Canada Thistle: A Prickly Invader of Meadows and Conservation Plantings

By Peter Landschoot, Professor Emeritus, The Pennsylvania State University

Canada thistle [Cirsium arvense (L.) Scop; family Asteraceae] is one of the most aggressive invasive species of meadows, conservation plantings, farmland, and pastures in central PA.  Introduced to North America from Europe as a contaminant in crop seed during the early 1600’s, this thistle species has spread rapidly throughout much of Canada and the northern U.S. and is listed as a noxious weed in at least 34 states, including PA (2).  Once introduced into an open stand of grass and/or other low-growing plants, Canada thistle can form large, dense patches that dominate the canopy and compete with desirable plants. The tall stems and spiney foliage of Canada thistle are a deterrence to grazing livestock and wildlife, and plants are difficult to remove using hand tools.  Although persistent mowing and other cultural practices can sometimes reduce populations, in cases where Canada thistle begins to overtake desirable species, land managers often apply herbicides for suppression or control.

Fig. 1. Canada thistle invading grass buffer strip.

Life Cycle and Reproduction

Successful management of Canada thistle requires an understanding of its perennial life cycle and reproductive characteristics.  This species is easily introduced into meadows and other open areas via seed produced from flower heads that begin to form in late spring.  A single plant produces multiple flower heads with each head yielding dozens of individual seeds, usually in July (6).  Studies in the western U.S. have shown Canada thistle produces an average of about 1,500 seeds per plant (5).  

Seed of Canada thistle is produced within a tiny fruit that is attached to a white, feathery plume called a pappus.  The pappus allows wind to carry some seeds to new locations.  When seeds and pappi are mature large numbers of white plumes can be observed drifting through air and littering the ground in what is sometimes referred to as “summer snow”.  Whereas many seeds never leave the seedhead, a few can travel up to 0.6 miles in wind currents (6).  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, 6).

Fig. 2. Seedheads of Canada thistle ready to disperse seeds (left) and seeds being dispersed from the seedhead by wind (right).

Fig. 3. Pappi of Canada thistle littering ground following dispersal by wind from parent plants.  In areas with large Canada thistle populations, pappi can create a white cast on ground. 

Once Canada thistle gains a foothold, it can spread rapidly throughout the stand due to its extensive root system, which can penetrate soil to a depth of 6 feet or more and extend horizontally to around 15 feet (1).  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 new shoots, which produce more roots, forming a massive network that can continue propagating new plants. 

New shoots begin to emerge from roots or seeds in mid-spring (late-April to early-May in the Mid-Atlantic region) forming small rosettes of leaves that quickly mature into bushy plants with branched stems.  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 meadows that are mowed only occasionally and not treated with herbicides. 

Fig. 4.  Seedling of Canada thistle emerging from soil in April (left) and new shoots arising from buds on roots in May (right).

Identification

Canada thistle plants produce rosettes of oblong, spatulate, or lance-shaped leaves with wavey, 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.

Fig. 5. Leaves of a mature Canada thistle plant showing smooth surfaces and spines projecting from lobes along leaf margins.

Fig. 6. Stem and leaves of a Canada thistle plant. Stems are devoid of spines, and leaves are attached directly to stems (no petioles) in an alternate arrangement.

Fig. 7. Canada thistle flower heads on branching stems (top) and an individual pinkish purple flower head (bottom).

Cultural Management  

Effective management of Canada thistle begins with fast establishment and maintaining a dense ground cover.  Because Canada thistle seedling growth is weakened when deprived of sunlight, desirable plants that maintain a dense canopy cover over successive growing seasons are most effective at suppressing this invasive weed species.  Keeping other invasive species to minimum levels can also help reduce voids in stands and deprive Canada thistle from gaining ground.

Fig. 8. A five-year-old conservation planting trial at Penn State showing differences in Canada thistle encroachment between strips of two fine fescue species. Sheep fescue (left) was less dense than creeping red fescue (right), which allowed more space for thistle development.

Mowing practices can help deplete carbohydrate food reserves in root systems and weaken Canada thistle plants, especially when combined with herbicide applications.  Research in pasture systems has shown that mowing on monthly intervals over several growing seasons can provide effective Canada thistle control (1).  Unfortunately, most meadows and conservation plantings are not mowed often enough to make a strong impact on Canada thistle populations.  Mowing only one or two times per year gives this weed enough time to produce new foliage and continue storing carbohydrates for future shoot growth. 

For small areas, hand-held brush trimmers with steel blades can remove stems and leaves above ground level, creating more space for desirable plants. This practice requires persistent scouting to keep up with regrowth from remaining tissues and new shoot emergence.

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 (4).  Precise timing of spring applications varies with location and site conditions and should be determined based on growth stage of Canada thistle plants.  In most cases, spring herbicide applications 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 most effective in killing foliage in meadows and conservation plantings. 

Late-season herbicide applications on Canada thistle plants are effective in translocating herbicide to the root system, which can kill entire plants and hence, limit new shoot growth from root buds in spring. However, late-season applications alone will do nothing to control seed production, which typically occurs in early summer. Thus, late-season applications are typically combined with mowing and/or spring herbicide applications to limit seed production and dispersal.  

Many herbicides are available to control Canada thistle, and users should follow label instructions carefully to avoid injury to desirable plants, especially trees, wildflowers, and other dicot plants.  Herbicide products containing clopyralid (Telar XP, Lontrel, Confront, etc.)

Fig. 9. Applications of broadleaf herbicides just prior to terminal flower bud development (left) are usually effective in killing Canada thistle foliage. Canada thistle plants treated with herbicide after seedheads form typically remain visible in the stand for much of the summer and can release viable seed (right). 

have served as standards for Canada thistle control in the Mid-Atlantic region. Products containing aminopyralid (Milestone and NativeKlean) are also very effective on Canada thistle (3). 

Although implementation of cultural and chemical control practices over several years can reduce populations of Canada thistle to manageable levels, complete control is very difficult to achieve. Thus, be sure to have some idea of an acceptable threshold level in mind before beginning a control program.

Author: Peter Landschoot, Professor Emeritus, The Pennsylvania State University

References

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

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

3. 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. https://doi.org/10.1614/WT-07-004.1

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

5. Hey, W.D. 1937. Canada thistle seed production and its occurrence in Montana seeds.  Seed World, March 26, pp. 6-7.

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