Canada thistle plants. Photo: Peter Landschoot, Penn State

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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 34 states. Once introduced into turfgrass areas, Canada thistle can spread rapidly, forming patches that detract from the visual appearance of the stand. In infrequently mowed rough on golf courses, the tall stems and spiny foliage of Canada thistle are a nuisance to golfers attempting to retrieve balls. Although persistent mowing and other cultural practices can sometimes reduce populations, in many cases, herbicides are required for adequate control.

Figure 1. Canada thistle plants growing in a low-maintenance lawn in central Pennsylvania. Photo: Peter Landschoot, Penn State

Life Cycle

Canada thistle is a member of the aster family (Asteraceae) and has a perennial life cycle. New shoots of Canada thistle begin to emerge from seeds or root buds in mid-spring (mid-April to mid-May in Pennsylvania), forming small rosettes of leaves. New plants continue to emerge during late spring, summer, and fall, provided that adequate soil moisture is present. In grass areas that are not mowed or mowed only occasionally, new plants become bushy with branched stems. Once Canada thistle gains a foothold in a stand of grass, it can spread rapidly due to its extensive root system, which can penetrate the soil to a depth of 6 feet or more and extend horizontally to around 15 feet. The root system of Canada thistle is unique in that it forms buds that give rise to new shoots. The result is the formation of patches or colonies of thistles which produce a massive root network that can continue propagating new plants.

Canada thistle is easily introduced into grass areas via seed produced in flower heads that begin to form in June. A single plant produces multiple flower heads, with each head yielding dozens of individual seeds, usually in July. Seed of Canada thistle is produced within a tiny fruit called an achene that is attached to a white, feathery plume known as a pappus. The pappus allows wind to carry the seed to a new location. When seeds and pappi are mature, large numbers of white plumes can be observed drifting through the air and littering the ground. Whereas many seeds never leave the seedhead, some can travel up to 0.6 miles in wind currents. 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 the soil.

Identification

Canada thistle plants produce rosettes of oblong, spatulate, or lance-shaped leaves with wavy, spiny margins. Margins of older leaves develop deep triangular sinuses. Leaves have a smooth (non-hairy) surface. In stands that are not mowed or mowed infrequently, plants become erect with branching stems that do not have spines or thorns. Leaves are sessile (no petioles) with bases partially encircling the stem and are arranged alternately on stems. Pink, pinkish-purple, or lavender flower heads (height ~ 1 to 1.5 inches and width ~ ½ inch) are produced in clusters at the tips of stems and are composed of numerous disk flowers with projecting pistils. A vase-shaped assembly of soft, spine-tipped bracts (phyllaries) subtends the disk flowers.

Figure 2. Newly emerged Canada thistle plant in a low-maintenance lawn. Photo: Peter Landschoot, Penn State

Figure 3. Leaves of a mature Canada thistle plant showing smooth surfaces and spines projecting from lobes along leaf margins. Photo: Peter Landschoot, Penn State

Figure 4. Stem and leaves of a Canada thistle plant. Stems are devoid of spines, and leaves are attached directly to stem (no petioles) in an alternate arrangement. Photo: Peter Landschoot, Penn State

Figure 5. Canada thistle flower heads on branching stems (left) and an individual pink flower head. Photos: Peter Landschoot, Penn State

Figure 6. Seedheads of Canada thistle ready to disperse seeds (left) and seeds being dispersed from the seedhead by wind (right). An individual seed is produced within a small dry fruit called an achene that is attached to a white, feathery pappus. Some pappi and seeds are dispersed by wind, whereas others remain clumped together in seedheads. Photos: Peter Landschoot, Penn State

Cultural Management

Effective cultural management of Canada thistle begins with fast turf establishment and maintaining dense turf cover. Because Canada thistle seedling growth is weakened when deprived of sunlight, grass species and cultivars that maintain a 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 grass stands and prevent Canada thistle from gaining ground.

Figure 7. A five-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. Photo: Peter Landschoot, Penn State

Mowing can help deplete carbohydrate food reserves and weaken Canada thistle plants, especially when combined with herbicide applications. Unfortunately, most infrequently mowed grass rough areas on golf courses are not mowed often enough to make a strong impact on thistle populations. Mowing rough 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.

Chemical Control

Postemergence broadleaf herbicide applications for Canada thistle control in lawns are best made during mid to late spring after most plants have emerged and again in late summer or fall. The precise timing of spring applications varies with geographic location.

In infrequently mowed grass rough on golf courses, a spring broadleaf herbicide application should take place after most Canada thistle shoots have emerged but before plants become tall and bushy and before seedheads are produced. Application of herbicides before most terminal seedhead buds form tends to be effective in killing foliage and reducing the visibility of thistles in rough areas. 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 8. Large and well-developed Canada thistle plants treated with a herbicide often remain visible in the stand — Figure 8. Application of a broadleaf herbicide before most terminal seedhead buds form (left) is usually effective in killing Canada thistle plants and reducing visibility in infrequently mowed grass stands. Large and well-developed Canada thistle plants treated with a herbicide often remain visible in the stand for much of the summer (right). Photos: Peter Landschoot, Penn State

Limited information is available comparing broadleaf herbicide products for control of Canada thistle in infrequently mowed golf course rough. Herbicide products containing clopyralid, such as Lontrel and Confront, have served as standards for Canada thistle control on golf courses in the Mid-Atlantic region. A recent study at Penn State demonstrated that NativeKlean (aminopyralid + 2,4-D) is also a very good herbicide for control of Canada thistle in infrequently mowed rough. The same study showed metsulfuron (Manor and other metsulfuron products) control of Canada thistle, albeit very slowly. Metsulfuron products are labeled for Canada thistle suppression and not control.

Some postemergence herbicide products labeled for control of Canada thistle.
Active ingredients	Product name(s)*
2,4-D and 2,4-DP	Patrone 170
2,4-D, clopyralid, and dicamba	Millenium Ultra 2
2,4-D, fluroxypyr, and triclopyr	Momentum FX2
2,4-D, fluroxypyr, triclopyr, and flumioxazin	Sure Power (ester formulation)
2,4-D, MCPP, dicamba	Trimec Classic, Lesco Three-Way Ester II (ester formulations)
2,4-D, fluroxypyr, and dicamba	Escalade 2 Herbicide, Escalade
2,4-D, quinclorac, and dicamba	Quincept, 2DQ Herbicide, Triad QC Select
2,4-D and triclopyr	Chaser (ester formulation)
2,4-D, triclopyr, dicamba, and pyraflufen-ethyl	4-Speed XT (ester formulation)
2,4-D, triclopyr, fluroxypyr, and sulfentrazone	Momentum 4-Score
Aminopyralid and 2,4-D	NativeKlean***
MCPA, fluroxypyr, and triclopyr	Battleship III
MCPA, fluroxypyr, and dicamba	Change Up
MCPA, MCPP, and dicamba	Tri-Power Selective Herbicide
MCPA, triclopyr, and dicamba	Cool Power (ester formulation), Horse Power
MCPA, triclopyr, and dicamba	Eliminate
clopyralid	Lontrel**
mesotrione	Tenacity
triclopyr	Turflon Ester Ultra (ester formulation), Triclopyr 4 (ester formulation), Tryera
triclopyr and clopyralid	Confront, 2-D**

*Follow label precautionary statements, restrictions, and directions regarding tolerant turfgrass species, rates, and timing of applications.

**Certain clopyralid-containing products should not be used on residential turf but can be used for treating weeds in non-residential turf, including infrequently mowed golf course rough.

***NativeKlean (aminopyralid and 2,4-D) is labeled only for use only in “native or natural areas that are not regularly mowed or maintained.” This herbicide should not be used on home lawns, ornamental plantings, or other high-maintenance, fine turfgrass areas.

References

Beck KG. 1998. Canada thistle. Colorado State University Cooperative Extension Bulletin 3.108.

Bravo, 2011. Canada thistle (Cirsium arvense): Pennsylvania noxious weed alert. Pennsylvania Department of Agriculture.

Enloe, S.F., R.G. Lym, R. Wilson, P. Westra, S. Nissen, G. Beck, M. Moechnig, V. Peterson, R.A. Masters, and M. Halstved. 2007. Canada thistle (Cirsium arvense) control with aminopyralid in range, pasture, and noncrop areas. Weed Technol. 21:890-894. Doi.org/10.1614/WT-07-004.1

Gover, A., J. Johnson, and J. Sellmer. 2007. Managing Canada thistle. Conservation Reserve Enhancement Program (CREP) Technical Assistance Series. Factsheet #1.

Landschoot, P. 2023. Canada thistle control in fine fescue rough. Golf Course Management. May, pages 58-63.

McDonald, S. and D. Loughner. 2009. Canada thistle control with selective herbicides. Golf Course Management. 77:86-92.

Moore, R.J. 1975. The biology of Canadian weeds. 13. Cirsium arvense (L.) Scop. Canadian Journal of Plant Science 55: 1033-1048.

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