Spotted knapweed is a member of the Aster family. Native to Central Europe, it is a simple perennial that reproduces from seed and forms a new shoot each year from a taproot. The plant can have one or more shoots up to 4 feet tall. Flower color is usually lavender to purple. Spotted knapweed occupies dry meadows, pastures, stony hills, roadsides, and the sandy or gravel flood plains of streams and rivers, where soils are light textured, well-drained, and receive summer precipitation. Spotted knapweed tolerates dry conditions, similar to diffuse knapweed, but will survive in higher moisture areas as well. There is a small infestation up Independence Pass above Tagert Lakes.

Comment: One of the most invasive, aggressive weeds to plague the western United States. Very rare in Pitkin County, the largest infestation is found at the base of Independence Pass along State Highway 82 above Tagerts Lake. It is imperative that this infestation be monitored yearly and treated as needed. This infestation must not be allowed to spread in the fragile sub-alpine ecosystem.

Biological controls: The seedhead flies Urophora affinis and Urophora quadrifasciata have been released in many Front Range counties. These insects cause plants to produce fewer viable seeds and abort terminal or lateral flowers. Root feeding insects may have more of a detrimental effect on knapweed populations than seed feeding insects. Larvae of the yellow winged knapweed moth feed in the roots of both knapweed species.

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

Cultural control: If desirable grass competition is evident in spotted knapweed stands, judicious herbicide application that does not injure grasses may release them to compete effectively with the weeds. Irrigation may help stimulate grass competition in these cases. Seeding suitable perennial native grasses is necessary to prevent weed re-invasion.

Mechanical controls: None available.

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