Canada thistle
Cirsium arvense

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

Class C noxious weed. Canada thistle is widespread throughout the state. In King County, control is recommended, but not required.

Additional information on Canada thistle

• Washington State Noxious Weed Control Board (external link)

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