y59 ff3 fs8 fc0 sc0 ls0 ws3">fested alfalfa (Medicago sativa L.) field twice annually
produced an 86% reduction in Canada thistle density af-
ter the first year [25]. This treatment provided complete
control after 4 years, demonstrating that desirable com-
petitive plant species (e.g. alfalfa) may enhance the suc-
cess of mowing. Overall however, these variable results
demonstrate the need for further evaluation of mowing as
part of a successful control strategy.
Revegetation has shown promise as a secondary tool
for long-term control of Canada thistle following other
control measures, and is becoming increasingly popular
as a component of integrated weed management pro-
grams. Regardless of the target weed, a lack of compete-
tion from desirable plants post-treatment often leads to
reestablishment of unwanted weeds [26,27]. Several
forbs and grasses have shown promise as effective com-
petitors with Canada thistle, including tall fescue (Sche-
donorus phoenix [Scop.] Holub), crownvetch (Securigera
varia [L.] Lassen), sudangrass (Sorghum bicolor [L.]
Moench ssp. drummondii [Nees ex Steud.] de Wet &
Harlan), annual ragweed (Ambrosia artemisiifolia L.),
common sunflower (Helianthus annuus L.), alfalfa, bien-
nial sweetclover (Melilotus sp.), and a mixture of peren-
nial ryegrass (Lolium perenne L.) and white clover (Tri-
folium repens L.) [1,28-32]. One study reported that the
use of competitive grasses for Canada thistle control was
as effective as herbicide application over a 3-year period
[33]. Unfortunately, most plants tested thus far as com-
petitors against Canada thistle are non-native species.
While use of non-natives is understandable because they
are frequently more aggressive, seeding non-native plants
is often strongly discouraged or prohibited in sensitive
settings. Clearly, land managers of sensitive settings face
a variety of challenges when attempting Canada thistle
control. The lack of effective long-term control measures
combined with the lack of research specific to their needs
translates into limited options for people tasked with re-
storing infested lands.
The following experiment was conducted to test the
effectiveness of clipping (to simulate field mowing) and
seeding plant species acceptable for use in sensitive set-
tings as control measures for Canada thistle. Specifically,
the objectives of this study were to: 1) determine the ef-
fects of clipping and grass competition on Canada thistle
growth, 2) compare the effectiveness of the different
grass seeding treatments for reducing Canada thistle
growth, and 3) determine the effect of Canada thistle on
the growth of each of the seeded grasses. Two North
American native grasses (western wheatgrass [Pascopy-
rum smithii {Rydb.} A. Löve] and streambank wheat-
grass [Elymus lanceolatus {Scribn. & J.G. Sm.} Gould
ssp. lanc eola tus ]) and one sterile commercial hybrid
cross between common wheat (Triticum aestivum L.) and
tall wheatgrass (Thinopyrum ponticum [Podp.] Z. W. Liu
& R. C. Wang) called Regreen were chosen for this re-
search.
We hypothesized that both clipping and grass compe-
tition would reduce Canada thistle shoot biomass, and
that the effect of the two factors together would be
greater than either alone. We also predicted that the grass
seeding treatments containing Regreen would reduce
Canada thistle growth more than the native grasses
seeded alone with Canada thistle because of the more
aggressive nature of Regreen. Additionally, we predi-
cted that grass growth would be inhibited by the presence
of Canada thistle.
2. Materials and Methods
A greenhouse study was conducted on potted Canada
thistle plants treated with combinations of clipping (used
to simulate mowing) and grass seeding. Two response
variables were measured: Canada thistle shoot biomass
and grass shoot biomass. Canada thistle biomass was
analyzed using a two by five factorial design consisting
of two levels of Canada thistle clipping (clipped, un-
clipped) and five levels of grass seeding (no grass,
streambank wheatgrass, western wheatgrass, Regreen,
or western wheatgrass + Regreen). Grass biomass was
analyzed using a three by four factorial design consisting
of three levels of Canada thistle (clipped, unclipped, ab-
sent) and four levels of grass seeding (streambank wheat-
grass, western wheatgrass, Regreen, or western wheat-
grass + Regreen). Combined, these two analyses invest-
tigated a total of 14 treatment combinations applied to
Canada thistle or grass plants (6 replicates per treatment)
grown in potting soil for 51 weeks in a greenhouse.
2.1. Selection of Grass Species
Western wheatgrass and streambank wheatgrass were
chosen for their aggressive underground growth habit,
early spring germination prior to Canada thistle growth,
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Canada Thistle (Cirsium arvense) Response to Clipping and Seeding of Competitive Grasses
1254
wide geographic and habitat range throughout the west-
ern United States, drought and extreme temperature tol-
erance, and broad availability [34-36]. These two species
have demonstrated prior success for control of other
weeds including Russian knapweed (Acroptilon repens
[L.] DC.) [37,38], leafy spurge (Euphorbia esula L.) [39],
cheatgrass (Bromus tectorum L.), and musk thistle (Car-
duus nutans L.) [40,41]. Western wheatgrass has also
provided effective control on sites infested with multiple
weed species [42]. Most importantly, western wheatgrass
has previously demonstrated some success against Can-
ada thistle [33]. The hybrid grass Regreen was chosen
for its ability to establish aggressive root growth more
quickly than native grasses, and its sterility increases
acceptability for use in sensitive settings.
2.2. Source of Plant Material
Western wheatgrass seed (variety “Arriba”) was obtained
from Pawnee Buttes Seed Inc. (Greeley, CO, USA).
Streambank wheatgrass seed (variety “Sodar”) was ob-
tained from Granite Seed (Lehi, UT, USA). Regreen
seed was obtained from Rainier Seed Company (Daven-
port, WA, USA). Canada thistle horizontal roots were
collected from a site near Fort Collins, Colorado, USA
(lat 40˚33'46"N, long 105˚00'24"W; 1491 m above sea
level). Horizontal roots were collected 26 August 2004,
placed in sealed plastic bags with soil collected from the
same location, and transported to the laboratory in a
cooler. The bagged soil was moistened and stored at 6˚C
in the dark for 8 weeks to prevent sprouting before use in
the experiment. Plant nomenclature follows the USDA
PLANTS Database [36].
2.3. Plant Preparation and Treatment
On 23 October 2004, refrigerated Canada thistle hori-
zontal root sections were cut into 2.5-cm long pieces
(diameter ranging from 0.2 to 0.65 cm) with a minimum
of one bud per piece. Pieces were soaked in 2.54 cm of
tap water in a refrigerated (6˚C) covered tray in the dark
for 28 h, then planted in rows at a depth of approximately
1.3 cm in 25- × 52-cm flat plastic trays filled with 2.5 cm
of wetted Scotts MetroMix 350 potting soil (Sun Gro
Horticulture, Bellevue, WA, USA). Each tray was tho-
roughly watered after planting and kept moist.
Germinated horizontal root pieces were transferred to
164 ml conetainers in December 2004. In late January
2005, the plants were transferred to 1.3-L pots and grown
for 7 weeks. Surviving plants were then transferred to
3.8-L plastic pots filled with Scotts MetroMix 350 pot-
ting soil (1 plant per pot), and grown there for the re-
mainder of the experiment. After 10 weeks of postemer-
gence growth, 12 grass seeds were added to 48 of the 60
Canada thistle pots (Canada thistle control pots had no
seeded grasses). Each pot assigned to a grass seeding
treatment received 12 grass seeds (six of each species for
the two-species treatment). Western wheatgrass and stream-
bank wheatgrass seeds were planted approximately 1.3
cm deep, while Regreen seeds were planted approxi-
mately 0.6 cm deep. More seeds were planted than
needed to assure establishment of a sufficient number of
plants. Twenty four grass control pots of the same size
and growth medium as the Canada thistle pots were
seeded with only grasses as described above. The result
was six replicate pots for each treatment combination.
Six weeks after initial seeding, adequate grass seed
germination was obtained and pots were thinned to four
grass plants per pot. Regreen + western wheatgrass
treatments were thinned to two grass plants per species in
each pot. Because of limited project resources, only one
grass species combination treatment was evaluated.
Western wheatgrass was chosen for this combination
with Regreen because of its more aggressive rhizome-
tous growth habit than streambank wheatgrass [34]. At 17
weeks of Canada thistle growth when plants were
blooming, the single clipping treatment was performed,
cutting plants with hand shears 9 cm above the soil sur-
face to simulate mowing. This clipping height was cho-
sen because it is the approximate mowing height used in
field studies for mowing Canada thistle [2,43]. Clipped
shoot biomass from each Canada thistle plant was placed
in individual paper bags, dried at 55˚C to constant mass
and weighed to determine Canada thistle shoot biomass.
Grasses were not clipped.
Experimental plants grew in the greenhouse for an ad-
ditional 34 weeks post-clipping at approximately 22˚C ±
5˚C, and were watered as needed and weeded for nonex-
perimental species. Plants received natural light supple-
mented with 400-W high pressure sodium vapor bulbs
(1.5 m above greenhouse benches) to obtain a 16-h pho-
toperiod. Pot locations on the greenhouse bench were
re-randomized and pots were moved every 6 weeks to
minimize effects of potential differences in light or tem-
perature. For the final harvest, experimental plants were
clipped at the soil surface and separated into grass shoot
biomass or Canada thistle shoot biomass for each pot and
placed in separate paper bags. It was not possible to ac-
curately separate root biomass when multiple species
grew in the same pot, thus root biomass was not consi-
dered further. Plant material was dried at 55˚C to con-
stant mass and weighed to determine shoot biomass for
each plant species.
2.4. Statistical Analyses
Two dependent variables were analyzed: Canada thistle
shoot biomass and grass shoot biomass. Canada thistle
biomass was analyzed using a two by five factorial de-
Copyright © 2012 SciRes. AJPS
Canada Thistle (Cirsium arvense) Response to Clipping and Seeding of Competitive Grasses 1255
sign consisting of two levels of Canada thistle clipping
(clipped, unclipped) and five levels of grass seeding (no
grass, streambank wheatgrass, western wheatgrass, Re-
green, or western wheatgrass + Regreen). Grass bio-
mass was analyzed with a three by four factorial design
consisting of three levels of Canada thistle (clipped, un-
clipped, absent) and four levels of grass seeding (stream-
bank wheatgrass, western wheatgrass, Regreen, or
western wheatgrass + Regreen). The data were analyzed
using two separate univariate two-way analyses of vari-
ance (ANOVA); one for each dependent variable. Can-
ada thistle shoot biomass and grass shoot biomass data
were natural log transformed to meet assumptions of the
analyses. Post hoc pair-wise comparisons of interest were
conducted using Tukey’s Honestly Significant Difference
(HSD). All data were analyzed using R 2.8.1 statistical
software [44] and an alpha level of 0.05.
3. Results
Clipping reduced Canada thistle shoot biomass (F1,50 =
126.54, P < 0.001). Mean shoot biomass of Canada this-
tle in the clipped treatments (3.12 ± 0.20 g [mean ± SE],
n = 30) was lower than the unclipped Canada thistle
treatments (8.02 ± 0.41 g, n = 30), regardless of the pre-
sence of grass or species seeded. Clipped plants produced
less than half the shoot biomass of their unclipped coun-
terparts, despite having 34 weeks for regrowth. Grass
seeding did not affect shoot biomass of Canada thistle
(F4,50 = 0.78, P = 0.544), and there was no interaction
between clipping and grass seeding on Canada thistle
shoot biomass (F4,50 = 0.85, P = 0.500).
Grass shoot biomass was affected by the presence and
clipping status of Canada thistle (F2,56 = 43.47, P < 0.001),
and varied by grass species seeded (F3,56 = 37.35, P <
0.001), as indicated by the interaction between Canada
thistle treatment and grass species seeded (F6,56 = 2.38, P
= 0.040). The presence of unclipped Canada thistle re-
duced grass shoot biomass below that of the control
(Canada thistle absent) when grasses were grown indi-
vidually regardless of species (Figure 1). When grass
species were paired (Regreen + western wheatgrass),
however, the presence of unclipped Canada thistle did not
significantly reduce grass shoot biomass.
Western wheatgrass was the only grass treatment that
produced more grass shoot biomass when grown in the
absence of Canada thistle than when grown with clipped
Canada thistle (P = 0.024, Figure 1). Streambank wheat-
grass (P = 0.124), Regreen (P = 0.231), and the grass
combination (Regreen + western wheatgrass) (P =
0.999) produced similar amounts of shoot biomass re-
gardless of whether grown in the absence of Canada this-
tle or with clipped Canada thistle.
The two most successful grass treatments overall in
Figure 1. Mean grass shoot biomass by treatment for grass
species grown in the presence or absence of Canada thistle
(Cirsium arvense [L.] Scop.). Means (± SE, n = 6 except for
one treatment group where one outlier was eliminated) with
letters in common across all grass species and treatments
are not significantly different using Tukey’s HSD (α = 0.05).
terms of shoot biomass produced were Regreen, and
Regreen + western wheatgrass. Regreen + western
wheatgrass was the only grass treatment where the pre-
sence of unclipped Canada thistle did not produce a de-
tectable effect (P = 0.404). In fact, Regreen + western
wheatgrass was the only grass treatment where no de-
tectable difference in shoot biomass was found regardless
of Canada thistle presence or clipping status (Figure 1).
All other grass treatments (including western wheatgrass
and Regreen grown separately) produced less shoot
biomass in the presence of Canada thistle, at least in its
unclipped state.
4. Discussion
4.1. Canada Thistle Response to Clipping
The significant reduction in aboveground Canada thistle
biomass as a result of clipping, despite adequate time for
regrowth, confirms our hypothesis that clipping reduces
Canada thistle shoot biomass and supports the potential
utility of mechanical cutting as a control measure for
Canada thistle. Similar results have been demonstrated in
field studies. One study reported field mowing reduced
Canada thistle growth by 85% [2], while another reported
a 95% reduction in Canada thistle growth after 2 years of
field mowing [45]. In some cases, mowing has virtually
eliminated Canada thistle after 4 years [46].
Conversely, others have found that a one-time field
mowing treatment did not affect shoot biomass in the
year of treatment or 2 years later [47]. In a German study
similar to ours, a 2-year field experiment was conducted
where potted Canada thistle plants were clipped once
annually to simulate mowing [48]. The first year, clip-
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Canada Thistle (Cirsium arvense) Response to Clipping and Seeding of Competitive Grasses
1256
ping resulted in an increase in Canada thistle shoot bio-
mass compared to the unclipped control. It was only in
the second year that the clipped Canada thistle plants
responded in a manner similar to our study, where clip-
ped plants produced less shoot biomass than unclipped
controls.
One difference between the German study and ours
was the height of clipping. Clipping height was 30 cm in
their study [48], leaving a significant amount of photo-
synthesizing foliage behind, which likely facilitated first
year regrowth. Those authors theorized that their mild
clipping treatment mimicked moderate herbivory, where
plant growth would be stimulated by removal of top
growth. In our study, Canada thistle was clipped at a
standard mowing height of 9 cm, leaving little foliage
behind. This more substantial removal of plant material
likely inhibited regrowth by reducing photosynthetic car-
bohydrate production. Removal of plant top-growth
through activities such as mowing is believed to weaken
Canada thistle plants by inhibiting photosynthetic carbo-
hydrate production and transport, while also forcing de-
pletion of root carbohydrate reserves to support regrowth
after cutting [49]. This may explain the inability of the
clipped plants to fully recover after the second clipping in
the German study [48].
Another difference between the German study and ours
was the timing of clipping. Root carbohydrate reserves of
Canada thistle are considered to be lowest at the initiation
of flowering [46,50]. In our study, Canada thistle was
clipped when 99% of the plants were flowering. Clipping
our plants when carbohydrate reserves were low may
have enhanced the effectiveness of our clipping treatment.
In the German study, plants were clipped in June each
year, before flowering [48]. In their study, flowering oc-
curred later in the growing season, with only 6% of con-
trol heads beginning to flower by August the first year,
and 43% of control heads beginning to flower by August
the second year [48]. With earlier flowering the second
year, root carbohydrate reserves may have been lower at
the time of clipping than the first year, perhaps also con-
tributing to the decrease in biomass of clipped plants the
second year. It may be that height of clipping and timing
of clipping relative to flowering play a critical role in the
relative success of clipping treatments for control of
Canada thistle.
4.2. Canada Thistle Response to Grass Seeding
Although the presence of grass plants did not inhibit
Canada thistle shoot growth in this study (contrary to the
hypothesis), the effect of the grasses on Canada thistle
root growth was not assessed. A key requirement for
grass species selection in this study was an aggressive
underground growth habit. Native grasses such as alkali
sacaton (Sporobolus airoides [Torr.] Torr.) have been
shown in greenhouse studies to inhibit Canada thistle root
growth while demonstrating no effect on Canada thistle
shoot biomass [51]. A 1-yr greenhouse experiment may
be too short for root competition to result in changes in
aboveground Canada thistle biomass. Canada thistle
shoot biomass fluctuated significantly for the first 2 years
of a field study when grown in the presence of plant
competitors, and those authors concluded that plant com-
petitors require more than two seasons of growth before
they can effectively suppress Canada thistle [28]. Con-
versely, others report that seeding the competitive exotic
grasses perennial ryegrass (Lolium perenne L.), Italian
ryegrass (Lolium perenne L. ssp. multiflorum [Lam.]
Husnot), and orchardgrass reduced shoot biomass of pot-
ted Canada thistle plants each year of their 2-year study
[52].
The grasses for this experiment were also selected for
their drought tolerance, a benefit untested under green-
house conditions, but potentially important under hot, dry
field conditions. For example, western wheatgrass estab-
lishes and maintains cover across a range of soil moisture
availabilities [53], while Canada thistle growth may be
suppressed when soil moisture availability is reduced
[54]. Hot, dry years may allow grasses such as western
wheatgrass to gain a foothold over Canada thistle under
field conditions.
4.3. Canada Thistle Response to Grass Seeding ×
Clipping
Contrary to our hypothesis that the combined effect of
clipping and grass competition on Canada thistle biomass
would be greater than either factor alone, there was no
synergistic effect of clipping and grass seeding in our
study, although different results may be expected in field
trials. One field study demonstrated that seeding peren-
nial grasses and mowing twice annually for 3 years can
reduce Canada thistle density by more than 90% [33].
Other testing of grass seeding and field mowing for con-
trol of Canada thistle has found that while the compete-
tive ability of the grasses was important for controlling
Canada thistle in the early stages of the experiment, as
the age of the grass stand increased and mowing contin-
ued, the effects of mowing became more important than
competition between the grass and Canada thistle [22].
4.4. Grass Response to Canada Thistle Presence
and Species of Grass Seeded
The negative effect of unclipped Canada thistle on shoot
biomass of each of the single-species grass treatments in
our study was hypothesized and expected because pre-
vious researchers have demonstrated the negative im-
pacts Canada thistle can have on the growth of other
plants [55-57]. What was surprising in our study was the
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Canada Thistle (Cirsium arvense) Response to Clipping and Seeding of Competitive Grasses 1257
failure of Canada thistle to reduce shoot biomass of the
paired grass species (Regreen + western wheatgrass),
regardless of Canada thistle clipping status. The mecha-
nism driving the resilience of the paired grass treatment is
unclear, but one likely explanation is the increased func-
tional group diversity [58,59] resulting from the pres-
ence of an annual and a perennial grass. Another possi-
bility, although more difficult to substantiate, is the cu-
mulative effect of allelochemicals produced by both Re-
green and western wheatgrass. Common wheat (one of
the hybrid components of Regreen) is believed to have
allelopathic potential against weeds in cropping systems
[60], and there is some evidence of phytotoxic allelo-
chemical production by western wheatgrass [61,62].
This study also elucidated the potential utility of me-
chanical cutting of Canada thistle to enhance grass
growth when only a single desirable species is seeded.
While shoot biomass growth of all of the single grass
species was inhibited by the presence of unclipped Can-
ada thistle, clipping Canada thistle resulted in greater
grass biomass for each single species treatment equiva-
lent to grass biomass in the complete absence of Canada
thistle (with the exception of western wheatgrass). Grass
growth may benefit from Canada thistle cutting not only
because of the weakened state of the Canada thistle plants,
but also from the decrease in competitive plant canopy.
The lack of benefit to western wheatgrass from Can-
ada thistle clipping may be a product of short study dura-
tion. Regreen establishes and produces growth more
quickly than many species considered for revegetation
[63]. Streambank wheatgrass also establishes and ma-
tures more quickly than western wheatgrass [34]. It may
take longer for the benefits of Canada thistle clipping to
translate into increased growth of western wheatgrass.
While the seeded grasses did not significantly inhibit
Canada thistle shoot biomass in this study, it is probable
that a field study, performed over a longer duration,
would more clearly determine the utility of these grasses
for restoration of Canada thistle infested sites. The ag-
gressive underground growth of these grasses may not
translate to observable aboveground effects on Canada
thistle for several seasons (regardless of additional con-
trol measures used), and the characteristics of these
grasses for which they were initially chosen (such as
drought tolerance and early season germination) may
translate to further advantages over Canada thistle in a
field setting.
The results of this experiment demonstrate the poten-
tial for both mechanical cutting and grass seeding as ef-
fective tools for restoration of Canada thistle infested
sites. The one-time clipping event resulted in a decrease
in Canada thistle biomass, and has implications for the
use of mowing as a field control measure. As revegeta-
tion tools, the grasses used in this experiment proved to
be tolerant of Canada thistle presence, and the combina-
tion of Regreen and western wheatgrass demonstrated
the ability to grow equally well regardless of Canada
thistle presence or cutting status. This is an important
finding, as it is generally considered that almost any con-
trol measure for Canada thistle requires multiple applica-
tions, with complete eradication impossible or at least
requiring multiple seasons. Thus any useful revegetation
species must be capable of growing in concert with Can-
ada thistle until it can be controlled. The success of this
combination grass seeding also emphasizes the potential
importance of the synergistic effects of using more than
one species for restoration of Canada thistle infested
sites.
5. Acknowledgements
We thank the Colorado Agricultural Experiment Station,
(Dr. Lee Sommers, Director) and Colorado State Uni-
versity, Fort Collins, CO, USA for funding this experi-
ment. We also thank Dr. Phil Westra, Dr. Paul Duffy, and
Lonnie Pilkington for their assistance and contributions
to this project, as well as Rainier Seed Company and
Granite Seed Company for donating grass seed.
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