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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 WA. Many western WA 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 follow this link to the WA State Noxious Weed Control Board.
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.
Program offices are located at 201 S. Jackson St., Suite 600, Seattle, WA 98104. To contact a staff member at the King County Noxious Weed Control Program, please call 206-296-0290 or by reach them by email.
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 WA. Many western WA county weed programs report Scotch thistle, but the sites are limited. 
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 Scotch thistle.