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Home>Plant Species>Weed Species>Canada thistle

Drawing of Canada thistleCanada thistle
Cirsium arvense

question markDid you know?

Canada thistle can produce up to 364 feet of roots in only 18 weeks!

How did Canada thistle get here?
Canada thistle is a native of southeastern Eurasia. It was introduced to Canada as a contaminant of crop seed as early as the 1600s. The rapid spread of Canada thistle led to control legislation as early as 1795 in Vermont and 1831 in New York. It is probably the most widespread of all thistle species.

Canada thistle flowerheadsWhat are its characteristics?
Canada thistle is a colony-forming perennial forb from deep horizontal roots. Stems are 1 to 4 feet tall. Canada thistle differs from other thistles in that there are separate male and female plants. Purple flowers of 1/2 to 3/4 inch in diameter are produced during July or August.

Canada thistle has a reputation for producing few viable seeds. Seeds are short lived, with most germinating during the 1st year. However, a single Canada thistle plant can produce up to 364 feet of roots and 26 additional stem shoots after only 18 weeks. Canada thistle grows best with 16 to 30 inches of precipitation per year. In the Great Basin, Canada thistle is often restricted to swales or other areas of deep, moistCanada thistle flowerheads soils. Canada thistle has been used in the northeastern United States in remedies for worms, as a mouthwash, and a tonic for gastrointestinal ailments.

Its extensive root system gives it the uncanny ability to survive major disturbance. At Mt. St. Helens, after the 1980 eruption, it survived landslides and resprouted from root and stem fragments after the blast.

Why is Canada thistle a problem in the Great Basin?
Canada thistle can decrease forage and livestock production on rangelands and can make recreation areas unpleasant to visit. Natural communities that are threatened by Canada thistle are typically those impacted by disturbance as well as those undergoing restoration efforts. Canada thistle may produce toxins that inhibit growth of other plants. Hiking and horse-back trails are major invasion pathways for Canada thistle. This species often establishes after fire, disking, and herbicide treatments that have reduced cover of other plants.

Canada thistle plant

How can we fight this weed?
Maintaining a healthy native community is the best defense against Canada thistle. Canada thistle should be removed from lightly infested areas when first observed, because it is hard to control once established.

Because Canada thistle has large root reserves, it recovers from most types of stress, including many control methods. Breaking up the roots by plowing only serves to increase the number of plants. Herbicides can be effective if properly timed and given repeated application. The Weed Management Handbook on the University of Wyoming Extension website offers current herbicide use information. To date, biological control of Canada thistle has not been successful, although it may be effective when combined with other control methods.



Images:
-Drawing: USDA-NRCS PLANTS Database / Britton, N.L., and A. Brown. 1913. Illustrated flora of the northern states and Canada. Vol. 3: 553.
-Flower: USDA ARS.
-Plant #1: Gary A. Monroe @ USDA-NRCS PLANTS Database.
-Plant #2: Mary Ellen (Mel) Harte, (http:www.forestryimages.org).

Text references:
-USDA, NRCS. 2006. The PLANTS Database (http://plants.usda.gov, 26 September 2006). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.
-Whitson, T. D., et.al. 2000. Weeds of the West., 9th ed. Western Society of Weed Science, Newark, CA.
-Zouhar, Kris 2001. Cirsium arvense. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: (http://www.fs.fed.us/database/feis/, 26 September 2006).