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Canada thistle flowers and seed head
Canada thistle has small purple flower heads found in clusters, and the bracts beneath the flower heads do not have spines. This perennial can reach 5 feet tall. Canada thistle spreads by seed and an extensive root system. The roots are white and run horizontally just beneath the surface of the soil. These roots produce shoots that produce new plants and Canada thistle can also spread by root fragments breaking off.


There are several biological control agents available to release on Canada thistle. If interested in participating in a biocontrol program for this thistle, please contact our office.

Legal status in King County, Washington

Public and private landowners are not required to control infestations of Canada thistle that occur on their property in King County. Canada thistle is a Class C Noxious Weed in Washington, first listed in 1988.  Because control is not required in the county, it is on the list of Non-Regulated Noxious Weeds for King County. For more information, see Noxious Weed Lists and Laws or visit the website of the Washington State Noxious Weed Control Board.

Additional information on Canada thistle

What to do if you find this plant in King County, Washington

Because Canada thistle is so widespread, property owners in King County are not required to control it and we are not generally tracking infestations.  We can provide advice on how to control Canada thistle, but there is generally no legal requirement to do so.

Canada thistle photos - click on thumbnail for larger image

Canada thistle infestation

Related information

Related agencies

Program offices are located at 201 S. Jackson St., Suite 600, Seattle, WA 98104. To contact staff, see the Noxious Weed Control Program Directory, send an email, or call 206-477-WEED (206-477-9333).