History and Impact
This exotic invader from Europe 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.

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.

Diffuse knapweed photos - click on thumbnail for larger image

As a Class B noxious weed, diffuse knapweed control is required in King County. For information on controlling diffuse knapweed in King County, please read the diffuse knapweed best management practices (204 KB Acrobat file, 29 seconds on 56K modem) and informational Selected Knapweeds of Washington (316 KB Acrobat file, 46 seconds on 56K modem).

For more information about diffuse knapweed, please see the written findings of the WA State Noxious Weed Control Board.

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