the end of experimental cycle when the plants were exhumed
after two growing seasons. Several 5-10 centimeter fleshy roots were dug out from at least two
crowns per partition with a hand trowel after use of soil cores to obtain root samples was judged
to be overly destructive to the plants.
Root samples were collected, washed, and placed into the freezer overnight to separate
fructose from cell tissue (Wilson et al. 2002). Just prior to analysis, root samples were thawed,

3535

dab dried with paper, cut into 0.5 cm sections and at least 2g were crushed in a garlic press to
collect root sap. The refractive index (RI) of extracted sap was evaluated using an Atago PAL-1
digital handheld refractometer. This device reports RI readings in degrees Brix based on the
relationship between RI and the sucrose concentration (% w/w) of a pure sucrose solution at
20°C. Care was taken to avoid sample dilution from thawed surface moisture and each sample
reading was taken three times and averaged.

Fern evaluation
Fern evaluation occurred on a regular basis throughout the growing season at roughly
two-week intervals. Stems were counted in each plot and either all stems were measured for
height or a subsampling was recorded to document the development stage of the fern. At several
sampling dates, stem counts were conducted separately for stems that were visually assessed to
be fully mature (older tissue based on leaflet color), newly mature (fully expanded cladophylls
but light green tissue), immature (small light green shoots with minimal cladophyll
development), and dead (aborted by the plant).
Fern growth was also evaluated at several critical stages for new shoot growth, by
measuring the length of new shoot tissue at 5 locations on the developing fern. New shoot
growth appears as a distinctly lighter shade of green. Randomly selected branchlets were
evaluated for amount of fresh growth at two points on both the north and south side of the fern,
and one at the pinnacle. This method was used to determine fern growth without destructive
sampling or reliance on a light sensor.
After the asparagus had reached 2500 GDD, boxes were either brought outdoors for frost
exposure or placed in a walk-in cooler to induce dormancy. After fern senescence dead fern was

3636

collected by cutting stems at ground level with hand pruners. From each plot stems were
combined, dried and weighed after air-drying in the greenhouse. It should be noted that during
the first cycle, stem and cladophyll tissue was largely intact at the time of fern biomass sampling.
However, during the second cycle most cladophyll tissue and some stem tissue had senesced at
the time of sampling, resulting in underestimates of fern dry weight and possible sample bias.

Yield and quality assessment
Following the dormancy period, boxes were returned to the greenhouse and re-watered to
field capacity (to mimic typical spring soil moisture conditions in Michigan) to initiate the
second season of growth (see Table 3-1 for timings). Fresh asparagus spears were harvested to
evaluate the effects of first season irrigation and drought stress treatments on yield, and to mimic
harvest pressure typical of second year asparagus. Spears were snapped at 20 cm height directly
above the soil level in the manner asparagus harvest occurs in Michigan. Spear number per plot
was documented along with weight of harvested spears. Spears were initially categorized as
either: marketable, small-diameter, or unmarketable appearance. A subsample of spears were
weighed fresh, dried and weighed again to determine the fresh to dry weight ratio.

Second year fern growth and root sampling
Following harvest in the second year, fern was allowed to develop and irrigation and
drought stress treatments were re-initiated, with the same plants receiving the same treatments
for a second year (see Table 3-1 for timing). Evaluations of fern development were conducted
during the second season of both cycles using the same protocols described above for the first
season with one exception: stem height during the second growing season was taken for each

3737

individual stem in each box, and the sum of all stem heights per box was calculated. This total
stem height was considered a reasonable non-destructive estimate of total fern growth, although
clearly it is not a perfect estimate, since stem diameter, branching, and cladophyll development
are not captured by this method.
After completing a second fern growth season, MACX boxes were disconnected from the
irrigation supply lines and exposed to frost one final time to return soluble carbohydrates to the
crowns from the above ground biomass. Following senescence, dead fern was removed, dried
and weighed as described above for season one. Boxes were then placed on a large sheet of
diamond metal grate and were flipped over with a forklift to separate soil and fleshy root tissue.
Asparagus crowns and fleshy root tissue remained above the grating as the soil was sieved out,
while fine roots and soil passed through the metal grate. Care was taken to make certain that
root and crown tissue from the two partitions did not mix on top of an open bin.
As each of the five crowns from each partition was freed of the majority of the soil they
were cleaned by hand brushing to remove sand. This process was adequate in most cases to
remove nearly all the soil particles. If necessary roots were rinsed off with a hose and dabbed dry
with towels and air-dried for 30 minutes. Fresh fleshy-root and crown tissue was subsequently
weighed. Approximately 5g samples of fleshy root were taken from each crown to assess
carbohydrate levels following the protocol described above. The remaining crown and root
material was dried and weighed.

Statistical analysis
The fixed effects of baseline irrigation (high vs. low) and drought (none vs. 2-3 wks.) on
asparagus stem number, stem height, stem dry weight, yield, root weight, and root soluble

3838

carbohydrate content (Brix) were analyzed using PROC MIXED procedures in SAS (SAS
Institute, 2009) with replicate (block) treated as a random effect. To improve assumptions of
normality and homogeneity, spear number data was log-transformed. All other responses did not
require transformation. One outlier for asparagus yield and stem number in this trial was
detected and removed from the data-set prior to analysis; justification for removal was based
both on its deviation from the mean, and the fact that outlier data came from the MACX box
closest to a heat source in the greenhouse. This appeared to promote earlier and more vigorous
spear emergence (data not shown) and subsequent fern growth. In most cases, significant
interactions of fixed effects with year were detected, so analyses were conducted separately by
year. However, for responses for which no significant year interactions were detected—including
yield—combined data for both years are presented. Given the high degree of variability in many
of the responses, mean separation was conducted whenever main or interactive effects had pvalues of less than 0.10. These p-values are noted at the bottom of each table of results.
Otherwise, significance is reported as “NS”. Differences between treatments were evaluated
using the pdiff LSMEANS option in the PROC MIXED procedure.

Results
This greenhouse experiment had a number of interesting results that helped explain and
expound on the data taken in related asparagus field trials. Our key findings were based on stem
number and weight data, yield, and root data taken over the three and a half growing seasons
completed so far in this trial.

3939

Stem number during first growing season
During the first growing season, neither baseline irrigation nor drought had any
detectable effect on stem number (Table 3-2). In the second repetition of the trial 79 days after
planting (DAP) crowns we had significantly higher stem numbers in the high water supply plots
over the low water supply. High irrigation increased stem number by over 20% in this second
trial repetition. This significant trend was still evident a month later at 111 DAP. By that point
in the growing season drought treatment had also significantly negatively impacted development
of asparagus fern. Drought stress reduced the total stem number by 14% by August 20, 2012. In
the two drought treatments there was a reduction of old (mature) stem numbers by 23% in the
drought treatments compared to the two non-drought stressed plots (Table 3-2). This implies a
photosynthetic surface disadvantage for the drought treatments during asparagus’s prime
photosynthetic period to recharge root storage carbohydrates. In a field situation, inability to
develop a strong fern canopy shortly after harvest can lead to detrimental late season growth as
the plants struggle to make up for a carbohydrate deficit before the winter season.

Fern dry weight
In both repetitions of the experiment, fern dry weight at the end of the first growing
season was not affected by baseline irrigation, drought stress, or their interaction (Table 3-3).
Similar to the stem number results in the fall of 2011 there was no significant difference between
treatments, although data indicated a trend likely was emerging between the drought treatments
with greater weight fern in no-drought plots. By April 2012 after high/low water treatments had
been imposed twice, there were significantly greater fern weights in the high water plots, 423g
per plot versus 285g. When compiled between both growing cycles this represented a 22%

4040

difference in cumulative fern weight between the high and low water treatments. In 2012 during
the second run of the trial fern weight was highly variable after the first season so no significant
differences were detected. This variability was probably related to how late in the season ferns
were collected: many cladophylls had already fallen off because of exposure to frost and wind
and all soluble carbohydrates had already been trans-located back to the crowns. This late
collection date was purposeful since we did not want to extract usable carbohydrates from the
more stressed treatments that had entered senescence more slowly than the high water nodrought plots.

Yield, spear number and spear size after one season
Asparagus yield following the first season was variable, resulting in low power to detect
significant differences. Nonetheless, when data was combined across both years, a 15%
reduction in yield was detected due to the short-duration drought stress imposed in the first
season (Table 3-4; P-value = 0.074). This yield reduction in acute drought stress treatments was
attributable largely to a decreased number of harvested spears rather than a reduction in spear
size; spear number was 14% lower in short-duration drought stress treatments (P-value = 0.064).
In contrast, neither spear number, spear size, nor yield were affected by chronic low water stress
treatments (Table 3-4).

Stem number and height in second growing season
Following the first dormant and harvest periods multiple counts of stems occurred to
document the development of the fern canopy. These plants were larger than they had been the
previous growing season and their growth habits altered somewhat. In the first repetition of the

4141

trial (2011-12), stem number at both 30 and 69 days after harvest (DAH) was significantly lower
in low irrigation compared to high irrigation treatments (Table 3-5). In the second run of this
greenhouse trial this effect was not detected, with stem number at 7 and 28 DAH unaffected by
either baseline irrigation or drought stress (Table 3-5). Stem number information later in the
season is not yet available.
Total stem height (sum of the height of all individual stems—see methods) during the
second growing season was unaffected by treatment in both years at the first two sampling dates
from 7 to 25 DAH (Table 3-6). In the first experimental repetition, total stem height at 35 DAH
was 22% lower in low irrigation compared to high irrigation treatments, but unaffected by
drought stress treatments. In contrast, total stem height at 36 DAH in the second experimental
repetition was unaffected by baseline irrigation level, but reduced 13% by short-duration drought
stress. Total stem height at 35-36 DAH in the two experimental repetitions was remarkably
similar, ranging from 2 to 2.8 m per box in both years (Table 3-6).

Root weight and soluble solids
At the time of destructive harvest of the plots following two growing seasons, low
irrigation baseline treatments resulted in a 25.5% decrease in total storage root biomass (P-value
= 0.034), while having no effect on the concentration of soluble sugars (brix) in root tissue
(Table 3-7). In contrast, short-duration drought stress had no effect on root biomass, but resulted
in a 11.9% reduction in brix levels (P-value = 0.058).

4242

Summary and Discussion
Overall, this trial demonstrated several interesting differences between drought stress
treatments of different durations on the commonly grown variety, Guelph Millennium. In
particular, our results demonstrated that prolonged low-level drought stress reduced fern growth
(Table 3-3) and root weight (Table 3-7), while short-duration intense drought stress, typical in
Michigan during July and August (Figure 3-1), had greater impact on root carbohydrate
concentration (Table 3-7) and short-term yield (Table 3-4). The trial confirmed our suspicion
that monitoring root carbohydrate levels is an inadequate method for guiding management
decisions in the absence of more intensive sampling to evaluate changes in storage root biomass.
The study also reinforced the conclusion from our field study, that targeted irrigation to avoid
short-duration drought stress can improve asparagus yields. In particular, we observed a 15%
improvement in yields one year after establishment when drought stress was avoided through
irrigation. Moreover, although long-term yield impacts could not be evaluated, increases in root
biomass and carbohydrate content due to irrigation, suggest that yield boosts in subsequent years
would also be likely as a result of irrigation.
Although these results are interesting and shed light on the effects of different types of
drought stress, they should be interpreted cautiously due to several problems encountered along
the way. The basis of our greenhouse production methodology was from field practices used on
my farm; professional greenhouse growers may have made different choices more adapted to
indoor production. Among the major challenges encountered in this trial were: adapting field
data collection methods and devising establishment and production protocols for a crop that’s
rarely produced indoors. The most complex issues were uncertainty about how to: initiate

4343

senescence in the asparagus plant, adapt to atypical fern growth habits, and find effective nondestructive evaluation tools for determining net plant productivity.

Dormancy
Typically asparagus fern in the field browns off in the middle of autumn. Krug (1999)
reported that either freezing temperatures or severe drought stress could initiate “ripening” in the
asparagus plant. Drought is the primary method of initiating senescence in desert climates in
South America and California’s Imperial Valley. Due to the large cumulative water holding
capacity of the soil within the MAXC boxes involved in this trial completely depleting available
moisture would have altered the seasonal growth pattern too far beyond what could reasonably
occur in Michigan’s climate. Drought conditions that severe could easily alter the crown
development to an extent that would have rendered trial data un-transferable into local field
conditions.
Walk in coolers were available for use but larger freezers could not be located on campus
to freeze plastic containers. It was assumed that removal from daylight and acclimatization to
near freezing temperatures would bring about dormancy. Ku and Woolley had found that
asparagus had a chilling requirement of three weeks at 5°C to break dormancy (2008). Once the
initial 2500 GDD had been achieved in the greenhouse plants were brought to the coolers and
left in unlit 3°C coolers for over a month with no strong sign of senescence beginning. Addition
of fifteen liters of ice to the soil surface per plot did not activate dormancy cycle by bringing
temperatures down to freezing.
After the initiation of this greenhouse trial Landry and Wolyn (2011) published a report
breaking down the senescence pattern of Millennium asparagus into three stages. Cues for each

4444

stage were broken down into a mixture of: amount of crown nitrogen, moisture percentage in
fleshy root, short day length signaling, and level of various proteins and carbohydrates in the
crown. We concluded the simplest way to satisfy enough of these factors to induce dormancy
was to bring the plants into contact with natural frosts after a full growing season as occurs in the
field. After waiting another month in the unlit cooler ambient outdoor temperatures dropped and
all boxes were placed outdoors and following one hard overnight frost event fern dormancy
quickly occurred.
This natural frost method was not always possible, since the trial’s time frames of
dormancy induction did not always correspond to periods of cold outdoor temperatures (Table 31). If the growing season ended between October and April this was not a problem in Michigan
but during the summer in the greenhouse 2500 GDD accumulated very quickly resulting in at
least one problematic dormancy shift per calendar year. Potentially lower cumulative light
absorption occurred in these growing seasons due to abbreviated growth time frames and
asparagus’s maximum photosynthetic capacity of around 1000 u mole (Faville et al.1999).

Temperature regulation
The headhouse was selected to accommodate the very large containers used in the
experiment and forklift, but ventilation systems and temperature control were suboptimal,
resulting in variable temperatures ranging from 10°C to 37°C between the seasons. Greenhouse
temperatures were documented at levels well above plant photosynthesis shutdown threshold
during the day in the summer because side venting or air-conditioning of the building was not
possible.

4545

Growth time had to be supplemented to some growing seasons to compensate for the
hours over 35°C that rarely occur in the 2500 GDD Michigan growing season. In the summer of
2012 an additional 500 GDD was allowed to compensate for this shutdown time. Also Takatori
(1971) noted finding that root growth dramatically slows when temperature exceed 30°C so the
heat levels may have confounded differences in root development between the various treatments
because humidity levels could have altered the soil temperature at root level. In a Michigan field
situation this never would have occurred because of the consistently cool soils.
Another challenge conducting this trial was related to uneven heat dispersion due to
placement of heaters at the ends of building and the lack of cooling capability in the greenhouse.
This temperature induced growth disparity was particularly evident when outdoor temperatures
were below freezing causing a greater temperature gradient in different areas of the building.
Asparagus spear elongation rates are directly correlated with temperature (Ku 2008). Asparagus
in plots in the first repetition (particularly the south-most box) often left dormancy more quickly
than the rest of the trial because of heating disparities resulting from the forced air system’s
location. If this trial was repeated, the inclusion of a guard row on the first plot would be
warranted, as would re-randomization of the plots before every growing season to improve data
quality.
Radiative heat loss through the plastic layer was theorized to be the reason for frost
damage shortly after harvest periods during the winter as quickly growing stems are very frost
susceptible once no longer protected by proximity to the ground. Once fern was formed fully and
hardened off it was resilient to minor radiative frost damage in the greenhouse.
Similar cosmetic frost damage to winter harvested spears led to the consolidation of the
marketable and unmarketable spear categories. Combining all larger sized spears into one

4646

production amount was more reflective of actual plant yield potential. The small diameter spear
category was retained to represent similar small spears that emerge from minor growth buds in
the field. Likewise we did not engage in trimming of over length spears and discount that weight
as other trials have, since in a production field that mass could have been harvested between 20
and 30 cm with a daily harvest crew.

Unusually tall ferns in second season
By the second season in the greenhouse trials fern heights were significantly taller than
field-grown asparagus of similar age. The height of some individual ferns measured well over
three meters while most field fern heights are closer to two meters. Ferns were very leggy, with
weak stems resulting in pronounced canopy drooping by the end of the growing season. This
may have been the result of either insufficient total light levels or altered light-spectrum
composition from the supplemental metal halide lighting with natural light filtered through
plastic. The lack of wind, which is known to discourage vertical growth, may have also played a
role, as could have soil pH.

Future work
Asparagus is a difficult crop to trial scientifically under unpredictable field conditions, so
greenhouse work has value to complement field data. Further repetitions of similar large-scale
greenhouse studies, where all tissue root can be captured could be used to better understand how
asparagus responds to environmental pressures. Additional information on the response of
soluble solid concentration and root growth in combination with measurements of stomatal
conductance would be a logical extension of this work, and would help inform asparagus
decision-tools like Aspire.

4747

Rhizocam tubes had been installed in this trial at its inception to monitor feeder root
development but the process proved cumbersome and beyond the resources of this trial. Use and
analysis of digital rhizocams with root tracking software during both drought events and
dormancy periods would be very useful to the body of knowledge. In addition, inclusion of a root
pathogen factor, like Fusarium or Phytophthora could help explain the commonly referenced but
little researched relationship between drought stress and asparagus soil pathogens.

4848

CHAPTER 4
Discussion

Field and greenhouse trial
Differences in fern number, cladophyll weight, and total dry fern weight were apparent
during both field seasons from irrigation treatments, although the drought conditions in Oceana
County weren’t as severe as they were in much of the nation. The yield increases of 6 to 21%
with irrigation treatments during the 2012 field season were an indication to me that there are
effects in asparagus that cascade from one season to the next. I’d hypothesize that long-term this
positive trend, resulting from irrigation, will continue benefitting both plant survival rates and
yields; making irrigated plantings more lucrative enough to justify additional expenses. Cultivar
reactions to environmental stress and irrigation treatments showed this may not be a “one size
fits all” situation. Our Diviner 2000 data illustrated different water requirements from various
soil depths for the two cultivars: UG Millennium was more responsive to surface watering,
making it a good match with fields with center pivots installed. Planting Millennium seedlings or
small crowns in sand under a center pivot would likely be more beneficial than planting large
Millennium crowns in heavier soil (Sinton and Wilson 2008).
Our results suggest that some alterations in spray program may be warranted in irrigated
fields, since irrigation resulted in large amounts of fresh fern growth that beetles appeared to
prefer for shelter and feeding (Lamparski et al. 2010). The same holds true with purple spot and
other foliar diseases related to leaf wetness hours, although we saw less of this than we expected
in our trial. Inclusion of chemical stickers with fungicide sprays may also be warranted under
center pivot irrigation.

4949

Future work
Typically trials of this variety end at the point when production is really coming into
maturity, when growers sunk costs are covered and profits are made. Continuation of this field
trial for several more years will yield a great deal of usable information; even if data collection
rates are downsized the long-term trends observed in those plots will be much more applicable to
the industry.
Both increased usage of cover crops and altered herbicide programs are options with
overhead irrigation that would be logical extensions of future research. Soil health in our
existing production system would probably benefit from including a cover crop besides the
winter rye commonly employed, on alternating years with normal residual herbicide use to keep
down perennial weed populations. Identifying systemic insecticides, fungicides, and nutrient
packages that could be chemigated into trickle irrigated asparagus is another practical topic for
future research.
Attempting higher density plantings with several modern varieties in irrigated Michigan
fields would also be interesting; other irrigated production areas have already moved in this
direction. Although Ku (2006) and Schaller and Paschold (2009) did some recent work on
asparagus’s photosynthetic rate, further investigation of modern North American cultivars in
field conditions would be helpful to optimizing water supply scheduling.
It appears that the Aspire soluble carbohydrate model won’t be commonly utilized by
Michigan growers. One alternative approach, which may have practical application and warrant
future research would be development of a temperature and accumulated light growth model to

5050

better predict carbohydrate accumulation. Already growers depend on weather stations to
monitor fungal development and hopefully will have insect pest modeling available soon.
Use of high tunnels both for drought research and altered harvest schedule production is
another area for prospective future work. Given that asparagus does not readily exhibit drought
stress symptoms, another topic that might benefit growers would be identification of easily
visibly wilting “indicator-plants” that could be planted into the edges of irrigated asparagus
fields in the place of electronic sensors for irrigation scheduling.

Industry implications
The USDA NASS reported in 2010 that 1843 of Michigan’s 4310 asparagus producing
hectares were planted prior to the year 2000; most of these were replant situations with the New
Jersey Giant cultivar not known for its longevity in those situations. From 2005 to 2009 only
1112 hectares were planted in Michigan because of the depressed markets (USDA NASS 2010).
The combination of stand age (+12 years), water stress during the 2011 and 2012 growing
seasons, increased harvest pressure due to high prices, and continued attrition from soil diseases
will quickly remove many of these older fields from production. Additionally, growers intended
to plant only 1355 hectares from 2010 to 2015, and since 2009 increased seed costs and limited
availability have further restricted new plantings (Bakker 2010-2012). A new challenge has
developed for Michigan growers: supply of both spears and young plants during the past several
years have been tight and there is no indication that this trend will reverse soon.
There is room for market growth as Michigan only supplies 2% of the nation’s asparagus;
providing the Great Lakes region with its May and June fresh asparagus supply is a feasible goal
(Martin 2010). However, maximizing production per unit of land is essential to make asparagus

5151

profitable with the hand harvest system. Michigan’s asparagus industry has to focus on
consistent high quality production and improving yield per acre to meet this existing demand or
other production areas will step in and fill the void. Midwestern asparagus production levels
have the potential to rebound in the next several decades but significant adaptation of production
methods may be required in order for a full resurgence to occur.
Michigan is not an arid environment, but even with a drought-hardy crop like asparagus,
supplemental irrigation can be of substantial value for ensuring consistent, optimum production
levels. Results from this study show that under weather conditions similar to the past several
years, yield increases from irrigation, in addition to increased overall plant health are likely to
justify the additional cost of irrigation for Michigan asparagus growers. Irrigation systems will
not necessarily provide substantial benefit every year in Michigan, but the increased yield in dry
years in conjunction with high plant survival rates over time due to greater plant health should
keep fields in higher production ranges longer.

5252

APPENDIX

5353

Table 2-1 Schedule of major events, 2010-2012

Dec.March
April
May

June

Hart, MI field trial

2010
NA

2011
Senescence

2012
Senescence

NA

Field mowed/ Spear emergence/
Fertilized/Harvest on 28th

Field mowed/ Spear
emergence/ Multiple frosts

Field
cultivated/Crowns
planted
Canopy Development/
Weed control

Harvests on 9&12th/ Layby
herbicides applied

Harvest period of several
weeks

Canopy development/ First
generation Miners/ Fungicides
applied/ Irrigation started
Full mature fern/ Drought period:
multiple irrigation events/ Root
samples
Full fern/Second generation
canopy ferns develop/Irrigation

Fern canopy development/
Multiple irrigation events

July

Full immature fern 1
Meter apx.

August

Full immature fern

September

Full immature fern

Final irrigation/Senescence
initiated in Millennium

October

Senescence initiated

Semi-dormant/First fern sampling

Senescence initiated/ First
Fern sampling

November

Fern dormant/ Root
Samples

Full senescence/ Second fern
sampling

Semi-dormant/Second fern
sampling

5454

Full mature fern/irrigation

Secondary canopy
development/ Drought:
irrigation events
Full fern/Miner
evaluation/Irrigation pulled

Table 2-2 Stem number by category, 8 August 2011 and 2 August 2012.
Hart, MI
2011
2012
Treatment
Total
New
Mature
Dead
Total
New
Mature
Dead
----------------------------------------------------#/m-row------------------------------------------------Irrigation Main Effect
None
28.6
1.6
25.4
1.6
28.3
2.4
6.3 b 21.0
Overhead
28.5
2.6
24.7
1.3
29.1
2.1
4.4 a 23.8
26.6
Drip
28.9
2.0
26.0
0.9
31.1
2.1
3.6 a
Variety Main Effect
2.5
1.7
4.8
2.5
Jersey
30.8 a
33.2 a
27.5 a
35.1 a
1.5
0.8
4.7
1.9
Millennium
19.9 b
25.8 b
20.1 b
22.3 b
Interactive Effects
Jersey
None
2.0
2.0
30.5
6.9
22.3
3.4
35.3 b
31.3 ab
Overhead
3.1
1.8
33.0
4.0
28.5
2.1
32.0 ab
27.0 b
Drip
2.5
1.3
36.1
3.3
31.6
2.0
38.0 a
34.3 a
Millennium
None
1.1
1.1
26.0
5.8
19.8
1.5
21.9 c
19.6 c
Overhead
2.0
0.6
25.3
4.8
19.1
2.1
25.0 c
22.4 c
1.5
0.6
Drip
25.9
3.9
21.5
2.1
17.8 c
19.9 c

---------------------------------------------P-value-------------------------------------------ANOVA
Variety
0.236
0.126
0.720
0.300
<0.0001
<0.0001
0.012
0.010
Irrigation
0.974
0.561
0.723
0.862
0.705
0.201
0.964
0.044
Variety x Irrigation
0.887
0.931
0.671
0.540
0.549
0.208
0.066
0.008
Different letters within a column and effect category indicate significant differences according to LSMeans (α=0.10).

5555

Table 2-3. Light interception by asparagus fern, 2011 and 2012.
Hart, MI
2011
2012
Treatment
19-Aug
6-Sep
4-Nov
9-Jul
6-Aug
14-Sep
-------------------------------------------% light intercepted---------------------------------------Irrigation Main Effect
None
68.3
65.3
83.3
77.2 b
74.7 b
69.7 b
Overhead
66.8
71.7
81.1
82.1 ab
85.0 a
78.7 ab
68.3
Drip
72.6
84.2
84.1 a
80.1 a
82.7 a
Variety Main Effect
Jersey
86.1 a
85.4 a
79.5 a
71.9 a
75.3 a
87.3 a
Millennium
75.2 b
77.1 b
72.8 b
63.7 b
64.4 b
78.4 b
Interactive Effects
Jersey
None
82.0
78.0
72.2
74.1
88.8
65.5 b
Overhead
86.1
89.0
80.9
68.9
87.9
78.2 a
Drip
90.2
89.2
84.9
72.5
85.2
82.1 a
Millennium
77.8
None
72.4
71.4
66.6
62.5
65.0 b
Overhead
78.2
81.1
76.4
64.7
80.6
65.2 b
Drip
75.2
78.9
75.2
64.0
76.9
63.1 b

---------------------------------------------P-value-----------------------------------------ANOVA
Variety
<0.0001
0.001
0.009
0.001
0.001
0.002
Irrigation
0.792
0.102
0.583
0.078
0.019
0.053
Variety x Irrigation
0.346
0.702
0.583
0.359
0.823
0.027
Different letters within a column and effect category indicate significant differences according to LSMeans (α=0.10).

5656

Table 2-4. Fern stem and cladophyll dry weight, 2011 and 2012.
Hart, MI
Treatment

Early Fall

Late Fall
2011
2012
2011
2012
Cladophyll
Stem
Total
Cladophyll
Stem
Total
Total
Total
------------------------------------------------grams biomass/m2-----------------------------------------------

Irrigation Main
Effect
None
883
1018
116
999
1115
862
595
136 b
Overhead
986
1210
141
953
1093
945
652
183 ab
1024
Drip
1235
150
1003
1153
962
670
211 a
Variety Main Effect
Jersey
195
1065
134
678
1283 a
1106 a 1240
1028 a
Millennium
157
852
138
600
1010 b
864 b 1001
818 b
Interactive Effects
Jersey
None
132
935
1024
966
618
971 ab 1122 ab
89 b
Overhead
202
1278
1011
710
1110
940 ab 1187 ab
168 a
Drip
250
1418
1107
708
1282 a
1539 a
144 ab 1273
Millennium
1205
758
572
None
141
795 b
915 b
143 ab 1062
Overhead
159
795
909
880
595
1046 ab 1241 ab
114 ab
Drip
172
733
889
817
633
764 b
931 b
156 ab
---------------------------------------------P-value-------------------------------------------ANOVA
Variety
NS
NS
NS
NS
0.3043
0.0592
0.088
0.0207
Irrigation
NS
NS
NS
NS
NS
NS
0.6031
0.0896
Variety x
Irrigation
NS
0.084
NS
NS
NS
0.9644
0.0898
0.099
Different letters within a column and effect category indicate significant differences according to LSMeans (α=0.10).

5757

Table 2-5. Root soluble solid concentration, 2011-2012.
Hart, MI
2011

2012

Treatment

Summer

Summer

Fall

Fall

-----------------------Percent BRIX----------------------Irrigation Main Effect
None
Overhead
Dri
p
Variety Main Effect
Jersey
Millennium
Interactive Effects
Jersey
None
Overhead
Drip
Millennium
None
Overhead
Drip
ANOVA
Variety
Irrigation
Variety x Irrigation

N/A
9.6
7.5

16.4
16.5
16.0

10.2
9.3
9.2

12.6
12.5
12.1

7.3 b
10.4 a

16.2
16.4

8.9
10.2

12.6
12.2

N/A
9.7
N/A

14.4 c
16.2 bc
18.0 ab

9.4
8.6
8.9

12.5
13.6
11.8

10.4
9.5
11.6

18.3 ab
16.8 bc
14.1 c

11.1
10.1
9.4

12.7
11.5
12.3

-----------------------P-value---------------0.085
0.800
0.102

0.798
0.914
0.006

0.632
0.969
0.775

0.537
0.826
0.362

Different letters within a column and effect category indicate significant
differences according to LSMeans (α=0.10).

5858

Table 2-6. Weed and disease severity, 2011.
Hart, MI
Marestail
Density

Treatment

# weeds per m sq.
Irrigation Main Effect
None
Overhead
Drip
Variety Main Effect
Jersey
Millennium
Interactive Effects
Jersey
None
Overhead
Drip
Millennium
None
Overhead
Drip
ANOVA
Variety
Irrigation
Variety x Irrigation

Purple spot
severity
visual rating

0.32
0.19
0.25

4.9
4.5
4.3

0.15 b
0.36 a

4.5
4.6

0.13
0.14
0.17

4.9
4.3
4.4

0.51
0.24
0.34

4.8
4.8
4.3

-------------------P-value-------------0.0399
NS
NS

NS
NS
NS

Different letters within a column and effect category indicate significant differences
according to LSMeans (α=0.10).
Visual rating (0=none; 10=severe)

5959

Table 2-7. Density and fern damage from key insect pests, 2012.
Hart, MI
Insect density (7/9)
Asp.
Japanese
Treatment
beetle
beetle
# insects per m of row
Irrigation Main Effect
None
0.5
0.3 b
Overhead
0.6
1.9 a
Drip
0.8
0.6 b
Variety Main Effect
Jersey
0.9
0.7
Millennium
0.8
0.6
Interactive Effects
Jersey
None
0.1 b
0.8
Overhead
2.8 a
0.0
Drip
0.3 b
1.1
Millennium
None
0.5 b
0.0
Overhead
1.0 ab
1.2
Drip
0.9 ab
0.5
ANOVA

Asparagus Miner damage
7/9

9/8
% of stems damaged

10.1
11.8
11.2

26.4
23.3
19.8

13.4
8.2

18.7 b
27.6 a

bc
c
ab

12.9
14.5
13.0

23.4
16.1
16.5

c
ab
bc

4.5
9.2
9.4

29.3
30.5
23.1

-------------------------P-value--------------------------

Variety
0.933
0.740
0.289
0.026
Irrigation
0.562
0.753
0.452
0.025
Variety x Irrigation
0.992
0.5428
0.067
0.079
Different letters within a column and effect category indicate significant differences according to LSMeans (α=0.10).
Visual rating of percentage of stems with significant miner damage

6060

Table 2-8. Asparagus yield and quality, 2012.
Hart, MI
Yield
kg/ha
Irrigation Main Effect
None
Overhead
Dri
p
Variety Main Effect
Jersey
Millennium
Interactive Effects
Jersey
None
Overhead
Drip
Millennium
None

Quality assessment
Number
#/ spear
plot

Spear wt
g/spear

Size category
Small
Large
Jumbo
% of harvested spears

2,916 b
3,279 a
3,236 a

153
160
164

14.6
15.6
15.4

25.8
22.3
24.6

73.2
76.9
74.0

1.0
0.8
1.4

3,297 a
2,991 b

187 a
131 b

14.1 b
16.3 a

25.0
23.5

74.1
75.3

0.9
1.2

3,144
3,317

13.8 c
14.9 bc

26.0
23.9

72.9
75.3

1.1
0.8

3,429

178 b
180 a
b
204 a

13.6 bc

25.0

74.1

0.9

2,688

129 c

15.3 a
b
16.3 a
17.1 a

25.7

73.4

1.0

Overhead
3,242
20.7
78.5
0.8
140 C
Drip
3,043
24.2
73.9
1.9
125 c
ANOVA
---------------------------------------P-value-------------------------------------Variety
0.1681
0.2294
0.3927
0.0028
<0.0001
0.0001
Irrigation
0.4419
0.5143
0.5255
0.4433
0.4461
0.0141
Variety x Irrigation
0.1427
0.4639
0.3008
0.4524
0.082
0.0607
Different letters within a column and effect category indicate significant differences according to LSMeans (α=0.10).
Quality measures were taken from subsample at three harvest dates only.
6161

Table 2-9. Soil chemical properties, 2012.
Hart, MI
Treatment
pH
Irrigation Main Effect
None
Overhead
Drip
Variety Main Effect
Jersey
Millennium
Interactive Effects
Jersey
None
Overhead
Drip
Millennium
None
Overhead
Drip

K
P
parts per million

Mg

Ca

6.6 b
7.0 a
6.6 ab

111.4
91.5
106.4

98.1
115.8
103.4

77.6 b
97.4 a
73.3 b

447.6
488.5
448.5

6.7
6.7

103.8
102.4

105.9
105.6

83.5
82.0

473.3
449.5

6.6
7.0
6.7

124.3 a
85.5 c
101.5 bc

91.0
119.5
107.3

78.5
93.3
78.8

467.3
478.3
474.5

6.6
7.0
6.6

98.5 bc
97.5 bc
111.3 b

105.3
112.0
99.5

76.8
101.5
67.8

428.0
498.0
422.5

ANOVA
-----------------------------------------P-value-------------------------------------Variety
NS
NS
NS
NS
0.080
Irrigation
NS
NS
NS
0.082
0.015
Variety x Irrigation
NS
NS
NS
0.016
0.080
Different letters within a column and effect category indicate significant differences according to LSMeans (α=0.10).

6262

4.00

ET - Rainfall (in)

3.00

2.00

1.00

0.00

-1.00
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 201 1 2012
-2.00

Figure 2-1. Estimated evapotranspiration (ET) minus rainfall between July and August, Hart, MI, 1997-2012. ET
estimates were calculated as product of PET and crop coefficient for asparagus fern.
6363

4
Irrigation

3.5

Rainfall or irrigation (inches)

Rainfall
3
2.5
2
1.5
1
0.5
0
4/3

4/17

5/1

5/15

5/29

6/12

6/26

7/10

7/24

8/7

8/21

9/4

9/18

Date

Figure 2-2. Rainfall (black bars) and irrigation (gray bars) events during 2011 growing season, Hart, MI.

6464

10/2

5.0
Irrigation

4.0
Rainfall or irrigation (inches)

4.5

Rainfall

3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
4/3

4/17

5/1

5/15

5/29

6/12

6/26

7/10

7/24

8/7

8/21

9/4

9/18

Date

Figure 2-3. Rainfall (black bars) and irrigation (gray bars) events during 2012 growing season, Hart, MI.
irrigation

6565

10/2

13
12
11
10
Volumetric water content (%)

9
8
7
6
5
4

Drip

3
None
2
1
0
25-Jun

Overhead

5-Jul

15-Jul
Jul

25-Jul

4-Aug
Date

14-Aug

Figure 2-4. Mean volumetric water content by irrigation treatment, 2011.

6666

24-Aug

3-Sep
Sep

13-Sep

13
12

Volumetric water content (%)

11
10
9
8
7
6
5
4

Drip

3

None

2

Overhead

1
0
5/6

5/16

5/26

6/5

6/15

6/25

7/5
Date

7/15

7/25

Figure 2-5. Mean volumetric water content by irrigation treatment, 2012

6767

8/4

8/14

8/24

9/3

16
None
Overhead
Drip

Volumetric Water Content (%)

14
12
10
8
6
4
2
0
0

10

20

30

40

50
60
Depth (cm)

70

80

90

100

Figure 2-6. Mean volumetric water content by depth on August 28, 2011. This pattern of moisture distribution
by depth was typical of conditions prevailing during most of the month of August, 2011 as well as early July
2012.

6868

0.3

Water loss (mm/day)

0.3
0.2
0.2
0.1
0.1

Jersey
Millenium

0.0
Depth in soil (cm)
Figure 2-7. Mean daily water loss by depth between July 11 and July 24, 2011 from
water
un-irrigated treatments containing either Jersey Supreme or Millennium asparagus
irrigated
varieties.

6969

0.4
0.4

Water loss (mm/day)

0.3
0.3
0.2
0.2
0.1
Jersey
0.1

Millenium

0.0
Depth in soil (cm)
Figure 2-8. Mean daily water loss by depth between June 14 and July 17, 2012 from
un-irrigated treatments containing either Jersey Supreme or Millennium asparagus
irrigated
varieties.

7070

Table 3-1. Calendar of Activities.
Activity

2011-12

2012-13

Crowns planted

3/15/11

5/1/12

First stem emergence

4/15/11

6/1/12

Baseline irrigation differences initiated

6/1/11

6/27/12

Drought stress initiatied (trts xy)

6/13/11

6/28/12

Drought stress terminated

6/28/11

7/16/12

NA

7/23/12

Cold dormancy initiated

8/2/11

8/20/12

Fern dry weight samples

11/3/11

11/14/12

Moved back to greenhouse and watered

11/3/11

1/9/13

Spears up

11/7/11

1/21/13

Harvest initiated

11/8/11

1/21/13

Harvest terminated

12/5/11

2/13/13

12/16/11-2/10/12

2/20/13-3/19/13

Baseline irrigation differences initiated

1/20/12

2/6/13

Drought stress initiatied (trts xy)

1/27/12

3/7/13

Drought stress terminated

2/17/12

NA

Cold dormancy initiated

3/21/12

NA

Fern dry weight samples

4/6/12

NA

Roots excavated

4/10/12

NA

Root Brix sample

4/10/12

NA

Season 1

Root Brix sample

Season 2

Fern stem counts and heights taken

7171

Table 3-2. Total stem number during first season.
2011-12
2012-13
5/23/11 6/23/11 7/28/11
6/18/12 7/19/12
8/20/12
69 DAP 100 DAP 135 DAP 48 DAP 79 DAP
111 DAP
Treatment
Total
Total
Total
Total
Total
Total
Old
Fern New Dead
-----------------------------------------------------------------------#/ box--------------------------------------------------------------------------Irrigation Main Effect
High
13.8
32.6
45.4
11.1
20.3 a 33.6
20.4 a
9.8
1.5
2.0
Low
12.4
30.8
44.1
12.8
16.8 b 29.0
15.0 b
9.8
2.1
1.4
Drought Stress Main Effect
No drought
13.3
32.6
46.8
12.3
19.5
33.8 a
20.0 A 10.1
2.3
1.4
Drought
12.9
30.8
42.8
12.6
17.5
28.9 b
15.4 B
9.4
1.4
2.0
Interactive Effects
High Irrigaiton
No drought
14.8
33.5
47.5
11.3
21.5
36.8
23.5
9.8
2.0
1.5
Drought
12.8
31.8
43.3
13.0
19.0
30.5
17.3
9.8
1.0
2.5
Low Irrigation
No drought
11.8
31.8
46.0
13.3
17.5
30.8
16.5
10.5
2.5
1.3
Drought
13.0
29.8
42.3
12.3
16.0
27.3
13.5
9.0
1.8
1.5
ANOVA
------------------------------------------------------------------P -value-----------------------------------------------------------------Irrigation
NS
NS
NS
NS
0.045
0.123
0.019
NS
NS
NS
Drought
NS
NS
NS
NS
0.226
0.038
0.001
NS
NS
NS
Irrig. X Drought
NS
NS
NS
NS
0.755
0.498
0.071
NS
NS
NS
Different letters within a column and effect category indicate significant differences according to LSMeans (α=0.10).
DAP = Days after planting; Old = Fully mature; Fern = Newly mature; New =Not fully developed.

7272

Table 3-3. Fern dry weight
Treatment
Irrigation Main Effect
High
Low
Drought Stress Main Effect
No drought
Drought
Interactive Effects
High Irrigaiton
No drought
Drought
Low Irrigation
No drought
Drought

2011-12
2012-13
10/27/11 4/11/12
Total
11/6/12
-------------------------g/box-------------------------309
288

423 a
285 b

732 a
572 b

165
151

314
283

366
341

680
624

159
156

326
293

423
423

749
715

174
155

301
274

310
260

611
533

144
158

------------------------------------------------------------------P -value---------------------------------------------------ANOVA
Irrigation
0.2356
0.0002
0.0007
NS
Drought
0.1089
0.2900
0.1156
NS
Irrig. X Drought
0.8599
0.2900
0.5107
NS
Different letters within a column and effect category indicate significant
differences according to LSMeans (α=0.10).
fern had dropped cladophylls at time of 2012-13 sampling

7373

Table 3-4. Spear weight and number
Treatment

Spear Yield
Spear Number
Weight per spear
2011-12 2012-13 Combined 2011-12 2012-13 Combined 2011-12 2012-13 Combined
g/box
#/box
g/spear

Irrigation Main Effect
High
152.0
151.8
151.9
19.6
16.6
11.3
7.4
9.6
Low
147.9
143.2
145.7
19.1
15.5
11.7
7.7
10.0
Drought Stress Main Effect
No drought
160.2
161.5
160.9 a
20.9
17.3 a
11.6
7.7
9.9
Drought
139.6
132.1
136.1 b
17.7
14.8 b
11.4
7.5
9.7
Interactive Effects
High Irrigation
No drought
159.0
165.5
162.3
ab 18.5
17.1
11.7
7.5
9.9
Drought
144.9
138.0
141.5
a
20.8
16.1
10.8
7.3
9.3
Low Irrigation
No drought
161.4
157.6
159.5
a
21.0
17.5
11.5
7.8
9.9
Drought
134.4
124.1
130.0
b
16.7
13.3
12.0
7.7
10.1
ANOVA
Irrigation
NS
NS
NS
NS
NS
NS
NS
NS
NS
Drought
NS
NS
0.074
NS
NS
0.064
NS
NS
NS
Irrig. X Drought
NS
NS
NS
0.099
NS
NS
NS
NS
NS
Different letters within a column and effect category indicate significant differences according to LSMeans (α=0.10).

7474

Table 3-5 Total stem number following harvest.
DAH = Days after harvest
2011-12
2012-13
12-Dec
11-Jan
22-Feb
20-Feb
13-Mar
Treatment
7 DAH 30 DAH 69 DAH
7 DAH 28 DAH
--------------------------------------#/box----------------------------------Irrigation Main Effect
High
8.3
13.1 a
13.6 a
9.3
10.8
Low
6.8
10.1 b
10.3 b
9.3
11.6
Drought Stress Main Effect
No drought
7.8
12.0
12.4
9.5
11.5
Drought
7.3
11.3
11.5
9.0
10.9
Interactive Effects
High Irrigaiton
No drought
7.3
13.3
13.8
9.8
11.5
Drought
9.3
13.0
13.5
8.8
10.0
Low Irrigation
No drought
7.3
9.0
11.0
9.3
11.5
Drought
6.3
8.5
9.5
9.3
11.8
------------------------------------------------------------------P -value-----------------------------------------------------------------ANOVA
0.108
0.004
0.001
NS
NS
Irrigation
Drought
0.567
0.365
0.271
NS
NS
Irrig. X Drought
0.108
0.540
0.424
NS
NS
Different letters within a column and effect category indicate significant differences
according to LSMeans (α=0.10).

7575

Table 3-6. Total stem height after harvest
DAH = Days after harvest
Sum of spear heights
Maximum height
2011-12
2012-13
2012-13
16-Dec 30-Dec 10-Feb 20-Feb 27-Feb 20-Mar 20-Feb 27-Feb
13-Mar 20-Mar
Treatment
11 DAH25 DAH 36 DAH 7 DAH 14 DAH35 DAH 7 DAH14 DAH 28 DAH 35 DAH
cm/plot
cm
Irrigation Main Effect
High
394
682
2,802 a
435
1,255 2,282
114.0 208.0
252.0 a 264.0
Low
360
544
2,174 b
395
1,099 2,205
99.0 191.0
229.0 b 254.0
Drought Stress Main Effect
No drought
337
607
2,580
412
1,231 2,395 a 102.0 202.9
244.2
265.0
Drought
417
620
2,395
418
1,123 2,091 b 110.7 196.5
236.5
251.0
Interactive Effects
High Irrigaiton
No drought
294
633
2,825
384
1,283 2,478
106.0 210.0
259.0
274.0
Drought
477
731
2,779
486
1,225 2,086
122.0 206.0
244.0
253.0
Low Irrigation
No drought
378
580
2,337
440
1,179 2,312
98.0 196.0
229.0
257.0
Drought
333
509
2,013
350
1,020 2,097
100.0 187.0
229.0
251.0
ANOVA
P-value
Irrigation
NS
NS
0.003
NS
NS
NS
NS
NS
0.048
NS
Drought
NS
NS
NS
NS
NS
0.029
NS
NS
NS
NS
Irrig. X Drought NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
Different letters within a column and effect category indicate significant differences according to LSMeans
(α=0.10).
Note drought stress in second year was applied at 58 DAH in 2012, and 22 DAH in 2013

7676

Table 3-7. Final root weight and soluble solids, 2011-12
2011-12
Treatment
Root weight Sol. Solids
g/box
% Brix
Irrigation Main Effect
High
1698 a
16.2
Low
1352 b
15.2
Drought Stress Main Effect
No drought
1549
16.7 a
Drought
1501
14.7 b
Interactive Effects
High Irrigaiton
No drought
1755
16.3
Drought
1640
14.1
Low Irrigation
No drought
1342
17.1
Drought
1361
15.3
------------------------------------------------------------------P -value-------------------------ANOVA
0.034
NS
Irrigation
NS
0.058
Drought
Irrig. X Drought
NS
NS
Different letters within a column and effect category
indicate significant differences according to LSMeans
(α=0.10).

7777

LITERATURE CITIED

7878

LITERATURE CITED
Bakker J. 2010 Michigan Asparagus Industry. Presentation at Great Lakes Expo Dec
2010 in Grand Rapids, MI. MI Asparagus Ad. Board.
Bakker J. 2012 Michigan Asparagus Industry. Presentation at Great Lakes Expo Dec
2012 in Grand Rapids, MI. MI Asparagus Ad. Board.
Ball, T., R. Folwell, and D. Holmes. 2002. Establishment and Annual Production Costs
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