Scotch thistle is a Class B noxious weed and control is required in King County. This species is also on the Washington quarantine list (known as the prohibited plants list) (external link) and it is prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts, seeds in packets, blends or "wildflower mixes" of this species, into or within the state of Washington. The majority of Scotch thistle infestations in Washington occur in eastern Washington. Although Scotch thistle has been found growing in a number of western Washington counties, the sites are generally small and isolated.
Scotch thistle can be confused with bull thistle (Cirsium vulgaris), which is also sometimes called "Scotch thistle", but is much more common in western Washington. Scotch thistle is more densely covered with white hairs, tends to be taller, and the stems have "wings" along the entire length, unlike bull thistle.
Biology and morphology
During its first year it develops a large spiny rosette, eliminating potential competing plants around its taproot by shading. In the second year Scotch thistle may grow 5 feet in diameter with branched flower stems 8 feet tall. The leaves are lobed with spiny margins that extend down the stems as spiny wings. The entire plant appears grayish-green because of a covering of fine white hairs. Purple flower heads are 1 to 2 inches in diameter with stiff spiny bracts. Scotch thistle differs from musk thistle in the upright flower heads and the leafy stems below the flower heads.
Additional information on scotch thistle
What to do if you find this plant in King County, Washington
Please notify us if you see scotch thistle growing in King County. Our program staff can provide the property owner or appropriate public agency with site-specific advice on how best to remove it. Also, because scotch thistle is not established in King County, we have an opportunity to stop it from spreading if we act quickly. We map all known locations of regulated noxious weeds such as scotch thistle in order to help us and others locate new infestations in time to control them.