Scotch Thistle
Onopordum acanthium

History and Impact

Scotch thistle is a Class B noxious weed and control is required in King County. The majority of Scotch thistle sites in the State occur in eastern Washington. Many western Washington county weed programs report Scotch thistle, but the sites are limited.

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. 

For more information about Scotch thistle, please see the WA State Noxious Weed Control Board website.

If you find Scotch thistle in King County, please notify us through our online infestation form.

To find out where we have records of this weed in King County, use our interactive noxious weed map and search for Scotch thistle.