Diffuse knapweed occurs in every state west of the Rocky Mountains and is particularly widespread in the Pacific Northwest; it also occurs in Nebraska and is scattered in several eastern states. The worst infestations are reported to be in Idaho, Montana, Oregon, and Washington. It is very aggressive and can quickly dominate disturbed habitats. Diffuse knapweed produces an allelopathic chemical that inhibits the root growth of other species and prevents them from competing for soil moisture and nutrients. It is most abundant in disturbed and overgrazed lands but it can also invade undisturbed grasslands, shrublands, and riparian communities. Management is difficult, and usually requires a combination of methods over more than five years.