Noxious weeds in Gilpin County
Diffuse knapweed (Centaurea diffusa) and Spotted knapweed
(Centaurea maculosa)
  
Diffuse knapweed - left (USDA ARS) and middle
pictures -------------- Spotted knapweed on right
(USFS)
State law: Refer to the State
containment map (scroll down to page 23) to see where it must
be eradicated or contained in Gilpin County.
Quick Facts
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.
Other Links For Further Information:
http://www.ext.colostate.edu/PUBS/NATRES/03110.html
http://www.colostate.edu/Depts/CoopExt/Adams/weed/dknapweed_id.htm
http://www.colostate.edu/Depts/CoopExt/Adams/weed/dknapweed_mgt.htm
http://www.mtweed.org/Identification/Diffuse/diffuse.html
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