Photographer: Leslie J. Mehrhoff, University of Connecticut Source: Bugwood.org Copyright: CC BY 3.0 US
Despite its name Canada thistle, this plant is not native to North America. Cirsium arvense is a perennial herb that grows up to 3 feet tall. The plant and leaves are spiny, and new buds and roots can arise anywhere on their extensive horizontal and vertical root system. Canada thistle is the only thistle to have unisexual flower heads with male flowers being staminate while female flowers are pistillate. Florets are usually a purple-lavender color but sometimes can be white. The seeds are light brown achenes approximately 1/8 inch long. Leaves are the largest on the power portion of the plant, and reduce in size upwards along the stem.
Canada thistle grows vigorously allowing it to form dense colonies. With it being able to sprout from the horizontal and vertical root system it is able to spread quickly. Canada thistle is able to overcome and outcompete native grasses and plants quickly.
Canada thistle flowers from June through August. This plants are dioecious, and seed production depends on cross pollination between male and female plants. Most seeds germinate under a year but some have been documented to remain dormant but viable for up to 20 years. The deeper the seed is buried in the soil the longer it can remain viable. A single plant can produce 40,000 seeds per year. Entire stands of this plant may not reproduce because male and female plants are too far apart. The plants only live about 2 years, but they are continuously replace be new shoots from the buds on the extensive root system. In those same two years a plant can produce over 60 feet of new roots!
First brought to North America in the 1700s possibly through farm seed shipments. It spread so quickly that legislature in Vermont was created in 1795 to require landowners to control Canada thistle. It is listed as a noxious weed or prohibited noxious weed in over 30 states that it is present in.
U.S. Habitat: It is able to establish in a wide range of soil types and moisture conditions: pastures, ditches, prairies, forest edged, open and abandoned lands. It is intolerant of swampy lands and complete shade.
U.S. Present: AK, AL, AR, AZ, CA, CO, CT, DC, DE, GA, IA, ID, IL, IN, KS, KY, MA, MD, MI, MN, MO, MT, NC, ND, NE, NH, NJ, NM, NV, NY, OH, OR, PA, RI, SD, TN, UT, VA, WA, WV and WY.
For a state and county distribution map provided by EDD MapS click here
Resembles other thistle species in the genus Cirsium, including Cirsium ochrocentrum and C. vulgare.
In Canada several weevils and one fly species were imported as biological controls. However, a few of them are no longer recommended due to their impact on native thistles. Prescribed burning also not proven to be as effective as hoped. Mechanical pulling or tilling is not recommended due to the plants ability to quickly sprout from remaining roots. Integrated management strategies are most effective than any single method. Clopyralid or clopyralid + 2,4-D has shown to be a good herbicide for Canada thistle. The plant also responds to glyphosate and triclopyr, but those two herbicides can seriously affect other grasses and plants.
Fact Sheet: Canada Thistle (Jan 2014; PDF | 481 KB)
Alberta Invasive Plants Council (Canada).
Hayden, A. 1934. Distribution and reproduction of Canada thistle in Iowa. American Journal of Botany 21: 355-373.
Royer, F. and R. Dickinson. 1999. Weeds of the Northern U.S. and Canada. The University of Alberta press. 434 pp.
Trumble, J. T., & Kok, L. T. 1982. Integrated pest management techniques in thistle suppression in pastures of North America. Weed Research, 22(6):345-359.