Diffuse knapweed is an aggressive, introduced weed species that can rapidly invade pasture, rangeland and fallow land and cause a serious decline in forage and crop production. Knapweed infestations can be traced to seed or hay brought in form neighboring states. Controlling Diffuse knapweed plants when they are first observed and monitoring the site for several years to prevent reinfestation from seed are important. Knapweeds are related to thistles and can spread even faster.
Diffuse knapweed is a short-lived perennial or sometimes a biennial reproducing solely by seeds. Seeds remain viable for five years or more, so infestations may occur a number of years after vegetative plants have been eliminated. The seeds can germinate from spring through early fall. Fall seedlings often overwinter as a rosette of leaves, resuming growth again in the spring. Plants grow 2 to 4 feet tall with one or more stems. The leaves are deeply lobed, pale green in color and 3 to 4 inches long. Diffuse knapweed can be distinguished from the other knapweeds by the bracts below the flower. Diffuse knapweed has a rigid terminal spine about one-third of an inch long with four to five pairs of shorter lateral spines (crablike). Flower color is usually light purple with occasionally white or pink flowers.
Control on non-crop lands:
Early detection and a rapid response is best to confine the infestation to a small area. Hand pulling of all visible plants and destroyed by burning or mulching to prevent seed formation. The infested area should be treated with herbicides to prevent reinfestation from seedlings.
Chemical: The most effective herbicides are Milestone, Tordon, and dicamba. Treat an extra 10 to 15 feet around the infestation area to control seedlings. A careful follow-up program is necessary to control missed plants and seedlings.
Biological: Biological control agents have been introduced in neighboring states with limited success.