Spotted knapweed is a serious threat to the rangelands of the western United States. It was probably introduced to North America from Europe as a contaminant in alfalfa seed or in ships’ ballast. A common invasive weed in much of Montana, Spotted knapweed grows best along ditches, field margins, over-grazed pastures, and in disturbed dry, arid soils.
Spotted knapweed produces its own natural herbicide called “catechin” that eradicates plants around it. Each plant can produce 300 flower heads and 140,000 seeds. In the correct conditions, it spreads rapidly. Knapweed grows in a bush like plant up to four feet high. Its flowers are usually a lavender color but can range from pink to purple. Large infestations appear as a continuous hazy purple mass.
Spotted knapweed plants are generally short lived. They spread primarily by seeds, which can remain viable in the soil for up to eight years. Control with herbicides can be effective while the plant is young through budding stage. Generally, June is an effective treatment month. Herbicides which provide residual control are the most effective in hindering re-growth.
Some biological control insects have proven to be effective in the management of knapweed. Root boring weevils and flower head eating bugs, when combined properly, can reduce knapweed densities but generally will not eradicate the plant.