Spotted knapweed has few natural enemies and is not preferred by livestock as forage. Infestations can be traced to seed or hay brought in form neighboring states. Infestations along railroad right-of-way can be traced to wooden ties brought in from infested storage areas. Spotted knapweed may remain in a confined location for several years and then spread rapidly to adjacent areas. Controlling and monitoring the site for several years to prevent reinfestations from seed are important.
Spotted knapweed is a short lived perennial or sometimes biennial reproducing solely by seed. Seed remains viable in the soil five years or more, so infestation may occur a number of years after vegetative plants have been eliminated. Seeds can germinate from spring through early fall. Seedlings emerging in the fall often overwinter as a rosette of leaves, resuming growth again in the spring. The plants grow 2 to 4 feet tall with one or more stems. The leaves are pale green and 3 to 4 inches long. Rosette leaves are deeply lobed. Plants flower from early July through August and produce 1,000 or more seeds per plant.
Spotted knapweed is easily distinguished from other knapweeds by the bracts below the flower. Spotted knapweed has stiff, black-tipped bracts giving the appearance of having spots. Flower color is usually light purple with some having pink or white 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.