Spotted knapweed (Centaurea steobe) is a perennial tap-rooted plant introduced to North America about from Eastern Europe probably as a contaminate of alfalfa seed or ship ballast.
It is currently reported in all but three states in the U.S. and infests an estimated 7 million acres. Knapweed spreads by seed and produces an average of 1000 seeds per plant.
Spotted knapweed reduces grass production 60 to 90% impacting wildlife and livestock resources. It also: increases soil erosion; reduces species richness; threatens rare and sensitive species; increases human allergies; and reduces land values. In 1996 the economic impact of spotted knapweed in Montana was calculated at $42 million annually in direct and secondary losses.
Considered a noxious or prohibited weed in 16 states.