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.
Other Links For Further Information:
http://extension.colostate.edu/topic-areas/natural-resources/diffuse-and-spotted-knapweed-3-110/
http://www.mtweed.org/Identification/Diffuse/diffuse.html