Weeds - Spotted Knapweed (Centaurea maculosa)

Photo: Spotted KnapweedSpotted Knapweed is a biennial or short-lived perennial. A multi-stemmed plant with several stems arising from the crown, flowers are light purple to violet, or occasionally cream colored. The tips of the flower head bracts are black, making the seed head appear spotted. The flower head is slender, about 1/2 inch long and urn shaped which is typical of most Centaurea species. Each seed head can produce hundreds of seeds each. Develops a slender taproot reaching as deep as 2 feet in ideal conditons.  

Spotted Knapweed will form dense stands on any open ground, excluding more desirable forage species. This plant prefers moister conditions than Diffuse knapweed, but can survive at fairly dry sites, also tends to choose gravelly to sandy areas.  Once established, the necessary extensive control measures are often more expensive than the potential income of the land.

This species is relatively easy to control with herbicides. All phenoxy type products, (2,4-D, Tryclopr, Dicamba) all work well. Test plots by the OSU Extension Service have shown that residual chemicals are preferable when trying to control the weed. Picloram (Tordon 22K), dicamba (Banvel), 2,4-D LV and Amine are effective. Always read and follow the label. Biological controls are increasing, check with local weed control personnel for more information.