Diffuse knapweed (Centaurea diffusa) and Spotted knapweed (Centaurea maculosa)
State law: Diffuse knapweed must be eradicated or contained in Gilpin County. If you live in a “containment” area, it does NOT mean you can let the knapweed grow freely on your property. Containment means that you need to suppress the weed (reduce its vigor and seed production) and keep it from spreading any further. If at all possible, it should slowly be eradicated, but work must be done every year nonetheless.
Spotted knapweed must be eradicated everywhere in the County.
- Diffuse knapweed is a biennial or, occasionally, an annual or short-lived perennial that reproduces and spreads solely from seed.
- Flowers can be white or light purple. There is a long spine on the edges of the bracts underneath the flower, which distinguishes it from spotted knapweed.
- Spotted knapweed is a short-lived, noncreeping perennial that reproduces from seed and forms a new shoot each year from a taproot. Flowers are usually purple. Bracts have a characteristic black edge to them (see picture).
- Both are readily controlled both by pulling and with herbicides. Unless cultural techniques are used, however, the weeds will reinvade.
- Extremely high seed production and germination potential. In Gilpin County, it usually does not go to seed until mid to late August.
- Allelopathic (releases chemicals into the soil which inhibit growth of other plants)
- Competes well in dry and especially in disturbed sites.
- If desirable grass competition is evident in diffuse or spotted knapweed stands, judicious herbicide application that does not injure grasses may allow them to compete effectively with the weeds. Seeding suitable perennial grasses is necessary to prevent weed reinvasion.