Description:
Bighead knapweed, Centaurea macrocephala, is a member of the thistle tribe (Cynareae) in the sunflower family. This perennial species is the tallest knapweed growing in the Pacific Northwest, ranging from two to five feet tall, depending on the habitat. The plant stems are upright and unbranched, terminating in a single flower head. The leaves are broadly lance shaped with toothed edges and pointed tips, and they have a rough surface. Basal or rosette leaves are stalked and they can reach 15 inches long and three inches wide. The leaves and leaf stalks are progressively smaller upward on the plant stem, with the top leaves being stalkless. The solitary flower heads are globe shaped, and one to three inches in diameter. The bracts beneath the flower head have thin, papery, fringed margins. The lower bracts show evidence of spines. The flowers are yellow. The seeds are medium brown and ridged, with a ring of light-colored bristles. C. macrocephala has a taprooted woody crown.

• Mechanical: C. macrocephala is a taprooted perennial, and pulling as a control option is impractical for large plants. When the flowering stem is broken off, a new stem will grow from the woody crown, producing another flower head later in the season. Repeated mowing will reduce seed production and will eventually diminish root reserves. Small plants can be dug, but the site needs to be monitored until the seed banks are depleted.
• Biological: None known.
• Herbicide: Not known. Refer to the State Noxious Weed Control Board site

For More Information:
Detailed information about Bighead Knapweed is available at the Washington State Noxious Weed Control Board Web Site.