There are 25 knapweed species in North America. Two species are native and the rest were introduced accidentally from other countries. Knapweed competes with native rangeland plants and can form dense monocultures, which degrades habitat for wildlife and reduces forage for livestock.
Diffuse knapweed (Centaurea diffusa) and spotted knapweed (Centaurea maculosa) have existed for decades in the rangelands of Montana, Idaho, Washington, and Oregon. Researchers say that today over two million acres of rangelands, mainly in Montana and Idaho, are heavily infested.
National Agricultural Pest Information System. < http://www.ceris.purdue.edu/napis/pests/misc/fknap.txt>.
Accessed 2006 Aug 27.