Diffuse Knapweed (Centaurea diffusa)

Diffuse knapweed is a member of the Aster family. Diffuse knapweed was introduced from Europe and is a biennial or short-lived perennial forb, which reproduces only by seed. The plant usually produces a single main multi-branched stem that is 1 ½ to 2 feet tall. The flower is white or pink with bracts.

Comments: Very few recorded infestations of diffuse knapweed at this time in Pitkin County; the early detection and early treatment philosophy could eradicate this weed in the county.

Biological controls: Two seed head flies Urophora affinis and U. quadrifasciata are available. They reduce seed production. A root-boring moth Agapeta zoegana causes considerable damage to roots.

Chemical controls: For additional information contact your local city, county or town natural resource office. (See section VII for phone numbers.)

Cultural controls: Reseeding of disturbed sites with fast growing grasses helps prevent diffuse knapweed establishment.

Mechanical controls: None available.

Locations of Infestations of Diffuse Knapweed on Roads and Open Space in Pitkin County: