EFFECTIVENESS OF SUMMER MOWING FOR WEED CONTROL IN PLANTINGS OF
NATIVE C4 PRAIRIE GRASSES
By
Colin James Phillippo

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
Plant Biology
2011

ABSTRACT
EFFECTIVENESS OF SUMMER MOWING FOR WEED CONTROL IN PLANTINGS OF
NATIVE C4 PRAIRIE GRASSES
By
Colin James Phillippo

Land managers often face seemingly conflicting needs to use land for species
conservation or to produce economically valuable goods. Diverse prairie plantings harvested for
biomass offer an opportunity to achieve both goals. Persistence of these disturbance-adapted
prairie communities requires management strategies that promote desirable native plant diversity
and reduce abundance of undesirable weedy species. This is sometimes achieved through
prescribed burning or mowing, though mowing effects have not been well studied. To address
this knowledge gap, I compared two summer mowing treatments to a no-mowing control in
experimental prairies on both loamy and sandy soils for two years. The communities examined
contained mature (4- and 5-yr-old) prairie grasses (Andropogon gerardii Vitman, big bluestem,
and Schizachyrium scoparium Michx., little bluestem), as well as young colonizers of both
desirable and weedy species. Overall, I found that June mowing reduced weed biomass and seed
production on loamy soil. Biomass of native grass colonizers was greatest in mowed blocks,
suggesting that mowing stimulated native colonization. However, these benefits of mowing came
at the cost of some reduction in the size and reproductive output of mature prairie grasses. June
and July mowing substantially reduced size and flowering of native grasses without additional
weed control benefits. I therefore suggest that June mowing could be a useful component of
adaptive management in tallgrass prairie in the Great Lakes region, with careful consideration of
effects on summer-nesting bird species, but I advise against mowing in both June and July.

ACKNOWLEDGMENTS

I would like to thank my advisor, Dr. Carolyn Malmstrom for her guidance throughout
this project. I am also grateful to my committee members, Dr. Jennifer Lau and Dr. Kelly
Millenbah, for their support. Assistance from multiple people made this research possible.
Abbie Schrotenboer established the sites I used for my research, and I am very grateful that I was
able to benefit from her hard work. Field and laboratory assistance was provided by Jason
Martina, Josh Springer, Lisa Stelzner, Elizabeth Schultheis, Anna Busch, Ricky Juritsch, Erin
Linderman, Mark Parayil, Stephanie Carpenter, Alexander Park, and Andrew Wood. Drafts of
my thesis were edited by Jamie Phillippo, Jason Martina, Josh Springer, and Laura Planck.
I would also like to thank Bill Schneider for use of research sites on his property, and for
his advice during our multiple discussions about ecological restoration. Bill Chase and the staff
at the MSU Horticulture Teaching and Research Center were also very accommodating of my
research needs. Funding for this research was provided by the Great Lakes Bioenergy Research
Center (GLBRC), the MSU Graduate School‟s Research Enhancement Award, the Paul Taylor
Endowment Fund, and the Department of Plant Biology.
I am very grateful for the support I have received from my family. Most importantly, I
would like to thank my wife, Jamie. Without her help and support none of this would have been
possible.

iii

TABLE OF CONTENTS

LIST OF TABLES ......................................................................................................................... vi
LIST OF FIGURES ...................................................................................................................... vii
CHAPTER 1
MOWING IN PRAIRIE RESTORATION AND MANAGEMENT ..............................................1
1. Distribution of North American tallgrass prairie: past and present .....................................1
2. Diversity and invasion in tallgrass prairie ...........................................................................2
3. Disturbance regimes and prairie management .....................................................................3
a. Historic disturbance regimes....................................................................................3
b. Current management options ...................................................................................5
4. Plant responses to mowing...................................................................................................6
a. Influence of timing of mowing ................................................................................6
i. Effects on dominant species.........................................................................6
ii. Community effects .......................................................................................9
b. Influence of mowing frequency .............................................................................10
5. Need for research ...............................................................................................................11
CHAPTER 2
EFFECTIVENESS OF SUMMER MOWING FOR WEED CONTROL IN PLANTINGS OF
NATIVE C4 PRAIRIE GRASSES ................................................................................................12
1. Introduction ........................................................................................................................12
2. Methods..............................................................................................................................17
a. Site descriptions and land use history ....................................................................17
b. Categorization of colonizing plants .......................................................................19
c. Mowing treatment descriptions..............................................................................20
d. Growth of colonizers..............................................................................................20
e. Community seed production ..................................................................................20
f. Growth and flowering of mature grasses ...............................................................21
g. Statistical analysis ..................................................................................................21
3. Results ................................................................................................................................26
a. Colonizing species .................................................................................................26
b. Mowing effects on weeds ......................................................................................26
c. Mowing effects on desirable natives......................................................................28
d. Mowing response differences between soil types ..................................................30
4. Discussion ..........................................................................................................................44
a. Successional processes...........................................................................................45
b. Influence of summer mowing on weedy colonizers ..............................................45

iv

c. Influence of summer mowing on native species ....................................................48
d. Duration, frequency, and timing of mowing ..........................................................49
e. Mowing in adaptive management ..........................................................................51
CHAPTER 3
ASSESSING SEED ABUNDANCE IN PRAIRIES .....................................................................55
1. Methods for conducting a seed bank study ........................................................................55
2. Seed trap design and use ....................................................................................................56
3. Seed photographs ...............................................................................................................59
LITERATURE CITED ..................................................................................................................70

v

LIST OF TABLES

Table 1. Plant species found in study plots. Each species was categorized as either a Michiganlisted noxious species (MI-noxious), a nuisance species, or a desirable native. All species names
follow Reznicek et al. (2011). ........................................................................................................32
Table 2. Yr 2 responses of seedlings of individual plant species to mowing. The one-mow effect
was calculated as the % difference between the response of control plants and that of Junemowed plants. The two-mow effect was calculated as the % difference between the response of
control plants and that of plants mowed in June and July. “0” indicates no change in seedling
counts. One symbol in either direction (“-“ or “+”) represents a 10–25% change. Two symbols
(“--“ or “++”) represents a 25–50% change. Three symbols (“---“ or “+++”) represents a change
> 50%. ............................................................................................................................................35
Table 3. Abundance of species found in seed banks of sandy and loamy plots in Mason,
3
Michigan (600 cm of soil sampled per plot). Seven species did not grow beyond the seedling
stage and could not be identified. Here they are listed separately as unknowns. .........................57

vi

LIST OF FIGURES

Figure 1. Sites in Ingham County, Michigan. East Lansing and Mason sites are approximately
11 km apart For interpretation of the references to color in this and all other figures, the reader is
referred to the electronic version of this thesis.. ............................................................................23
Figure 2. Plot placement at each site: a) East Lansing, and b) Mason. ........................................24
Figure 3. Representative plot layout containing 19 blocks. Outer border represents plot
boundaries. Solid circles are mature native grass plant individuals, dashed rectangles are
colonizer monitoring quadrats, and solid rectangles are block boundaries. ..................................25
Figure 4. Composition of colonizing plant community summed across mowing treatments, as
measured by a) Yr 1 % cover in August, b) Yr 2 % cover in August, c) number of seedlings per
2

m in May of Yr 2, and d) number of seeds trapped in 10.8 cm diameter seed traps from July to
October of Yr 2. .............................................................................................................................37
Figure 5. Percent cover of colonizing species in August of Yr 1 and Yr 2: a) MI-noxious
species, b) nuisance species, and c) desirable natives. Means ± one standard error are
presented. .......................................................................................................................................38
Figure 6. Seedling density of colonizing species in May of Yr 2: a) MI-noxious species,
nuisance species, and desirable natives; and b) low-profile nuisance species percent cover.
Means ± one standard error are presented. ....................................................................................39
Figure 7. Mowing effects on seed production of colonizers measured from July to October of Yr
2 using 10.8 cm diameter seed traps: a) MI-noxious; b) nuisance species on the loamy plot and
on the sandy plot; and c) desirable natives. Means ± one standard error are presented. ..............40
Figure 8. Mowing effects on colonizer biomass harvested in late August of Yr 2 (dry weight):
a) all species (excluding native C4 grasses) in the loamy plot, and in the sandy plot; and b) native
C4 grass biomass (A. gerardii and S. scoparium). Means ± one standard error are presented.. ...41
Figure 9. Mean responses of individual mature plants to mowing. In Yr 1, plants were three
years old. The panels show Yr 1 measures for S. scoparium and Yr 1 and Yr 2 measures for A.
gerardii: a) end-of-season height, b) end-of-season basal area, and c) aboveground biomass.
Panel c shows a further breakdown of values. In Yr 1, residual biomass was harvested in
October. In Yr 2, A. gerardii mowed biomass was also collected after the June („Ju‟) and July
(„Jy‟) mowing. Means ± one standard error are presented. ..........................................................42

vii

Figure 10. Flowering response of individual mature A. gerardii to mowing. a) Yr 1
inflorescence counts, and b) Yr 2 flowering tiller counts. Means ± one standard error are
presented. .......................................................................................................................................43
Figure 11. Seed trap used to assess plant community seed deposition. ........................................56
Figure 12. Seeds collected directly from plants and from seed traps in Ingham county, MI.
Photographs were taken using 10x–200x magnification. The scale shown is divided into 1 mm
increments. .....................................................................................................................................59

viii

Chapter 1
Mowing in Prairie Restoration and Management

Distribution of North American tallgrass prairie: Past and present
Prairies historically dominated much of the North American Great Plains region and large
portions of the Midwestern United States. Estimates suggest that prior to European settlement
these North American prairies exceeded 162 million ha in area, of which approximately 68.4
million ha were tallgrass prairie (Samson and Knopf 1994). Prairies are primarily defined by
continuous cover of herbaceous vegetation, and their lack of woody vegetation (<10%) (FaberLangendoen 2001). Tallgrass prairies are further defined by the presence of C4 grasses that
typically dominate the plant community, primarily big bluestem (Andropogon gerardii Vitman),
little bluestem (Schizachyrium scoparium Michx.), and Indian grass (Sorghastrum nutans (L.)
Nash) (Faber-Langendoen 2001). In the 1800s, seven percent of Michigan consisted of prairie
and savanna, primarily in the southwestern region (O'Connor et al. 2009).
As European settlers spread westward across North America in the 1800s, they cleared
land for settlement and agricultural development. Prairies were the easiest land to clear because
they lacked woody species, and they were particularly well-suited for agriculture because of their
fertile soils (Iverson 1988, Samson et al. 2004). Since the 1800s, nearly 99.9% of North
American prairies have been converted to other land covers, resulting in the greatest loss in
extent of any ecosystem in North America (Samson and Knopf 1994). In Michigan, the majority
of native prairie was converted to agriculture by European settlers, and remnants continue to be
converted to other land use types or degraded by human activities (O'Connor et al. 2009). The
remnant prairies in Michigan that persisted did so largely because they were situated in areas less

1

suitable for agriculture (Iverson 1988). For this reason, many remnants are found on lower
quality soils than was typical of historical prairies.
Diversity and invasion in tallgrass prairie
Prairies in the Great Lakes region once supported a large number of diverse plant and
animal species (O'Connor et al. 2009). The few long-term studies of prairie restoration have
shown that lost biodiversity can be difficult to replace. Even the oldest prairie restoration in
existence, the Curtis Prairie at the University of Wisconsin, is threatened by abundance of
invasive species and decreased native species (Wegener et al. 2008). Similarly, restored sites
continued to have lower species richness than remnant sites thirty-five years after restoration
began at a prairie in northeast Kansas, (Kindscher and Tieszen 1998), possibly because many
native forbs failed to establish in restored areas, despite being seeded in. Restored prairies in
Illinois ranging in age from 35 to 45 years old also had lower species richness than nearby
remnant sites, though both site types had similar floristic quality index (FQI) values and
percentage of alien species (Allison 2002). Restored sites likely had high FQI values in this
study because sites were planted with species characteristic of high-quality prairies, though
species distributions were much patchier in restored than in remnant sites. These three examples
illustrate that prairie restoration is a long and challenging process, which often fails to produce
communities with the same characteristics as high quality remnants. However, improvements to
prairie restoration strategies can increase the effectiveness of this process.
Restoration activities may temporarily increase species or genetic diversity by
deliberately introducing new species or genotypes into the system, but this diversity is often not
maintained over time. Polley et al. (2005) found that plant species diversity was greater in

2

remnant prairies than comparable restored prairies due to greater variability in species diversity
among restored plots than among remnant plots. While some restored plots may have greater
species diversity than remnant plots, species diversity in most restored plots is much lower, such
that average species diversity is lower in restored sites. Gustafson et al. (2002) showed that
remnant prairies were actually less genetically diverse than restored prairies. This was largely
attributed to the use of multiple seed sources in restored prairies, which causes an increase in
genetic diversity because multiple genotypes are mixed. Many remnant sites are isolated from
other restored or remnant patches, and may contain less genetic diversity due to inbreeding and a
lack of new genetic inputs.
Prairie diversity and ecosystem functioning in both remnant and restored sites are
currently threatened by invasive species (Millenium Ecosystem Assessment 2005), which are a
primary concern for land managers (Rowe 2010). These species rapidly colonize disturbed sites
(Larson et al. 2001), often causing ecological and economic harm by displacing desirable native
species (Bryson and DeFelice 2010, Randall 1996). Control of invasive species likely requires
management strategies similar to historical disturbance regimes that formerly maintained prairie
plant communities. Mowing is one possible management strategy that partially mimics effects of
historical disturbances, and could improve invasive species control in prairies
Disturbance regimes and prairie management
HISTORIC DISTURBANCE REGIMES
One way to address problems with reduced plant species diversity and an influx of
invasive species is to reestablish disturbance regimes to prairies. Prior to European settlement,
disturbance in prairies came from four sources: fires set in spring by Native Americans (Packard

3

and Mutel 1997), lightning-ignited fires in summer (Higgins 1984), grazing by bison (Bison
bison) and other ungulates (Knapp et al. 1999), and burrowing by small mammals (Gibson
1989), all of which differentially promoted some plant species and suppressed others. In
tallgrass prairie, fire and grazing were the most important forms of disturbance.
In the mid-1800s, the vast herds of bison that grazed prairie communities prior to
European settlement were nearly eradicated (Knapp et al. 1999). The number of bison in the
U.S. in the 1800s was at least in the millions, though their precise abundance is impossible to
determine (Shaw 1995), and this species played an important role in maintaining prairie plant
communities. Grazing activity by bison can increase plant species diversity in prairies, often by
reducing abundance of dominant C4 grasses (Hartnett et al. 1996, Veen et al. 2008) or increasing
microsite variability (Hartnett et al. 1996). While bison grazing had great influence on prairies
in the Great Plains region, bison are believed to have been much less abundant in the Great
Lakes region, and consequently the effects of grazing in Michigan may have been less important
than the effects of fire (O‟Connor et al. 2009).
th

In the 20 century, the perceived need to protect human lives and property from
uncontrolled wildfires prompted widespread fire suppression efforts in the United States. In
turn, fire suppression substantially altered the composition of fire-adapted native plant
communities. In Michigan, fire suppression substantially reduced the frequency of naturally
occurring fires (Cleland et al. 2004) and has led to degradation of prairies and other firedependent ecosystems. In Wisconsin, much of the species loss in prairie remnants has been
attributed to fire suppression in these ecosystems (Leach and Givnish 1996). Frequent fires are
also important in preventing the encroachment of woody species into prairies, and fire

4

suppression often results in significant increases in woody species abundance (Briggs et al.
2002). Without fire or bison to maintain prairie communities and suppress weed populations,
many remnant prairies may be unable to persist without human intervention (Randall 1996).
These remnants may experience reductions in diversity from invasive species colonization, or
may become forests over time if invaded by woody species.
CURRENT MANAGEMENT OPTIONS
In the tallgrass prairie, prescribed burns are conducted primarily in early spring, and are
among the most common prairie management practices in North America (Rowe 2010).
However, spring prescribed burning does not reflect patterns of natural and human-initiated
disturbances that historically shaped prairie ecosystems. These disturbances varied in type (e.g.
fire and grazing), frequency within and between years, and timing. Mowing could be an
effective tool to mimic some aspects of past disturbances, especially considering its flexibility in
terms of application season and necessary conditions for safe and effective use. Prescribed fire
use is more constrained by environmental conditions such as wind speed and direction than
mowing (Rowe 2010). Grazing is also more constrained than mowing, but is not frequently used
in prairie restorations (Rowe 2010).
Mowing and burning often have similar effects on tallgrass prairie vegetation, as
demonstrated in spring (Diboll 1986, Hover and Bragg 1981) and summer (Hover and Bragg
1981). Biomass removal is one of the most important roles of disturbances such as fire and
grazing (Hulbert 1969, MacDougall and Turkington 2007). Removing biomass near the time of
invasive flowering negatively affects dominant invasives by reducing their size and preventing
seed production (Benzel et al. 2009, MacDougall and Turkington 2007, Rinella et al. 2001), and

5

favors sub-dominant species by increasing light levels and temperature at the soil surface.
Removing inflorescences before invasive plants disperse seeds may be critical to prevent further
spread of weedy species. This is particularly true for annual weeds, since the parent plant will
not persist into the next growing season. However, both fire and grazing have additional impacts
that are not replicated by mowing treatments. Prescribed fire alters nutrient cycling and blackens
soil, which further increases soil temperatures (Hulbert 1988). In addition to biomass removal
from grazing activity, bison increase nitrogen deposition through waste elimination, and alter soil
characteristics by trampling and wallowing (Knapp et al. 1999).
Plant responses to mowing
Plant responses to mowing are largely mediated by plant traits. Mowing selects for and
against species based on height; taller species are often more negatively affected by direct
biomass removal, while short-statured species may benefit from increased light availability
(MacDougall and Turkington 2007). Timing of mowing is also critical. In general, summer
mowing (mid-June–August) favors cool-season species, and spring mowing (April–early-June)
favors warm-season species (Hover and Bragg 1981, Howe 1999, Wilson and Clark 2001).
Plants may also exhibit different responses depending on how frequently they are mowed within
a growing season.
INFLUENCE OF TIMING OF MOWING
Effects on dominant species
Most mowing studies have focused exclusively on the dominant plant species in a
community. This research has shown that summer mowing can be especially detrimental to the

6

size of native C4 grasses. For example, big bluestem and little bluestem biomass yields in
Missouri were reduced by summer clipping, particularly in July (Vogel and Bjugstad 1968).
Likewise, July mowing decreased cover of A. gerardii and S. scoparium in a Nebraska prairie
(Hover and Bragg 1981). Two summer clipping treatments in a Kansas prairie reduced biomass
production of S. scoparium, but a single summer clipping in this prairie did not affect biomass
(N‟Guessan and Hartnett 2011).

In contrast, mowing when C4 grasses are dormant or just beginning to grow may promote
their growth by suppressing other actively growing species and removing standing litter. May
mowing in Wisconsin increased frequency of S. scoparium and side-oats grama (Bouteloua
curtipendula (Michx.) Torr.) (Diboll 1986). Similarly, when big bluestem and little bluestem
plants were clipped in the dormant stage (October–November), biomass yields increased the
following season (Vogel and Bjugstad 1968).
Most native C3 grasses exhibit the opposite response to mowing as C4 grasses; they are
suppressed by early or late growing season (spring or fall) mowing, and promoted by summer
mowing, which may reduce the amount of standing litter in the following growing season. For
example, May mowing reduced cool season grass biomass production in Wisconsin prairies
(Diboll 1986). Similarly, July (late season) mowing in a Texas switchgrass stand decreased
shoot numbers (Haferkamp and Copeland 1984) and biomass production of big sacaton
(Sporobolus wrightii Munro ex Scribn.), but biomass of mowed plants rebounded within three
years after mowing ceased (Cox 1988). In contrast, summer mowing increased cover of
porcupine grass (Hesperostipa spartea (Trin.) Barkworth) in Nebraska (Hover and Bragg 1981),
and increased both cover and inflorescence production of California oatgrass (Danthonia

7

californica Bol.) in Oregon (Wilson and Clark 2001). In a Wisconsin prairie, August mowing of
the native forb, golden Alexanders (Zizia aurea (L.) W. D. J. Koch), doubled population size and
number of flowering plants, but May mowing had no effect (Howe 1999).
Summer mowing can also help control non-native invasive species that flower near the
time of treatment application. For example, July mowing in a Garry oak savanna reduced
growth and flowering of the invasive grasses Kentucky bluegrass (Poa pratensis L.) and orchard
grass (Dactylis glomerata L.), and increased native cover and flowering (MacDougall and
Turkington 2007). In an Oregon prairie, mowing in either late spring or early summer decreased
cover of the weedy species, tall oatgrass (Arrhenatherum elatius (L.) P. Beauv. ex J. Presl and C.
Presl) (Wilson and Clark 2001). Reductions were more apparent when plants were mowed
around the time of Arrhenatherum flowering. Similarly, two studies in Montana on invasive
spotted knapweed (Centaurea stobe L.) showed that mowing during the flowering stage is the
most effective way to decrease density and reproductive output of this species (Benefield et al.
2001, Rinella et al. 2001). Benefield et al. (2001) suggest a similar strategy for control of yellow
starthistle (Centaurea solstitialis L.) in California grasslands. Plants at the flowering stage have
invested much of their energy and resources to inflorescence production, and may not have the
time, energy, or resources to rebound from mowing treatments (Benzel et al. 2009). This control
strategy is widely accepted, but has only been directly studied for a few individual species.
Mowing may actually be counter-productive for control of some highly disturbance-adapted
weeds, such as white sweetclover (Melilotus alba L.), however. For example, mowing Illinois
prairies in late June increased white sweetclover biomass, but reduced total and grass biomass
(Randa and Yunger 2001).

8

Community effects
Multiple studies have shown that summer mowing treatments have little influence on
species richness and diversity in U.S. prairies. For example, neither species richness (Collins et
al. 1998, Foster et al. 2009, Randa and Yunger 2001) nor species diversity (Randa and Yunger
2001) are altered by June mowing. While it is well established that nitrogen addition alone
decreases plant species richness (Tilman 1987), mowing combined with fertilizer addition may
increase species richness (Foster et al. 2009), or prevent richness from decreasing in burned plots
(Collins et al. 1998). Removal of biomass is an important component of mowing, which may be
especially important in fertilized plots where biomass production is greater and may cause losses
of sub-dominant species (Collins et al. 1998).
Mowing has been found more often to influence species diversity and richness in
grasslands outside of the U.S. Summer mowing increased species richness in South African
grasslands (Fynn et al. 2004), in a Norwegian boreal hayfield (Antonsen and Olsson 2005), and
in a Garry oak savanna in British Columbia (MacDougall and Turkington 2007). However, these
increases may be largely attributable to germination of ruderal species from the seed bank, not to
an increase in native species abundance (MacDougall and Turkington 2007). A combination of
differently-timed mowing treatments may also effectively increase diversity by promoting plants
with different flowering phenologies (Antonsen and Olsson 2005). However, increases in weedy
(or ruderal) species are likely to contribute heavily to increased species richness following
disturbance since they are, by definition, the first colonizers of newly disturbed sites. These
results suggest that diversity metrics may be a poor indicator of mowing effects, particularly in
highly disturbed or newly established sites.

9

INFLUENCE OF MOWING FREQUENCY
Increased frequency of mowing within a growing season can have greater impacts on
plants than a single mowing. Repeated summer mowing can be especially damaging for native
C4 grasses, particularly for biomass production. For example, Aldous (1930) observed a
decrease in end-of-season biomass yield of A. gerardii and S. scoparium that became more
apparent as clipping frequency increased. Similarly, Wallace (1987) showed that S. scoparium
clipped every 6 weeks produced less end-of-season aboveground biomass than un-clipped
controls, though total biomass production was not calculated. In some cases, native grasses were
unaffected by single mowing treatments, but were affected by repeated mowing. Two or more
mows reduced biomass and tiller numbers of S. scoparium (N‟Guessan and Hartnett 2011). In
the same study, three or more mows reduced the ratio of flowering to vegetative tillers. In a
Kansas big bluestem stand, additional clipping treatments reduced tiller numbers of A. gerardii,
but did not further affect biomass production (Owensby et al. 1974).
In addition, frequent clipping may promote the establishment of forbs that are seeded into
prairie restorations. For example, Williams (2007) found that seedling numbers were greater,
and forbs were taller, in plots mowed every 1-2 weeks. This is likely because frequent mowing
reduces competitive pressure from tall plants, such as grasses. However, seedlings in mowed
plots also experienced greater over-winter mortality (Williams et al. 2007), possibly because less
grass litter was present to insulate seedlings during the winter. Both Benzel et al. (2009) and
Rinella et al. (2001) tested multiple mowing frequencies and found that repeated mowing has
little additional impact on invasive spotted knapweed (Centaurea stoebe L.) compared to single
mowing treatments. Both studies suggest that timing of treatments is most important for control
of this species, and repeated mowing is unnecessary as long as inflorescences are removed

10

before seeds are produced. Since plants have already invested the bulk of their energy and
nutrient reserves into inflorescences, they are typically unable to regenerate lost biomass near the
end of the growing season (Colin Phillippo, pers. obs.).
Need for research
Previous research on the effects of mowing in prairies has primarily focused on
individual dominant species. However, mowing either directly or indirectly affects all plants in a
community. Some studies have addressed the effects of mowing on the entire plant community,
but did so using richness and diversity metrics, which treat all species equally. This approach
neglects differences between species types, such as their desirability in restored prairies. There
is need for more research that both assesses whole community responses to mowing and
examines the consequences for each individual species.

11

Chapter 2
Effectiveness of Summer Mowing for Weed Control in Plantings of
Native C4 Prairie Grasses
Introduction
In the North American Midwest, native prairie communities add conservation and
agronomic value to the landscape. For example, native prairie plants benefit adjacent
agricultural fields by providing habitat for crop pollinators and arthropod natural enemies of crop
pests (Fiedler and Landis 2007, Isaacs et al. 2009, Kremen et al. 2004, Olson and Wackers
2007). The restoration and re-creation of prairies is a top priority for conservation organizations
throughout the U.S. because of their ecological and economic importance (The Nature
Conservancy 2011). In addition, prairie restoration is of increasing importance as natural
ecosystems are currently being considered to help meet demands for bioenergy. Prairie
communities recently have shown considerable potential as biofuel crops (Palmer 2006, Tilman
et al. 2006). However, there is much concern that biofuel crops will compete for land with
traditional agricultural food crops. This concern could be lessened if prairies and other
ecosystems could provide provisioning services such as plant biomass for biofuel production,
while continuing to support wildlife populations and ecosystem services important to agricultural
and urban areas. To achieve these economic and conservation goals simultaneously, diverse
native plant communities are needed.
Establishment and persistence of such diverse native plant communities faces multiple
challenges, particularly from invasive plant species. Invasive species, defined as non-native
species that cause economic or environmental harm (NISIC 2011), are currently one of the
primary threats to ecosystem structure and function (Millenium Ecosystem Assessment 2005,
Rowe 2010). “Weeds,” defined more generally as plant species growing where they are

12

unwanted and targeted for control (Blumenthal et al. 2003, Bryson and DeFelice 2010), refers to
many different types of plants, including invasives. Weeds can be either native or introduced,
and represent a wide range of life histories (Sutherland 2004) and taxonomic groups (Daehler
1998). Some native weeds may even provide food or habitat to animals, but generally spread
aggressively and exclude other plants (Larson 2002). Weed species may cause economic losses
in agricultural fields, decrease floristic diversity in native ecosystems, or threaten wildlife
populations by displacing plant species that provide food or shelter (Bryson and DeFelice 2010,
Randall 1996). Many weeds thrive in highly disturbed, low quality sites, and can rapidly invade
newly disturbed sites (Larson et al. 2001). Often, the goal in weed management is to simply
reduce the dominance and spread of weedy species (Rowe 2010), given that complete eradication
is often nearly impossible to achieve, and may in fact be undesirable for some species that
provide wildlife benefits at low densities.
Prairies are disturbance-adapted ecosystems in which community structure and plant
diversity were historically maintained by a combination of grazing by large ungulates (Knapp et
al. 1999), burrowing by small mammals (Gibson 1989), and landscape-scale wildfires (Higgins
1984, Packard and Mutel 1997). Fires included both i) fires deliberately set by Native
Americans in spring (Packard and Mutel 1997) and ii) lightning-ignited fires that burned in
summer (Higgins 1984, Howe 1994a). Since Europeans settled North America, large ungulate
herds have been almost entirely extirpated (Knapp et al. 1999), and landscape fragmentation and
fire suppression have significantly altered natural fire regimes (Cleland et al. 2004). Without
historical patterns of burning and grazing, many prairie remnants have been infiltrated by woody
species and aggressive non-native plant species (O'Connor et al. 2009).

13

Prairie managers sometimes attempt to re-establish historical patterns of natural
disturbance, which can stimulate growth of native species while suppressing growth and
proliferation of non-natives (Collins et al. 1998, Higgins 1984, Howe 1994b). One common tool
is prescribed fire, which is most often applied once per year when plants are dormant or just
beginning summer growth (Diboll 1986). In the absence of summer disturbance, plant
community composition can shift toward dominance of summer-flowering weeds, which are
unlikely to be suppressed by spring disturbances. Multiple studies have shown that effective
control of undesirable plant species requires treatment timing that corresponds to flowering
phenology of target plants (Benefield et al. 2001, Benzel et al. 2009, Wilson and Clark 2001). In
many areas, burning is either not possible or not practical due to weather and seasonal
constraints, and mowing is often used in its place.
Restoration practitioners generally view mowing as a more flexible technique that is not
weather-dependent, and that can be utilized during the summer when burning is often not
possible (Rowe 2010). Mowing can also be used to achieve alternative management goals such
as hay production (Foster et al. 2009). For example, end-of-season biomass harvests have long
been used to harvest hay in diverse Kansas prairies (Kindscher 2008) and could also be used to
harvest biofuel crops. Mid-season mowing may be an effective strategy to control summerflowering weeds, and could be congruent with biomass harvesting goals.
Despite its frequent use in prairie management, the effects of mowing on plant
communities have not been well studied. Most previous research has focused on the influence of
mowing on growth and reproduction of dominant species (e.g., Howe 1999, Vogel and Bjugstad
1968), and it has not addressed mowing effects on sub-dominants or on the plant community as a
whole. However, mowing affects all plant species in a community, either directly by removing

14

biomass or indirectly by increasing light availability (MacDougall and Turkington 2007). In the
studies that have been done, some positive effects of mowing on desirable native species have
been observed, such as increased population size of the May-flowering golden zizia (Zizia aurea
(L.) W.D.J. Koch) (Howe 1999), and enhanced establishment of native forbs from seed
(Williams et al. 2007). However, the size of individual native C4 grasses, Andropogon gerardii
Vitman (Owensby et al. 1974, Vogel and Bjugstad 1968) and Schizachyrium scoparium Michx.
(Vogel and Bjugstad 1968, Wallace 1987), is reduced by mowing or clipping during the growing
season. Additionally, increased mowing frequency within a growing season can have greater
negative effects on plants, especially dominant C4 grasses. (Owensby et al. 1974)
Mowing has had largely negative impacts on most weed species (e.g., Benzel et al. 2009,
Diboll 1986, Rinella et al. 2001, Wilson and Clark 2001), but can increase biomass of the nonnative weedy forb, white sweet-clover (Melilotus albus Medik.) (Randa and Yunger 2001).
Effects also vary with mowing timing; in fact, multiple studies suggest that timing may be more
important than treatment type (i.e., mowing, burning, or grazing) in determining influence on the
plant community (Hover and Bragg 1981, Howe 1994b, MacDougall and Turkington 2007).
Some mowing experiments have addressed the entire plant community, but have focused
mostly on richness and diversity metrics that treat both desirable natives and noxious weeds
equally. In general, mowing increases species richness (Antonsen and Olsson 2005, Fynn et al.
2004, MacDougall and Turkington 2007) and diversity (Antonsen and Olsson 2005), or
preserves diversity when added to a burning and fertilization regime, which reduces diversity
when implemented without mowing (Collins et al. 1998). However, such observed increases in
diversity may result from increased abundance of weedy species, not from recruitment of

15

natives. In one study that grouped plants by growth form and native status, mowing stimulated
growth of native species (Gonzales and Clements 2010). However, these groupings did not
acknowledge that both desirable and weedy species may have the same growth form, and that
many native plants are also weedy, so increased growth of native species could represent
increased native weed growth. No previous study has investigated mowing effects on the entire
plant community while addressing the value of individual species in tallgrass prairie.
Additionally, only a small number of mowing studies have been conducted in the North
American Midwest (most of those in Kansas). To address these deficiencies, I have grouped
species occurring in this Michigan prairie restoration experiment based on their potential
negative effects in natural and human-managed landscapes.
Species composition in prairies is also influenced by soil type, largely due to soil
moisture and nutrient content differences (Weaver 1968), and plant communities on different soil
types likely respond differently to mowing. This is particularly important since marginal lands
are being considered for biofuels production (Hill et al. 2006), which would reduce pressure on
food crops. However, plant responses to management treatments such as mowing on marginal
lands will likely differ from responses on more productive land, though the nature of these
responses is currently not well understood.
The following research investigated the effects of summer mowing on summer-flowering
weeds, naturally colonizing native plants, and two dominant warm-season prairie grasses, big
bluestem (A. gerardii) and little bluestem (S. scoparium Michx.), in experimental prairies
established on both loamy and sandy soil. Two mowing regimes intended to control summerflowering weeds in the Midwest (Kindscher 2008, Randa and Yunger 2001, Sargent and Carter
2001) were used: i) mowing in late June, when many summer-flowering weeds were producing

16

inflorescences, and ii) mowing in both late June and late July, when both weeds and native
grasses were flowering. The following questions were addressed in this research: 1) Does
summer mowing control weeds? 2) How does summer mowing affect desirable native species?
and 3) Is there any benefit from mowing twice within a growing season? I hypothesized that: 1)
summer mowing a) suppresses growth and reproduction of summer-flowering weeds, and b)
reduces size and seed production of mature native C4 grasses; and that 2) repeated mowing
within a growing season will cause greater size reductions of desirable plant species.
Methods
SITE DESCRIPTIONS AND LAND USE HISTORY
This study was conducted from May 2009 (Yr 1) to October 2010 (Yr 2) at two locations
in Ingham County, Michigan: i) Michigan State University‟s Horticulture Teaching and Research
Center in East Lansing, MI, and ii) Wildtype Native Plant Nursery in Mason, MI, approximately
11 km southeast of the East Lansing location (Fig. 1). Each site contained both sandy and loamy
soil plots: one sandy plot at each site, two loamy plots at East Lansing, and one in Mason (Fig.
2a, b). The East Lansing sandy plot contained excavated fill (loamy sand), and the Mason sandy
plot was established on Lamellic hapludalf (Alfisol) (Schrotenboer et al., in prep.). Soils in all
loamy plots were Aquic glossudalf (Alfisol) (Schrotenboer et al., in prep.). Mason plots were
planted in a corn-soybean-wheat rotation until prairie grass establishment in 2006, and were
adjacent to corn in Yr 1 and soybeans in Yr 2. In East Lansing, past cultivation of the loamy
plots varied annually due to research center use, and the sandy plot was never cultivated. East
Lansing plots were adjacent to various agricultural research crops. Average growing season
temperature in this region is 18.75°C and cumulative annual precipitation is 69.95 cm/yr. In Yr

17

1, average growing season temperature was 18.14°C and cumulative annual precipitation was
86.28 cm/yr. In Yr 2, average growing season temperature was 20.21°C and cumulative annual
precipitation was 63.83 cm/yr (Enviro-weather 2011).
Plots were initially established in June 2006 as a common garden experiment to assess
the influence of cultivation and local adaptation on native plant performance. Six varieties of A.
gerardii and five varieties of S. scoparium were planted to represent the range of seed sources
commonly used in Michigan prairie restorations (wild-Michigan, cultivated-Michigan and
cultivated-non-local populations) (Schrotenboer et al., in prep.). In summer of Yr 1, 1,035 of the
original 1,128 plants remained. All surviving A. gerardii and S. scoparium plants were in their
fourth growing season when mowing treatments were first applied. In summer of Yr 2, I
measured only A. gerardii; 607 of the original 616 plants remained.
All plots contained 19 blocks (2 m x 7 m each), except the East Lansing sandy plot,
which contained 18 (Fig. 3). Within each block, 12 individual grass plants were planted: one
each of the 11 varieties and a 12th randomly selected from among them. We harvested A.
gerardii plants annually in October 2006 through 2010. S. scoparium plants were harvested in
October of 2007 and 2009. Mowing treatments were applied in 2009 and 2010, so only data
from these years is presented in this paper. In spring of Yr 1, I established one quadrat in the
center of each block (0.5 m x 1 m) to monitor colonizing species. Flags marked the corner of
each quadrat to ensure consistent sampling of the same quadrats between sampling periods and
years.
Prior to this mowing experiment, all weeds within blocks were removed by hand and
spot-treated with glyphosate as needed from spring 2006 until spring 2009. Weeding ceased at

18

the end of May 2009 to allow time for summer-flowering weed establishment. No weeds were
removed or treated with glyphosate in 2010.
CATEGORIZATION OF COLONIZING PLANTS
All plant species were categorized into one of three groups for analysis: desirable native
species, Michigan-listed noxious species (all but one exotic; hereafter referred to as MI-noxious),
and nuisance species (native and exotic). Scientific names follow the Michigan Flora Online
(Reznicek et al. 2011). Desirable species included three native grasses (A. gerardii, S.
scoparium and Poa alsodes A. Gray) and three native forbs (Coreopsis lanceolata L., Epilobium
coloratum Biehler, and Symphyotrichum lanceolatum (Willd.) G.L. Nesom). Species were
placed in the MI-noxious category if they appeared on the Michigan Department of Agriculture
Prohibited and Restricted Weeds list (Pesticide and Plant Pest Management Division 2002).
These plants are regulated under Michigan state law because they have the potential to cause
significant economic losses or cause environmental problems.
Plants were categorized as nuisance species if they appeared in common weed keys for
the Midwest (Bryson and DeFelice 2010, Uva et al. 1997), excluding those also listed as
prohibited and restricted weeds. In addition, I included the one woody species encountered, Acer
rubrum L. because it is not a prairie species. Plants in the nuisance species category are not
regulated by state law, but are often problematic. Many of these species are common in
disturbed areas, on waste ground, or in agricultural fields. Some native species encountered (e.g.
Solidago canadensis L.) are regarded as weeds because they reproduce aggressively and have a
tendency to become excessively abundant in a site. For these species, the goal of management is
not to eradicate them completely, but to reduce their dominance.

19

MOWING TREATMENT DESCRIPTIONS
Treatments consisted of 1) a no-mowing control, 2) June mowing, and 3) June and July
mowing (n = 6–7 blocks/treatment/site). Blocks were randomly assigned to each mowing
treatment and mowed to 15–20 cm above ground level using a Stihl FS90 trimmer with a „steel
brush knife‟ blade (Stihl Inc., Waiblingen, Germany). In Yr 1, mowed material was left on the
ground. In Yr 2, mowed A. gerardii material in the Mason plots was collected immediately
following mowing and weighed to assess total seasonal biomass production; biomass of S.
scoparium and weeds was left on the ground. Blocks were randomly assigned to each treatment
and mowed on June 24 only, or June 24 and July 22 in Yr 1. Mowing was repeated in Yr 2 on
June 28 only, or June 28 and July 26, depending on treatment.
GROWTH OF COLONIZERS
To monitor how mowing influenced the growth and phenology of colonizing plants, I
placed a removable 1 m x 0.5 m sampling quadrat in each block between rows of mature grasses.
Percent cover of each species present was measured in quadrats in August of both years.
Seedlings of all plants in quadrats were counted in May of Yr 2, except seedlings of low-profile
species (e.g. Trifolium spp.), which could not be counted. For these species, percent cover was
measured instead. All colonizing plants were identified to species level when possible, and
classified as described above. Plant biomass was harvested from quadrats at the Mason site in
late August of Yr 2.
COMMUNITY SEED PRODUCTION
To assess the impact of mowing on seed deposition, seed traps were installed in each
block at the Mason plots in July of Yr 2. Traps consisted of a PVC pipe placed in the ground (1-

20

2 cm above soil surface), with a 10.8 cm diameter funnel placed on top, and a mesh bag attached
to the bottom of the funnel. Trap bags were exchanged every other week, July to October. All
seeds were identified to species level when possible. Seeds of several species from the family
Asteraceae could not be distinguished and were grouped.
GROWTH AND FLOWERING OF MATURE GRASSES
I measured the growth of A. gerardii in Yrs 1–2, and that of S. scoparium in Yr 1. Height
of the tallest tiller and basal area of each plant were measured in September. For biomass
measures, plants were cut at a 15 cm height in October, placed in a paper bag, dried at 70°C for
48-72 h, then weighed (In Yr 2, plants were harvested only from the Mason plots). Grass
biomass removed by mowing treatments was also collected in Yr 2 and similarly processed.
Number of inflorescences was counted on two varieties of A. gerardii on sandy and
loamy soils in Mason (40 individuals each) in late September of Yr 1. Number of flowering
tillers (all varieties) was counted in September of Yr 2 at both Mason plots.
STATISTICAL ANALYSIS
Data were analyzed as a randomized complete block design using analysis of variance in
PROC MIXED (SAS Institute, 2009). Statistical models for colonizer percent cover and
seedling counts included the fixed effects of mowing treatment, soil, and site, and the random
effect of block (nested in site*soil). Analysis of mature grass growth included fixed effects of
mowing treatment, population type, soil, and site (East Lansing or Mason), and random effects of
block (nested in site*soil) and population (nested in population type). A. gerardii and S.
scoparium were analyzed using separate ANOVAs. A. gerardii basal area and colonizer percent
cover were analyzed using repeated measures ANOVA to assess differences between years;

21

components of these models were as described above. Native grass flowering models included
mowing treatment, population type, and soil as fixed effects, with block (nested in soil) and
population (nested in population type) as random effects. Models of community seed production
included mow and soil as fixed effects and block (nested in soil) as a random effect. Plots were
analyzed separately when the mowing treatment*soil interaction was significant. Two preplanned contrasts were used in each analysis: mowing vs. no mowing, and June vs. June and July
mowing. When the main effect of mowing is significant, results of contrasts are presented.
Significance was at the α = 0.05 level for all tests. Results were considered marginally
significant if α ≤ 0.1. Square-root and log transformations were used when necessary to meet the
assumptions of normality.

22

Figure 1. Sites in Ingham County, Michigan. East Lansing and Mason sites are approximately
11 km apart. For interpretation of the references to color in this and all other figures, the reader
is referred to the electronic version of this thesis.

23

Figure 2. Plot placement at each site: a) East Lansing, and b) Mason.

24

Figure 3. Representative plot layout containing 19 blocks. Outer border represents plot boundaries. Solid circles are
mature native grass plant individuals, dashed rectangles are colonizer monitoring quadrats, and solid rectangles are block
boundaries.

25

Results
COLONIZING SPECIES
After weed control efforts ceased (spring of Yr 1), colonizers invaded all mowed and unmowed blocks. Seventy-three colonizing plant species were present in the study plots (Table 1).
Of these, 67 were categorized as weeds (13 MI-noxious species, 54 nuisance species), and six as
desirable natives. Four species could not be identified and were excluded from subsequent
analysis. Colonizer species richness was greater on loamy plots (36–40 species) than on sandy
plots (23–30 species). Here, I present the effects of mowing on these key categories of
colonizing plants; mowing effects on individual species are listed in Table 2.
MOWING EFFECTS ON WEEDS

Between Yr 1 and Yr 2, percent cover of MI-noxious species nearly tripled (F1, 91 =
17.42, p < 0.0001), though it remained the least abundant plant category. In Yr 1, MI-noxious
species constituted only 10% of total plant community cover (Fig. 4a). In Yr 2, this group
constituted 16% of total cover (Fig. 4b), 6% of total seedlings (Fig. 4c), and 4% of total seed
production (Fig. 4d). Mowing did not alter percent cover of MI-noxious species in Yr 1 or Yr 2
(F2, 82 = 0.32, p = 0.7257), although there was some suggestion in Yr 2 that mowing might
decrease cover with time (Fig. 5a). Mowing also did not influence May seedling counts of MInoxious species (F2, 82 = 0.47, p = 0.6253) (Fig. 6a), though plants had been mowed only once at
this point, or their seed production (F2, 32 = 1.39, p = 0.2646) (Fig. 7a).

26

Between Yr 1 and Yr 2, percent cover of nuisance species decreased by 10% (F1, 91 =
33.03, p < 0.0001), and this category was consistently the largest portion of the plant community.
In Yr 1, they constituted 88% of total plant community cover (Fig. 4a). In Yr 2 they constituted
53% of total cover (Fig. 4b), 59% of total seedlings (Fig. 4c), and 73% of total seed production
(Fig. 4d). Mowing marginally increased cover of nuisance species by about 25% in Yr 1 (F1, 167
= 3.74, p = 0.0547) and by 35% in Yr 2, relative to controls (F1,167 = 5.61, p = 0.019), but oncemowed and twice-mowed plants did not respond differently (Yr 1 F1, 167 = 0.23, p = 0.6348; Yr
2 F1, 167 = 0.18, p = 0.6714) (Fig. 5b). Mowing did not influence seedling counts of nuisance
species (F2, 81.8 = 0.57, p = 0.5697) (Fig. 6a), or percent cover of low-profile nuisance species
(F2, 66 = 0.59, p = 0.5591) (Fig 6b).

There was a significant interaction between mowing treatment and soil (which was
unreplicated) (F2, 32 = 3.24, p = 0.0521), so I analyzed data from each soil type independently.
In the loamy soil plot, mowing reduced nuisance species seed production by about 80% (F1, 16 =
16.78, p = 0.0008), with both mowing treatments having similar effects (F1, 16 = 0.02, p =
0.8848) (Fig. 7c). In contrast, mowing did not alter seed production in the sandy soil plot (F2, 16
= 0.07, p = 0.9305) (Fig. 7c). In both plots, Asteraceae species accounted for nearly all seed
production: 85% of nuisance seeds in the loamy plot and 89% of nuisance seeds in the sandy
plot. These seeds could not be positively identified to species, but only one Asteraceae species
was abundant in percent cover measurements from each plot. In the loamy plot, Solidago

27

canadensis constituted 52% of August nuisance species cover, while Erigeron annuus (L.) Pers.
constituted only 1%. In the sandy plot, Conyza Canadensis (L.) Cronq. constituted 5% of
nuisance species cover.
Mowing decreased colonizer biomass (excluding native C4 grasses) in the loamy plot by
53% (F1, 16 = 23.72, p = 0.0002), which was similar for both mowing treatments (F1, 16 = 0.01, p
= 0.9099), but had no effect on biomass in the sandy plot (F2, 16 = 1.64, p = 0.2256) (Fig 8a).
The colonizer biomass was primarily composed of weeds, since desirable natives (excluding
native C4 grasses) comprised only 6% of total percent cover in these plots.
MOWING EFFECTS ON DESIRABLE NATIVES

Desirable native colonizer cover increased 23-fold between Yr 1 and Yr 2 (F1, 91 =
269.27, p = p < 0.0001); the native cover increase was mostly A. gerardii in loamy plots, and
mostly S. scoparium in sandy plots. Basal area of the mature A. gerardii, which were three and
four years old, increased by 28% during the same period (F1, 603 = 420.28, p < 0.0001), but
height and end-of-season biomass did not change. Desirable native colonizers were the smallest
component of the plant community in Yr 1 (2% of total plant community cover) (Fig. 4a), but
were more abundant than MI-noxious species in Yr 2 (31% of total cover, 35% of total seedlings,
and 23% of total seed production) (Fig. 4b, c, d). Mowing increased percent cover of desirable
native species (F2, 82 = 3.13, p = 0.0490) (Fig. 5c), but only the June vs. JuneJuly contrast in Yr
2 was marginally significant (F1, 171 = 2.81, p = 0.0953). May seedling counts tended to be
greater in mowed blocks, but differences were not significant (F2, 81.7 = 0.22, p = 0.8015) (Fig.

28

6a). Mowing decreased desirable native seed production by 61–76% (F1, 32 = 34.03, p < 0.0001)
(Fig. 7c). Twice-mowed blocks were not significantly different from once-mowed blocks (F1, 32
= 1.19, p = 0.2828). Biomass of colonizing native C4 grasses increased by 55–247% in mowed
blocks (F1, 32 = 5.3, p = 0.0280) and blocks mowed twice contained 55% less biomass than
blocks mowed once (F1, 32 = 5.61, p = 0.0241) (Fig. 8b).

In mature A. gerardii, mowing reduced height by 12–25% in Yr 1 (F1, 81.1 = 125.51, p <
0.0001) and by 17–34% in Yr 2 (F1, 107 = 547.22, p < 0.0001) (Fig. 9a). In S. scoparium (only
measured in Yr 1), mowing reduced end-of-season height by 7–17% (F1, 75.1 = 35.23, p <
0.0001) (Fig. 9a). Grasses mowed twice in a season were shorter at the end of the season than
those mowed only once. Twice-mowed A. gerardii was 15% shorter than once-mowed plants in
Yr 1 (F1, 87.4 = 47.03, p < 0.0001), and 21% shorter in Yr 2 (F1, 134 = 189.45, p < 0.0001).
Twice-mowed S scoparium was 11% shorter in Yr 1 than once-mowed plants (F1, 78.4 = 20.63, p
< 0.0001).

Mowing reduced the basal area of mature A. gerardii by 14–15% in Yr 1 (F1, 103 =
15.69, p = 0.0001) and 9–21% in Yr 2 (F1, 103 = 26.45, p < 0.0001) (Fig. 9b). The second
mowing treatment had no effect on basal area in Yr 1 (F1, 112 =0.01, p =0.9153), but reduced it
by an additional 13% in Yr 2 (F1, 112 = 8.7, p = 0.0039). S. scoparium basal area was not altered
by mowing (F2, 86.3 = 1.63, p = 0.2023) (Fig. 9b).

29

In Yr 1, mowing substantially reduced end-of-season biomass of mature A. gerardii by
56–83% (F1, 80.7 = 500.21, p < 0.0001), and that of S. scoparium by 47–67% in Yr 1 (F1, 78.5 =
97.81, p < 0.0001) (Fig. 9c). The second mowing decreased A. gerardii biomass by an
additional 62% (F1, 88.5 = 104.63, p < 0.0001) and S. scoparium biomass by 37% (F1, 79.7 =
15.81, p < 0.0001). In Yr 2, total harvested biomass of A. gerardii in mowed blocks, which
included both biomass removed by mowing and residual biomass remaining in October, was 15–
50% lower than in control blocks (F1, 31.7 = 30.97, p < 0.0001), and was 40% lower in twicemowed blocks than in once-mowed blocks (F1, 32.3 = 15.77, p = 0.0004) (Fig. 9c).

Mowing reduced inflorescences and flowering tillers in mature A. gerardii, and these
mowing effects were greater for twice-mowed plants than for once-mowed plants. Plants in
mowed blocks had 51–87% fewer inflorescences at the end of Yr 1 (F1, 33.4 = 73.64, p < 0.0001)
(Fig. 10a) and 38–77% fewer flowering tillers in Yr 2 (F1, 32.4 = 106.93, p <0.0001) (Fig. 10b).
The second mowing event decreased inflorescence counts in Yr 1 by an additional 74% (F1, 33.6
= 21.33, p < 0.0001), and reduced flowering tillers in Yr 2 by an additional 64% (F1, 32.3 = 56.7,
p < 0.0001). Reductions in flowering tillers appeared to be correlated to reductions in vegetative
tillers (Colin Phillippo, pers. obs.).
MOWING RESPONSE DIFFERENCES BETWEEN SOIL TYPES
Plant responses to mowing treatments differed between soil types for some
measurements. End-of-season biomass of both A. gerardii (F2, 84.4 = 8.39, p = 0.0005) and S.

30

scoparium (F2, 78 = 5.01, p = 0.0090) in Yr 1 was more reduced by mowing in sandy plots than
in loamy plots. In contrast, Yr 2 A. gerardii height (F2, 101 = 7.12, p = 0.0013), total A. gerardii
biomass (F2, 32 = 9.35, p = 0.0006), and colonizer biomass (excluding native C4 grasses) (F2, 32
= 4.87, p = 0.0143) were reduced more by mowing in loamy soil plots than in sandy soil plots.

31

Table 1. Plant species found in study plots. Each species was categorized as either a Michigan-listed noxious species (MI-noxious), a
nuisance species, or a desirable native. All species names follow Reznicek et al. (2011).
Category
MInoxious
species

Scientific name
Abutilon theophrasti Medik.
Berteroa incana (L.) DC.
Centaurea stoebe subsp. micranthos (Gugler) Hayek
Cirsium arvense (L.) Scop.
Cirsium vulgare (Savi) Ten.
Convolvulus arvensis L.
Daucus carota L.
Elymus repens (L.) Gould
Plantago lanceolata L.
Rumex crispus L.
Sinapis arvensis L.
Solanum ptychanthum Dunal
Sonchus arvensis L.

Common name
Velvet-leaf
Hoary alyssum
Spotted knapweed
Canada thistle
Bull thistle
Field bindweed
Queen Anne's lace
Quack grass
Narrow-leaved plantain
Curly dock
Wild mustard
Black nightshade
Field sow-thistle

Native Status
Introduced
Introduced
Introduced
Introduced
Introduced
Introduced
Introduced
Introduced
Introduced
Introduced
Introduced
Native
Introduced

Duration
Annual
Ann./bienn./per.
Biennial/per.
Perennial
Biennial
Perennial
Biennial
Perennial
Ann./bienn./per.
Perennial
Annual
Annual
Perennial

Nuisance
species

Acer rubrum L.
Achillea millefolium L.
Amaranthus retroflexus L.
Ambrosia artemisiifolia L.
Anagallis arvensis L.
Arenaria serpyllifolia L.
Asclepias syriaca L.
Bidens frondosa L.
Bromus secalinus L.
Bromus tectorum L.
Carex vulpinoidea Michx.

Red maple
Yarrow
Rough amaranth
Common ragweed
Scarlet pimpernel
Thyme-leaf sandwort
Common milkweed
Common beggar-ticks
Cheat
Cheat grass
Sedge

Native
Native/introduced
Native
Native
Introduced
Introduced
Native
Native
Introduced
Introduced
Native

Perennial
Perennial
Annual
Annual
Annual/biennial
Annual
Perennial
Annual
Annual
Annual
Perennial

32

(Table 1. continued)
Nuisance Cerastium fontanum Baumg.
species Chenopodium album L.
Cichorium intybus L.
Conyza canadensis (L.) Cronq.
Crepis tectorum L.
Dactylis glomerata L.
Echinochloa crusgalli (L.) P. Beauv.
Equisetum arvense L.
Erigeron annuus (L.) Pers.
Euphorbia maculata L.
Fallopia convolvulus (L.) Á. Löve
Geranium maculatum L.
Hieracium caespitosum Dumort.
Juncus tenuis Willd.
Lactuca serriola L.
Lepidium virginicum L.
Lolium perenne L.
Medicago lupulina L.
Mollugo verticillata L.
Oxalis stricta L.
Panicum capillare L.
Persicaria maculosa A. Gray
Phleum pratense L.
Plantago major L.
Poa annua L.
Polygonum aviculare L.
Portulaca oleracea L.

Mouse-ear chickweed
Lambs-quarters
Chicory
Horseweed
Hawk's beard
Orchard grass
Barnyard grass
Common horsetail
Daisy fleabane
Nodding spurge
Black-bindweed
Carolina crane's-bill
Yellow hawkweed
Path rush
Prickly lettuce
Common peppergrass
Ryegrass
Black medic
Carpetweed
Yellow wood-sorrel
Witch grass
Lady's thumb
Timothy
Common plantain
Annual bluegrass
Knotweed
Purslane

33

Introduced
Native/introduced
Introduced
Native
Introduced
Introduced
Introduced
Native
Native
Native
Introduced
Native
Introduced
Native
Introduced
Native
Introduced
Introduced
Native
Native
Native
Introduced
Introduced
Introduced
Introduced
Introduced
Introduced

Biennial/perennial
Annual
Biennial/perennial
Annual/biennial
Annual
Perennial
Annual
Perennial
Annual
Annual
Annual
Annual/biennial
Perennial
Perennial
Annual/biennial
Ann./bienn./per.
Perennial/annual
Annual/perennial
Annual
Perennial
Annual
Annual/perennial
Perennial
Perennial
Annual
Annual/perennial
Annual

(Table 1. continued)
Nuisance Potentilla argentea L.
species Potentilla norvegica L.
Ranunculus sceleratus L.
Schedonorus arundinaceus (Schreb.) Dumort.
Setaria viridis (L.) P. Beauv
Silene antirrhina L.
Silene latifolia Poir.
Solidago canadensis L.
Sonchus asper (L.) Hill
Taraxacum officinale F. H. Wigg.
Tragopogon dubius Scop.
Trifolium pratense L.
Trifolium repens L.
Urtica dioica L.
Verbascum thapsus L.
Veronica arvensis L.
Desirable Andropogon gerardii Vitman
native
Coreopsis lanceolata L.
species Epilobium coloratum Biehler
Poa alsodes A. Gray
Schizachyrium scoparium Michx.
Symphyotrichum lanceolatum (Willd.) G. L. Nesom

Silvery cinquefoil
Rough cinquefoil
Cursed crowfoot
Tall fescue
Green foxtail
Sleepy catchfly
White campion
Canada goldenrod
Prickly sow-thistle
Common dandelion
Goat's beard
Red clover
White clover
Stinging nettle
Common mullein
Corn speedwell

Introduced
Native
Native
Introduced
Introduced
Native
Introduced
Native
Introduced
Introduced
Introduced
Introduced
Introduced
Native/introduced
Introduced
Introduced

Perennial
Ann./bienn./per.
Annual/perennial
Perennial
Annual
Annual
Biennial/perennial
Perennial
Annual
Perennial
Annual/biennial
Biennial/perennial
Perennial
Perennial
Biennial
Annual

Big bluestem
Sand coreopsis
Cinnamon willow-herb
Bluegrass
Little bluestem
Panicled aster

Native
Native
Native
Native
Native
Native

Perennial
Perennial
Perennial
Perennial
Perennial
Perennial

34

Table 2. Yr 2 responses of seedlings of individual plant species to mowing. The one-mow effect was calculated as the % difference
between the response of control plants and that of June-mowed plants. The two-mow effect was calculated as the % difference
between the response of control plants and that of plants mowed in June and July. “0” indicates no change in seedling counts. One
symbol in either direction (“-“ or “+”) represents a 10–25% change. Two symbols (“--“ or “++”) represents a 25–50% change. Three
symbols (“---“ or “+++”) represents a change > 50%.
Yr 2 seedlings
One mow
Two mow
Category Scientific name
Common name
effect
effect
MIBerteroa incana (L.) DC.
Hoary alyssum
----noxious Cirsium arvense (L.) Scop.
Canada thistle
0
-species Daucus carota L.
Queen Anne's lace
0
Plantago lanceolata L.
Narrow-leaved plantain
-+++
Rumex crispus L.
Curly dock
---Sonchus arvensis L.
Field sow-thistle
+++
++
Nuisance
species

Ambrosia artemisiifolia L.
Arenaria serpyllifolia L.
Bromus tectorum L.
Chenopodium album L.
Conyza canadensis (L.) Cronq.
Erigeron annuus (L.) Pers.
Geranium carolinianum L.
Hieracium caespitosum Dumort.
Juncus tenuis Willd.
Lactuca serriola L.
Lepidium virginicum L.
Lolium perenne L.
Medicago lupulina L.
Oxalis stricta L.
Panicum capillare L.
Schedonorus arundinaceus (Schreb.) Dumort.

Common ragweed
Thyme-leaf sandwort
Cheat grass
Lambs-quarters
Horseweed
Daisy fleabane
Carolina crane's-bill
Yellow hawkweed
Path rush
Prickly lettuce
Common peppergrass
Ryegrass
Black medic
Yellow wood-sorrel
Witch grass
Tall fescue
35

+++
+++
--+++
-----+++
--+++
+++
++
+++
--++

+++
---+++
--0
-----+++
+
+++
++
+++
-

(Table 2. continued)
Nuisance Solidago canadensis L.
species Sonchus asper (L.) Hill
Taraxacum officinale F. H. Wigg.
Tragopogon dubius Scop.
Trifolium pratense L.
Trifolium repens L.
Desirable Andropogon gerardii Vitman
species Coreopsis lanceolata L.
Epilobium coloratum Biehler
Schizachyrium scoparium Michx.
Symphyotrichum lanceolatum (Willd.) G. L. Nesom

Canada goldenrod
Prickly sow-thistle
Common dandelion
Goat's beard
Red clover
White clover

+++
---+++

++
+++
++
++
+++

Big bluestem
Sand coreopsis
Cinnamon willow-herb
Little bluestem
Panicled aster

+++
0
--+++
+++

-+++
--+++
+++

36

Figure 4. Composition of colonizing plant community summed across mowing
treatments, as measured by a) Yr 1 % cover in August, b) Yr 2 % cover in August,
2

c) number of seedlings per m in May of Yr 2, and d) number of seeds trapped in
10.8 cm diameter seed traps from July to October of Yr 2.

37

Figure 5. Percent cover of colonizing species in August of Yr 1 and Yr 2: a) MI-noxious
species, b) nuisance species, and c) desirable natives. Means ± one standard error are presented.

38

Figure 6. Seedling density of
colonizing species in May of Yr 2:
a) MI-noxious species, nuisance
species, and desirable natives; and
b) low-profile nuisance species
percent cover. Means ± one
standard error are presented.

39

Figure 7. Mowing effects on seed production of
colonizers measured from July to October of Yr 2
using 10.8 cm diameter seed traps: a) MI-noxious; b)
nuisance species on the loamy plot and on the sandy
plot; and c) desirable natives. Means ± one standard
error are presented.

40

Figure 8. Mowing effects on colonizer biomass harvested
in late August of Yr 2 (dry weight): a) all species
(excluding native C4 grasses) in the loamy plot, and in the
sandy plot; and b) native C4 grass biomass (A. gerardii and
S. scoparium). Means ± one standard error are presented.

41

Figure 9. Mean responses of individual mature plants to mowing. In Yr 1, plants were three
years old. The panels show Yr 1 measures for S. scoparium and Yr 1 and Yr 2 measures for
A. gerardii: a) end-of-season height, b) end-of-season basal area, and c) aboveground
biomass. Panel c shows a further breakdown of values. In Yr 1, residual biomass was
harvested in October. In Yr 2, A. gerardii mowed biomass was also collected after the June
(„Ju‟) and July („Jy‟) mowing. Means ± one standard error are presented.

42

Figure 10. Flowering response of
individual mature A. gerardii to mowing.
a) Yr 1 inflorescence counts, and b) Yr 2
flowering tiller counts. Means ± one
standard error are presented.
species in the loamy site, and all other
species in the sandy site.

43

Discussion
Grassland restoration and management is a growing industry worldwide because of the
capacity for grasslands to provide multiple ecosystem services and conservation value (Weigelt
et al. 2009; Tilman et al. 2006). Because grasslands are generally disturbance-adapted, a notable
focus of grassland management lies in application of disturbance regimes that optimize
development and maintenance of desired grassland characteristics, such as species composition
and productivity. In regions such as the United States, many guidelines for prairie management
emphasize the need for prescribed burns (e.g., NRCS 2006, Packard and Mutel 1997), but
implementing prescribed burns can be logistically difficult, and thus many practitioners have
employed mowing as an alternative management tool. For North American tallgrass prairies,
quantitative information about the effects of different mowing regimes has been limited, and
guidelines about the use of mowing reflect conflicts between different management goals (e.g.,
NRCS 2006 vs. NRCS 2011).
This study advances understanding of mowing strategies during key summer periods and
their effects on dominant warm-season grasses, other desirable native species, and noxious and
nuisance weeds. I found that two years of summer mowing treatments increased cover and
biomass per unit area of colonizing native species, primarily C4 grasses, while reducing weed
biomass per unit area and decreasing nuisance seed production. Mowing reduced the size of
mature native grasses within the community as well, but the effects of one mow were notably
less deleterious than that of two mows. Mowing twice within a growing season did not increase
weed control.

44

SUCCESSIONAL PROCESSES
Many of the nuisance species in this study are considered to be early successional and
often are found in disturbed, nutrient-poor sites (Bryson and DeFelice 2010, Reznicek et al.
2011, Uva et al. 1997). These annual and short-lived perennial plants are frequently displaced
over time by longer-lived perennial species. In this study, successional processes were evident in
the un-mowed control blocks, in which percent cover of nuisance species declined while basal
area of mature A. gerardii and percent cover of native species increased between years. These
changes indicate that the plant community at these sites would likely become increasingly
dominated by longer-lived perennials in the future if no further management were applied
(Kosola and Gross 1999).
INFLUENCE OF SUMMER MOWING ON WEEDY COLONIZERS
Mowing can both stimulate and inhibit plant growth, depending on the traits of the
species to which it is applied and the timing of its application (Hover and Bragg 1981, Vogel and
Bjugstad 1968, Wilson and Clark 2001). Mowing has been used to reduce the growth and
reproduction of non-native weedy species in numerous grassland ecosystems, and seems to be
most effective when weeds are mowed at or near the flowering stage. Much of this research has
focused on control of invasive grass species. For example, mowing was shown to reduce the size
of several non-native, cool-season grasses, including quack grass (Elymus repens (L.) Gould) in
Wisconsin prairies (Diboll 1986), tall oatgrass (Arrhenatherum elatius (L.) J. Presl & C. Presl) in
Oregon (Wilson and Clark 2001), and both Kentucky bluegrass (Poa pratensis L.) and orchard
grass (Dactylis glomerata L.) in British Columbia, Canada (MacDougall and Turkington 2007).

45

However, mowing had no effect on cover of the invasive warm-season King Ranch bluestem
(Bothriochloa ischaemum (L.) Keng) in Texas (Simmons et al. 2007).
Studies of mowing on weedy forbs in grasslands, such as those examined here, appear to
be fewer but also have found notable effects. For example, mowing reduced the density (Rinella
et al. 2001) and seed production (Benzel et al. 2009) of spotted knapweed (Centaurea stoebe L.)
in Montana, and reduced biomass and seedhead production of yellow starthistle in California
(Benefield et al. 1999). Likewise, I found that summer mowing reduced the biomass per unit area
of the weedy forb species (which were much more abundant than weedy grasses) invading my
experimental prairie grass plantings, with the most marked effect seen on loamy soils.
More critically, summer mowing also reduced seed production of nuisance weed species,
which were the dominant weed group at my sites. This effect was not evident for MI-noxious
weed species, in part due to their relative lack of abundance at the study sites; nearly 30% of
blocks contained no MI-noxious seedlings at the beginning of Yr 2, while only one block (out of
94) lacked nuisance species.
The response of nuisance species to mowing in tallgrass prairie communities has received
somewhat less attention than that of noxious weeds. In other grasslands, mowing typically
increases abundance of short-statured species such as white clover, which are not directly
affected by mowing, but benefit from increased light availability (Fulkerson and Michell 1987).
In this study, the observed reduction in weedy biomass and the reduction in nuisance species
seed abundance suggest that the effects of mowing might become more notable over time. If
seed rain or seed banks are limited, reduction in seed production could result in reduced seedling
recruitment in subsequent years (Rinella et al. 2001). At this site, as is likely at sites of other

46

agricultural field conversions, the seed bank was relatively species-poor (see Chapter 3) and thus
perhaps easier to manage than seed banks that have developed in weed-rich regions.
Evaluation of the weed metrics employed in this study suggests that biomass and seed
production are more useful measures than percent cover, which is strongly dependent on mowing
influence on plant structure. These mowing responses are influenced by the location of plant
meristems. In forbs, apical meristems are often located at stem tips and inhibit growth of other
branches. Removal of these meristems by mowing can cause rapid lateral branching, often
resulting in a prostrate growth form (Radosevich et al. 1997). In contrast, meristems on grasses
are located nearer to the base of plants, so this response to defoliation is less often observed.
Thus, mowing can often drive release of apical dominance in forbs when their apical meristems
are removed. For example, Randa and Yunger (2001) observed an increase in cover following
June mowing for the invasive white sweet-clover (Melilotus albus Medik.) in an Illinois prairie,
but they also observed a biomass reduction from mowing. In forb-dominated blocks in my
study, vertical plant growth removed by mowing was replaced by lateral branches from surviving
plants, which increased percent cover of nuisance species. However, the biomass of nuisance
species did not in fact increase in response to mowing; in the loamy plot it decreased, and in the
sandy plot it was not affected. Instead, the growth form of dominant forb species, most notably
Canada goldenrod (Solidago canadensis L.), became more lateral than vertical in response to
mowing (Colin Phillippo, pers. obs.). These results illustrate that mowing of some forb species
may appear to increase their abundance (i.e., more ground area covered), but biomass of these
plants is reduced.

47

INFLUENCE OF SUMMER MOWING ON NATIVE SPECIES
Consistent with management recommendations to mow newly-established prairies
(NRCS 2006, Packard and Mutel 1997), I found that June mowing promoted notable increases in
the cover and biomass per unit area of young plants of native C4 grass species, with A. gerardii
showing the greatest responses on loamy soil and S. scoparium on sandy soil. However, in
contrast to recommendations that young plants should be mowed as frequently as every 30 days
(NRCS 2006) the June mow treatment was more beneficial than the two-mow June and July
treatment, in which the biomass response wasn‟t significant.
Mowing has long been found to reduce size of more mature native C4 prairie grasses,
particularly during peak plant growth in the summer (Aldous 1930, Randa and Yunger 2001,
Vogel and Bjugstad 1968). However, Owensby (1974) saw no effect of a single clipping on A.
gerardii tiller density, but found that tiller density rose with two or more clippings. Consistent
with the former set of observations, the mature native grasses examined in this study (A. gerardii
and S. scoparium) were negatively affected by mowing treatments, with effects influenced by
soil type.
Flowering of native plant species can be notably altered by summer mowing treatments.
For example, June mowing decreased flowering stem density and height of A. gerardii and S.
scoparium in June-mowed plots in a Nebraska prairie (Hover and Bragg 1981). However,
MacDougal & Turkington (2007) observed an increase in the flowering of native plants from
July mowing. In Texas, July-mowed switchgrass (Panicum virgatum) (Haferkamp and Copeland
1984), and May-mowed S. scoparium and sideoats grama (Bouteloua curtipendula (Michx.)
Torr.) produced more tillers than un-mowed plants did, but these plants were mowed prior to

48

their flowering stages, which likely stimulated inflorescence production. Mature native grasses
in my study were mowed at peak flowering time, and mowed plants consequently had fewer
inflorescences, since little time remained for re-growth.
Frequent mowing during the growing season typically has greater influence on native
grasses than a single mowing. Previous studies of both A. gerardii (Aldous 1930) and S.
scoparium (Aldous 1930, N‟Guessan and Hartnett 2011) have shown that twice-mowed mature
grasses were more negatively affected than once-mowed plants. Frequent clipping has had
mixed results in other studies. Wallace (1987) showed that clipping treatments removing 1020% of leaf area every six weeks stimulate tillering of S. scoparium, but Owensby (1974)
reported that clipping more than once during the growing season can actually reduce tillering of
A. gerardii. In my June and July mowed plots, plants grew little after the second mowing
treatment, and few tillers of mature plants were apparent at the end of the growing season. Even
though a single mowing treatment increased colonizing native grass biomass, this increase was
no longer evident when blocks were mowed twice (June and July). Here, I have show that there
are increased costs of repeated mowing on native grasses, and no apparent benefits.
DURATION, FREQUENCY, AND TIMING OF MOWING
Mowing for several years may be necessary before differences in plant community
composition become apparent (MacDougall and Turkington 2007, Packard and Mutel 1997).
Short-term mowing can stimulate production of belowground structures such as rhizomes
(Dickinson and Polwart 1982), so a long-term commitment to mowing (i.e. more than two years)
is particularly important for controlling rhizomatous species, such as Canada thistle (Cirsium
arvense (L.) Scop. (Sheley et al. 2003).

49

In addition to the duration of mowing, the frequency of mowing needed for weed control
is an important consideration (Sheley et al. 2003). Two studies on spotted knapweed in Montana
prairies found that repeated mowing conferred few or no additional weed control benefits beyond
those obtained by mowing just once (Benzel et al. 2009, Rinella et al. 2001). Similarly, mowing
yellow starthistle in California at the early flowering stage is as effective as repeated mowing at
other growth stages (Benefield et al. 1999). Likewise, I found that mowing both in June and July
did not produce results that differed from those of a single June mowing for any measurement or
weed category. In contrast, de Kroon et al. (1987) observed increased abundance of hairy cat‟s
ear (Hypochaeris radicataI L.) as mowing frequency increased in a grassland in the Netherlands,
because removal of inflorescences induced another flowering period.
The June and July mowing treatment may have had a greater negative effect on mature
native grasses than the single June mowing not only because of the increased mowing frequency,
but also because of the difference in mowing timing. However, mowing timing was not directly
addressed in this study, as all mowed plants were first mowed in June. In general, summer
mowing favors C3 species by suppressing late-germinating C4 species, and spring mowing has
the opposite effect (Hover and Bragg 1981, Howe 1999, Wilson and Clark 2001). July mowing
can be particularly detrimental to biomass production of native C4 grasses (Haferkamp and
Copeland 1984, Vogel and Bjugstad 1968) because plants have little time to re-grow biomass
removed by mowing. For weed control, management strategies targeting flowering stages of
weeds are most effective for long-term population reductions (Benefield et al. 2001, Benzel et al.
2009). Rinella (2001) also demonstrated that a single mowing treatment, if timed appropriately,
can be as effective for weed control as repeated mowing treatment applications. Similarly, I
showed that June-mowed nuisance species produced significantly fewer seeds than controls, but

50

a second mowing did not further reduce seed production. Determining demographic
characteristics of weed populations can improve weed control techniques by targeting key stages
(e.g. floral development) of undesirable plants (Jordan 1992, Magda et al. 2004).
Current prairie mowing recommendations suggest using spring (e.g. Prairie Nursery
2011) or fall (e.g. Sargent and Carter 1999) treatments in the first few years after planting. This
strategy is suggested to reduce weed abundance, particularly annuals, and to avoid harm to
summer-nesting birds, such as the Henslow‟s sparrow (Ammodramus henslowii) and
Grasshopper sparrow (Ammodramus savannarum). However, spring or fall mowing treatments
would be unlikely to reduce seed production by summer-flowering weeds, which are frequently
problematic. In addition, prairies were historically maintained by multiple disturbance regimes
varying in timing, frequency, and extent (Knapp et al 1999, Packard and Mutel 1997), and a
management approach using multiple treatment options may most effectively aid prairie
restoration. Mowing is a valuable tool in prairie restoration that is particularly influential for
summer-flowering plant species, and could be used to partially simulate historical summer
disturbances, provided precautions are taken to avoid harm to bird communities.
MOWING IN ADAPTIVE MANAGEMENT
Response of grasslands to implementation of disturbance regimes is community specific
because it depends in large part on the phenological and structural differences of the plant
species present, and their relative susceptibility to the particular disturbance applied at a
particular time point (Vogel and Bjugstad 1968). From this perspective, disturbance can be
thought of as a filter that selects for particular species on the basis of their specific traits. The
challenge in management is how to select and implement strategies that can increase the
abundance of desired species and reduce those that are less desired. To achieve this, managers

51

can benefit from flexibility in choosing tools appropriate for their particular systems and desired
outcomes. However, in an attempt to balance requirements of conflicting management goals,
guidelines for tallgrass prairie management have become relatively prescriptive. For example,
summer mowing is generally recommended only in the first few years of prairie establishment
(e.g., NRCS 2006; Packard and Mutel 1997). To protect wildlife in established prairies, it is
recommended that mowing occur only in late summer (August 1-20 in Michigan), and only
every three years (NRCS 2011), and these restrictions are compulsory on Conservation Reserve
Program lands, which provide economic incentives to landowners for converting cropland into
perennial plant cover.
This study, in conjunction with studies of weed control in other grasses (Benefield et al.
(1999), Benzel et al. 2009, and Rinella et al. (2001)), suggests that mowing applied immediately
before or during the flowering periods of summer-flowering weeds can help control their
populations in tallgrass prairie plantings. Moreover, June mowing may facilitate native grass
colonization of newly planted sites, where rapid colonization by dominant grasses is desirable.
In addition, June mowing reduces seed inputs from nuisance species, which can improve longterm weed control by minimizing inputs to the seed bank. Preventing seed production in the first
few years of a restoration could reduce weed populations in future growing seasons, and would
be especially beneficial if the majority of weedy species at a site were annuals. In sites with
perennial weeds, mowing for multiple years may be necessary to effectively reduce weed
populations, but this may also reduce populations of natives with similar phenologies.
Mowing has potential to play important roles in the management of multiple-use
ecosystems, such as biofuel crops or hay meadows, by increasing both total productivity
(Weigelt 2009) and plant species diversity (Antonsen and Olsson 2005), and by providing

52

control of summer-flowering weeds, as demonstrated here. While residual, end-of-season A.
gerardii biomass was reduced 50% by June mowing, reductions in total biomass (residual and
that removed by mowing) were only 15%. Cox (1988) observed that July mowing reduced
forage production of big sacaton (Sporobolus wrightii Munro ex Scribn.), but by the third
growing season after mowing treatments had ceased, mowed plants were similar to controls. I
observed no lasting effects of either mowing treatment in Yr 1 on mature A. gerardii growth in
Yr 2, suggesting that this species may be tolerant to biomass removal without experiencing longterm negative effects.
My work here indicates that summer mowing could be a valuable addition to the adaptive
management tool kit, and thus merits more consideration in management guidelines. This study
suggests that June mowing would likely be particularly valuable for cases in which grasslands
contained i) weedy summer-flowering forbs, ii) well established native C4 grasses, iii) little or no
C3 grasses, which were not abundant at my sites but could be stimulated by June mowing in
other situations (Hover and Bragg 1981, Wilson and Clark 2001), and iv) few well established,
vegetatively propagating perennial weeds. In addition, it is likely that mowing treatments would
need to be sustained over multiple years to effect control in systems where weed seed banks were
highly developed, and could contribute to weed abundance for many years (Jordan 1992).
The biggest conflict that managers of tallgrass prairies face in implementing summer
mowing to control summer flowering weeds is the need to prevent harm to birds that ground-nest
in grasslands during this same period. While mowing likely has less negative influence on
invertebrate and aquatic communities than alternative methods such as herbicide application
might, preservation of limited grassland bird populations is a key priority (Van Dyke et al. 2004).

53

Given this study's quantitative demonstration that June mowing offers weed control benefits
while maintaining productivity of native grasses, it would be helpful to evaluate multiple means
for implementing June mowing strategies that would also protect summer ground-nesting birds.
Such strategies could include conducting breeding bird surveys prior to mowing, or mowing only
a portion of a prairie each growing season. In addition, it would be valuable to develop precision
agricultural methods for locating and avoiding nesting birds during harvest periods.

54

Chapter 3
Assessing Seed Abundance in Prairies

To assess the potential for mowing to influence weed populations, it is important to
evaluate the composition of the seed bank and seed rain at a site, since both are important
determinants of plant community composition. Here, I describe methods for assessing seed bank
composition, and methods for assessing seed deposition in prairie ecosystems. Seed bank
species composition data and photographs taken during my own seed trap study are presented.
Methods for Conducting a Seed Bank Study
I used the greenhouse germination methods of Gross (1990) to assess seed bank
composition in my research sites. Seven soil cores (1.9 cm diameter x 5 cm depth) were taken
from each block in May and homogenized into a single sample per block (19 blocks per site).
Soil cores were spread in pots on Fafard superfine seedling mix (Conrad Fafard Inc., Agawam,
MA). Pots were incubated in a greenhouse and seedlings were counted periodically until
December.
An alternative method could also be used to assess seed bank composition. Gross and
Renner (1989) proposed a soil elutriation method, in which soil cores are collected and sieved to
separate seeds from soil. Seeds are then identified under a microscope. This method yields
comparable numbers of large seeds, but may miss smaller seeds (Gross 1990). Additional testing
is also needed to determine if seeds are viable. I chose the greenhouse germination method
because many of the weeds present in my research sites have very small seeds, and may be
missed by the soil elutriation method. Additionally, the soil elutriation method includes seeds
that are not viable, while the greenhouse germination method necessarily only counts viable
seeds.

55

Seed Trap Design and Use
One seed trap was installed in each block of the Mason plots in July. My traps were
modeled after those developed by Chabrerie and Alard (2005). Each trap consisted of a 7.6 cm
diameter, 22.9 cm long PVC pipe placed in the ground (1-2 cm above soil surface), with a 10.8
cm diameter funnel placed on top, and a mesh bag attached to the bottom of the funnel (Fig. 8).
Trap bags were exchanged every other week until October. Seeds from known species were
collected directly from plants and were photographed using a Dino-lite digital microscope
(Model # 413T, AnMo Electronics corp., New Taipei City, Taiwan) to aid in identification of
seeds in seed traps (Fig. 9). Additional species found in seed traps were also photographed (Fig.
9). All seeds were identified to species when possible. Seeds from several species in the family
Asteraceae could not be easily distinguished and were grouped.

Figure 11. Seed trap used to assess plant community
seed deposition.

56

Table 3. Abundance of species found in seed banks of sandy and loamy plots
3

in Mason, Michigan (600 cm of soil sampled per plot). Seven species did not
grow beyond the seedling stage and could not be identified. Here they are
listed separately as unknowns.
Site
Sandy
plot

Loamy
plot

Species
Amaranthus retroflexus L.
Ambrosia artemisiifolia L.
Andropogon gerardii Vitman
Chenopodium album L.
Conyza canadensis (L.) Cronq.
Mollugo verticillata L.
Oxalis stricta L.
Portulaca oleracea L.
Schizachyrium scoparium Michx.
Taraxacum officinale F. H. Wigg.
Unknown 1
Unknown 2
Unknown 3
Unknown 4
Unknown 5
Veronica arvensis L.
Amaranthus retroflexus L.
Ambrosia artemisiifolia L.
Andropogon gerardii Vitman
Chenopodium album L.
Conyza canadensis (L.) Cronq.
Cyperus esculentus L.
Echinochloa crusgalli (L.) P.
Beauv.
Elymus repens (L.) Gould
Epilobium coloratum Biehler
Geranium maculatum L.
Medicago lupulina L.
Oxalis stricta L.
Panicum capillare L.

57

Total seedling
count
2
1
2
16
44
40
22
2
3
1
1
1
1
1
1
26
20
1
51
23
194
1
10
4
5
7
1
26
1

(Table 3. continued)
Loamy
plot

Plantago major L.
Platanus spp.
Persicaria maculosa A. Gray
Portulaca oleracea L.
Potentilla spp.
Solanum ptychanthum Dunal
Sonchus asper (L.) Hill
Taraxacum officinale F. H. Wigg.
Unknown 2
Unknown 5
Unknown 6
Veronica arvensis L.

58

196
1
9
1075
40
3
27
63
69
2
2
3

Seed Photographs
Figure 12. Seeds collected directly from plants and from seed traps in Ingham county, MI.
Photographs were taken using 10x–200x magnification. The scale shown is divided into 1
mm increments.

a) Abutilon theophrasti Medik

b) Amaranthus retroflexus L.

c) Ambrosia artemisiifolia L.

d) Andropogon gerardii Vitman

59

(Figure 12. continued)

e) Arctium minus Bernh.

f) Arenaria serpyllifolia L.

g) Berteroa incana (L.) DC.

h) Bidens frondosa L.

i) Bromus secalinus L.

j) Bromus tectorum L.

60

(Figure 12. continued)

k) Carex vulpinoidea Michx.

l) Centaurea stoebe L.

m) Cerastium fontanum Baumg.

n) Chenopodium album L.

o) Cichorium intybus L.

p) Cirsium arvense (L.) Scop.

61

(Figure 12. continued)

q) Cirsium vulgare (Savi) Ten.

r) Conyza canadensis (L.) Cronq.

s) Coreopsis lanceolata L.

t) Crepis capillaris (L.) Wallr.

u) Cyperus esculentus L.

v) Daucus carota L.

62

(Figure 12. continued)

w) Echinochloa crus-galli (L.) P. Beauv.

x) Epilobium coloratum Biehler
.

y) Erigeron annuus (L.) Pers.

z) Fallopia convolvulus (L.) À. Löve

.

.

aa) Juncus tenuis Willd.

ab) Lactuca serriola L.

.

.
63

(Figure 12. continued)

ac) Lolium perenne L.

ad) Medicago lupulina L.
.

.

ae) Melilotus albus Medik.

af) Melilotus officinalis (L.) Pall.
.

.

ag) Mollugo verticillata L.

ah) Oxalis stricta L.

.

.
64

(Figure 12. continued)

ai) Panicum capillare L.

aj) Persicaria maculosa A. Gray

.
.

ak) Phalaris arundinacea L.

al) Poa alsodes A. Gray

.
.

am) Portulaca oleracea L.

.

an) Potentilla norvegica L.

65
.

(Figure 12. continued)

ao) Plantago lanceolata L.

ap) Plantago major L.

.
.

aq) Ranunculus scleratus L.

ar) Rumex crispus L.

.
.

as) Schedonorus arundinaceus (Schreb.)
Dumort.

at) Schizachyrium scoparium Michx.

66

.

(Figure 12. continued)

au) Setaria viridis (L.) P. Beauv.

av) Silene alba Poir.

.
.

aw) Silene antirrhina L.

ax) Solanum ptychanthum Dunal

.

.

ay) Solidago canadensis L.

az) Sonchus arvensis L.

67

(Figure 12. continued)

ba) Sonchus asper (L.) Hill

bb) Symphyotrichum lanceolatum (Willd.)
G. L. Nesom

.

.

bc) Taraxacum officinale F. H. Wigg.

bd) Tragopogon dubius Scop.

.
.

be) Trifolium pratense L.

bf) Trifolium repens L.

68

(Figure 12. continued)

bg) Urtica dioica L.

bh) Verbascum thapsus L.

.
.

69

LITERATURE CITED

70

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