Canada thistle recently treated with herbicides in fine fescue rough. Photos by Peter Landschoot
Canada thistle (Cirsium arvense) is one of the most aggressive broadleaf weed species of infrequently mowed fine fescue rough areas on golf courses. Introduced to North America from Europe as a contaminant in crop seed during the early 1600s, this species
has spread throughout much of Canada and the northern U.S. and is listed as a noxious weed in at least 43 states (2). Once introduced into fine fescue stands, Canada thistle can form large patches that detract from the visual appearance of the area.
The tall stems and spiny foliage of Canada thistle are a nuisance to golfers attempting to retrieve balls, and plants are difficult to remove using hand tools. Although persistent mowing and other cultural practices can sometimes reduce populations,
in many cases herbicides are required for adequate control.
Successful control of Canada thistle requires an understanding of its perennial lifecycle and reproductive characteristics. This weed species is easily introduced into grass stands via seed produced from reddish-purple or pink flower heads that begin
to form in June. A single plant produces multiple flower heads, with each flower head yielding dozens of individual seeds when mature, usually in July (5).
Seed of Canada thistle is produced within a tiny fruit that is attached to a white, feathery plume called a pappus (Figure 1). The pappus allows wind to carry seeds to new locations. When seeds and pappi are mature, large numbers of white plumes can be
observed drifting through the air and littering the ground. Although many seeds never leave the seedhead, some can travel up to 0.6 mile (1 kilometer) in wind currents (5). Canada thistle seeds do not require a dormancy period to germinate if conditions
are favorable; however, some seeds can remain in a dormant state for up to 21 years when buried deep in soil (1, 5).
Figure 1. Seedhead of Canada thistle in flowering stage (left) and seedheads with seeds attached to pappi (right).
Once Canada thistle gains a foothold in a stand of grass, it can spread rapidly due to its extensive root system, which can penetrate soil to a depth of 6 feet (1.8 meters) or more and extend horizontally to around 15 feet (4.6 meters) (1). The root system
of Canada thistle is unique in that it forms buds that give rise to new shoots. The result is formation of patches or colonies of new shoots, which produce a massive root network that can continue propagating new plants.
New shoots begin to emerge from seeds or roots in mid-spring (late April to mid-May in the Mid-Atlantic region) forming small rosettes of leaves that quickly develop into bushy plants with branched stems (Figure 2). New shoots continue to emerge during
the summer and fall provided that adequate soil moisture is present. This staggered emergence pattern gives Canada thistle a competitive advantage in managed grass stands that are mowed only occasionally and treated with herbicides once per year.
Figure 2. Seedling of Canada thistle emerging from soil in late April (left) and new shoots arising from buds on roots in May (right).
Cultural management
Effective cultural management of Canada thistle begins with fast establishment and maintaining a dense ground cover of fine fescues. Because Canada thistle seedling growth is weakened when deprived of sunlight, fine fescue species and cultivars that maintain
dense canopy cover over successive growing seasons are most effective at suppressing this weed species. Keeping other weeds to minimum levels can also help reduce voids in fine fescue stands and prevent Canada thistle from gaining ground (Figure 3).
Mowing can help deplete carbohydrate food reserves and weaken Canada thistle plants, especially when combined with herbicide applications. Recommendations based on pasture research call for mowing on monthly intervals over several growing seasons for
effective Canada thistle control (1). Unfortunately, most fine fescue grass stands on golf courses are not mowed often enough to make a strong impact on thistle populations. Mowing fine fescue stands only one or two times per year gives this weed
enough time to produce new foliage and continue storing carbohydrates for future shoot growth. Allowing excess debris from mowing to remain on the canopy for long periods can smother fine fescues and create voids where Canada thistle can develop.
Figure 3. A 5-year-old fine fescue trial showing differences in Canada thistle encroachment between two species. Sheep fescue (left strip) has a more open canopy than creeping red fescue (right strip), which allowed more space for thistle development.
Control of Canada thistle with herbicides
Herbicide applications for Canada thistle control are best made during mid-to-late spring and again in late summer or fall. Precise timing of spring applications varies with geographic location and should be determined based on growth stage of weed plants.
In infrequently mowed fine fescue rough, a spring broadleaf herbicide application should take place after most shoots have emerged, but before plants become tall and bushy and before seed is produced. Application of herbicides just before most terminal
seedhead buds form tends to be effective in killing foliage and reducing visibility of thistles in fine fescue stands. Although older Canada thistle plants can be controlled with herbicides, stems of dead plants often remain rigid, upright and highly
visible in rough areas (Figure 4).
Limited information is available comparing broadleaf herbicide products used for control of Canada thistle in infrequently mowed fine fescue rough. Herbicide products containing clopyralid, such as Lontrel (Corteva Agriscience) and Confront (Corteva Agriscience),
have served as standards for Canada thistle control on golf courses in the Mid-Atlantic region (4). Studies in the western U.S. in pastures and rangeland revealed that the pyridine carboxylic acid herbicide Milestone (aminopyralid; Corteva Agriscience)
controlled Canada thistle 88% to 93%, whereas clopyralid-containing products provided 86% control following applications in late May and early June (3). Little published information is available comparing products containing aminopyralid + 2,4-D (NativeKlean;
Corteva Agriscience), 2,4-D + triclopyr (AquaSweep; Nufarm; and similar products), or metsulfuron (Manor; Nufarm; and other metsulfuron products) to clopyralid-containing products for control of Canada thistle in fine fescue rough. Such information
may provide superintendents with options for using different active ingredients for improved broad-spectrum weed control and resistance management.
Figure 4. Application of a broadleaf herbicide just before most terminal seedhead buds form (left) is usually effective in killing Canada thistle plants and reducing visibility in fine fescue stands. Large and well-developed Canada thistle plants treated with a herbicide often remain visible in the stand for much of the summer (right).
Experiments with spring-applied herbicides
The objective of this research was to evaluate the potential of NativeKlean, AquaSweep, Manor, Lontrel and Confront for spring and summer control of Canada thistle in fine fescue rough. Milestone was included in this experiment strictly for comparison
to other herbicide treatments because it is known for having strong efficacy on Canada thistle. According to information on the Milestone label, this product should not be applied to golf course properties. AquaSweep is not labeled for control of
Canada thistle but is sometimes used for broadcast applications in fine fescue rough during spring for controlling other broadleaf weeds and small trees. All other products are labeled for control or suppression of Canada thistle.
Experiments were conducted during 2021 and 2022 in adjacent areas at the Landscape Management Research Center in University Park, Pa. Both experiments were performed in an 8-year-old nonirrigated stand of Chewings fescue uniformly infested with approximately
50% Canada thistle at the time of application. The stand was mowed once per year in late October at a height of 5 inches (12.7 centimeters).
All herbicide treatments included 0.25% v/v nonionic surfactant and were applied once each season on June 5, 2021, and May 25, 2022, when most Canada thistle plants approached the bud initiation stage and plant heights ranged between 8 and 22 inches (20.3
to 55.9 centimeters). Total precipitation during the 2021 and 2022 evaluation periods were 8.4 and 4.3 inches (21.3 and 10.9 centimeters), respectively (Table 1).
All herbicides were applied using a sprayer equipped with a single boom fitted with a 9504E flat-fan nozzle at 40 pounds per square inch with a water carrier volume of 87 gallons per acre (814 liters per hectare). The experimental design was a randomized
complete block design, and each treatment was replicated three times. Criteria for evaluating herbicides included visual ratings of Canada thistle foliar injury using a 0-to-10 scale, with 0 indicating no injury and 10 representing complete foliar
desiccation. All data presented in this article were subjected to statistical analysis.
Table 1. Product names, active ingredient amounts and product rates used in 2021 and 2022 experiments. A non-treated control was included as a treatment during both years. †Product rates are based on label rates for Canada thistle control or suppression, except for AquaSweep, which is not labeled for Canada thistle control. Milestone was included for comparison purposes and should not be applied on golf courses.
Tables 2,3. Influence of herbicide treatments on Canada thistle injury following application on June 5, 2021, (top) and May 25, 2022, (bottom) at the Landscape Management Research Center in University Park, Pa. †Means within the same column and followed by the same letter are not significantly different as determined by Fisher’s Protected Least Significant Difference test at α=0.05.
Results
All herbicide treatments caused injury to Canada thistle when compared to the non-treated control in 2021 (Table 2). Manor showed the least injury of all herbicide treatments at 10, 17 and 23 days after treatment (DAT), but resulted in similar injury
to most other treatments at 32 and 40 DAT. At 10 DAT, the Milestone treatment showed the highest foliar injury rating, followed by Lontrel and the 34-fluid-ounces-per-acre (2.48-liters-per-hectare) rate of NativeKlean. At 17 DAT, injury from the Manor
treatment was not different from both rates of NativeKlean. All herbicide treatments provided similar injury ratings 32 DAT. At the conclusion of the 2021 evaluation period (40 DAT), all herbicide treatments, except for Confront, provided complete
foliar desiccation.
In 2022, all herbicide treatments caused injury to Canada thistle when compared to the non-treated control on all rating dates (Table 3). Manor caused the least injury of all herbicide treatments at 4, 14, 22, 28 and 35 DAT, but provided similar injury
ratings to AquaSweep, Confront and Lontrel at the conclusion of the evaluation period (55 DAT). At 4 DAT, Milestone, Confront, AquaSweep and both rates of NativeKlean showed the highest thistle injury ratings. Milestone and both rates of NativeKlean
produced higher Canada thistle injury ratings compared to Lontrel, Confront and AquaSweep at 22, 28 and 35 DAT. At the conclusion of the 2022 experiment (55 DAT), Milestone, Confront and both rates of NativeKlean treatments showed complete desiccation
of above-ground portions of Canada thistle plants, whereas all other treatments produced severe injury, with only a minimal portion of plants showing green tissue.
Products containing aminopyralid (Milestone and NativeKlean) generally provided fast burndown and complete desiccation of foliar portions of Canada thistle at the end of each evaluation period in 2021 and 2022. With one exception in 2021, the 34-fluid-ounces-per-acre
rate of NativeKlean was as efficacious as Milestone and provided equal or higher injury ratings than Lontrel and Confront. Although not labeled for Canada thistle, AquaSweep provided injury comparable to Lontrel and Confront in both years. Though
Manor was slow acting, this herbicide provided acceptable control of Canada thistle at the end of each evaluation period.
Results of this study demonstrate that different herbicide options exist for spring control of Canada thistle in fine fescue rough. However, practitioners should realize that spring herbicide applications do not typically provide season-long control of
this species and that a second application is often needed in late summer or fall to control new plants that may have grown from root buds or emerged following seed germination. When late-season applications are properly timed, herbicides are translocated
along with sugars to roots, where they interfere with shoot initiation from root buds of thistle plants the following spring. Hence, an herbicide application at this time of year is often beneficial for long-term control of Canada thistle (Figure
5).
Figure 5. Experiment area 55 days after application of herbicide treatments on May 25, 2022. Area in foreground shows beneficial effects of treatments vs. heavily infested area of Canada thistle in background that was not treated.
The research says
- Different herbicide options are available to golf course managers for control of Canada thistle, which is important for broad-spectrum weed control and management of herbicide resistance in infrequently mowed fine fescue rough.
- Spring applications of NativeKlean (aminopyralid + 2,4-D) provided relatively fast burndown and equal or greater injury to Canada thistle compared to Lontrel (clopyralid) and Confront (triclopyr and clopyralid), whereas AquaSweep (triclopyr + 2,4-D) produced Canada thistle injury similar to Lontrel and Confront.
- Manor (metsulfuron) was slow to injure Canada
thistle but provided acceptable control within 40 to 55 days.
Acknowledgements
This work was supported by the USDA National Institute of Food and Agriculture, Hatch project 1023300. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author and do not necessarily reflect the view
of the National Institute of Food and Agriculture or the USDA. Some financial support for this research was provided by the Pennsylvania Turfgrass Council. Some of the herbicide products used in this study were donated by Corteva Agriscience.
Literature cited
- Beck, K.G. 1998. Canada thistle. Colorado State University Cooperative Extension Bulletin 3.108. https://extension.colostate.edu/topic-areas/natural-resources/canada-thistle-3-108/
- Bravo, 2011. Canada thistle (Cirsium arvense): Pennsylvania noxious weed alert. Pennsylvania Dept. of Agriculture. https://www.agriculture.pa.gov/Plants_Land_Water/PlantIndustry/NIPPP/Documents/Canada%20Thistle%20Noxious%20Weed%20Alert%202011.pdf
- Enloe, S.F., R.G. Lym, R. Wilson, P. Westra, S. Nissen, G. Beck, M. Moechnig, V. Peterson, R.A. Masters and M. Halstvedt. 2007. Canada thistle (Cirsium arvense) control with aminopyralid in range, pasture, and noncrop areas. Weed Technology 21(4):890-894
(https://doi.org/10.1614/WT-07-004.1).
- McDonald, S., and D. Loughner. 2009. Canada thistle control with selective herbicides. Golf Course Management 77(7):86-92.
- Moore, R.J. 1975. The biology of Canadian weeds. 13. Cirsium arvense (L.) Scop. Canadian Journal of Plant Science 55:1033-1048.
Peter Landschoot (pjl1@psu.edu) is a professor of turfgrass science at Pennsylvania State University, University Park.