Centaurea stoebe (syn. C. biebersteinii, C. maculosa)
This exotic invader from Europe is a member of the sunflower family (Asteraceae). Spotted knapweed threatens wildlife habitat, pastures, and grasses, and causes problems for Christmas tree growers. Knapweed invasions cause losses averaging up to 63 percent of available grazing forage. Many spotted knapweed infestations start on rights of-way or from infested gravel or fill.
Legal status in King County, Washington
Spotted knapweed is a Class B noxious weed. Property owners in King County are required to control this plant. 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.
Biology and morphology
Spotted knapweed is a perennial with several branched upright stems growing from a stout taproot; usually to 5 feet tall. Leaves become smaller as they advance up the stem; heads of pink to purple (sometimes white) flowers grow at the ends of the branches; excluding flowers, the heads are about 1/4" in diameter by 1/2" tall. Bracts around the flower heads have obvious vertical veins below the black triangular spot on the bract tip. It flowers continuously from early summer into the fall, as long as moisture and temperatures permit.
Additional information on spotted knapweed
What to do if you find this plant in King County, Washington
Please notify us if you see spotted 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. Also, because spotted knapweed is 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 spotted knapweed in order to help us and others locate new infestations in time to control them.
Spotted knapweed photos - click thumbnail for larger image