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

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.

For more information about Canada thistle, please follow this link to the WA State Noxious Weed Control Board.