Spotted knapweed is an invasive weed. The weed is a prolific seed producer with 1000 or more seeds per plant. Seed remains viable in the soil five years or more, so infestations may occur a number of years after vegetative plants have been eliminated. Spotted knapweed has few natural enemies and is consumed by livestock only when other vegetation is unavailable. The plant releases a toxin that reduces growth of forage species. Areas heavily infested with spotted knapweed often must be reseeded once the plant is controlled. Historical records indicate that spotted knapweed was introduced from Eastern Europe into North America in the early 1900s as a contaminant in crop seed. It now infests several million acres of grazing land in the northwestern United States and Canada. Reference <a href="http://www.ag.ndsu.edu/pubs/plantsci/weeds/w842w.htm" rel="nofollow">www.ag.ndsu.edu/pubs/plantsci/weeds/w842w.htm</a>