The key to management is to prevent it from going to seed. A single knapweed plant can produce 500 – 1500 seeds! Seeds are disbursed by the wind, as the plant dries, it breaks off and becomes a tumbleweed, allowing seeds to be disbursed over large areas. The dried stalks can lodge under vehicles, which spreads the seeds even further. Diffuse knapweed invades overgrazed pastures, forms dense stands and may be toxic to horses. Herbicides should be used in the spring or fall, and then the pasture area should be allowed to recuperate by not grazing for a season or longer as needed. If grasses do not return it should be re-seeded so grasses can compete with any surviving knapweed plants.
Of special concern, is diffuse knapweed which is native to degraded non-cropland and seashores from the Mediterranean. It thrives in the semi-arid west and can’t tolerate flooding or shade. Diffuse knapweed has been reported to contain chemicals that can suppress competitive plant growth. Environmental disturbances encourage its invasion.
Weeds listed below with * are rare in Douglas County.