Canada Thistle

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Cirsium Arvense

Photograph by Gerald D. Carr
Photograph by Richard R. Old
Photograph by Richard R. Old
Photograph by Gerald D. Carr
Photograph by Gerald D. Carr
Photograph by Gerald D. Carr
Photograph by Ben Legler
Photograph by Ben Legler
Photograph by Craig Althen
Photograph by Richard R. Old
  • Canada thistle is a creeping perennial that reproduces from vegetative buds in its root system and from seed.
  • It is difficult to control because its extensive root system allows it to recover from control attempts.
  • Combining control methods is the best form of Canada thistle management.
  • Persistence is imperative so the weed is continually stressed, forcing it to exhaust root nutrient stores and eventually die.

Canada thistle is an aggressive, creeping perennial weed that infests crops, pastures, gardens, rangeland, roadsides, parks, and other noncrop areas.  Generally, infestations start on disturbed ground, including ditch banks, overgrazed pastures, tilled fields or abandoned sites.  Cattle typically will not graze near infestations. 

One plant can grow up to 6 feet in diameter in two years and will grow in a variety of soils. 

Canada thistle develops from seed or from expanding its root system.  Horizontal roots may extend 15 feet or more and vertical roots may grow up to 15 feet deep.  Canada thistle emerges from its root system in mid-to-late spring (late April through May) and forms rosettes. 

Canada thistle begins to flower in late spring to early summer and female flowers produce a sweet odor. Insects pollinate patches up to 200 feet apart.

Canada thistle doesn’t develop many seeds compared to other thistles. Most spread is due to their hardy root systems. When they seed, the seeds can be transported long distances by water, or attached to animals, clothing, farm equipment and other vehicles, and in contaminated crop seed. The seeds can remain viable in soils for up to 22 years, and deep burial promotes survival longevity.

The key principle to Canada thistle control is to stress the plant and force it to use stored root nutrients. Canada thistle can recover from almost any stress, including control attempts, because of root nutrient stores. Therefore, returning infested land to a productive state occurs only over time. Success requires a sound management plan implemented over several years.

This information was primarily derived from the Colorado State University Extension website.  For additional images and information on other management methods, please visit: http://extension.colostate.edu/topic-areas/natural-resources/canada-thistle-3-108/