This European plant is a member of the sunflower family (Asteraceae). Diffuse knapweed threatens wildlife habitat, pastures, grasses, and causes problems for Christmas tree growers. Knapweed invasions cause losses averaging up to 63 percent of available grazing forage.

Legal status in King County, Washington

As a Class B noxious weed, diffuse knapweed control is required in King County. For more information on noxious weed regulations and definitions, see Noxious weed lists and laws.

Biology and morphology

Diffuse knapweed is a biennial or weak perennial

that is shorter than spotted knapweed. Growing from a deep taproot, its upright stems have numerous spreading branches giving the plant a ball shape. Heads of white (sometimes pink to purple) flowers are solitary or in clusters of two or three at the ends of the branches. Excluding flowers and spines, heads are about 3/16" in diameter and 5/16" tall. Bracts around the flower heads are yellowish green with a light brown margin, fringed in spines ending with a longer spine at the tip. It flowers continuously from early summer into the fall, as long as moisture and temperatures permit.

Additional information on diffuse knapweed

• Washington State Noxious Weed Control Board (external link)
• Diffuse knapweed best management practices (204 KB Acrobat file)
• Selected Knapweeds of Washington (316 KB Acrobat file).

What to do if you find this plant in King County, Washington

Please notify us if you see diffuse knapweed 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. We map all known locations of regulated noxious weeds such as diffuse knapweed in order to help us and others locate new infestations in time to control them.

Diffuse knapweed photos

Report diffuse knapweed in King County, Washington

• Please notify us through our online infestation form

Locate diffuse knapweed in King County, Washington

• Use our interactive noxious weed map and search for diffuse knapweed

Related information

• Agriculture in King County, Washington
• Northwest yard and garden
• Animals, plants and habitat

Related agencies

• Dept. of Natural Resources and Parks
• Water and Land Resources Division

---

Program offices are located at 201 S. Jackson St., Suite 600, Seattle, WA 98104. To contact staff, see the Noxious Weed Control Program Directory, send an email, or call 206-477-WEED (206-477-9333).