War on Weeds—weeds are everybody’s problem
Photo courtesy of Cindy Roche, Bugwood.org
THE ISSUE: Squarrose knapweed
Squarrose knapweed (Centaurea virgata) is a long-lived perennial native to the eastern Mediterranean area. It invades dry disturbed areas as well as grasslands and rangelands. It tolerates dry and cold conditions better than other knapweed species. Its ability to form dense monocultures allows it to out compete native species. It is not palatable to livestock or wildlife when plants are mature which gives it an advantage over desirable vegetation.
Squarrose knapweed stems are highly branched, rough in texture, and can grow 1-3 ft. tall. It has a woody crown and thick taproot. It has deeply lobed lower and rosette leaves. Upper leaves are smaller, linear, have smooth edges and have a bract-like appearance. A single pink to pale-purple flower is produced at branch tips. Squarrose knapweed has pale green to straw-colored bracts at the base of each flower head. Spines on seed heads cling to animals and clothing aiding in dispersal.
Look- A- Likes
Diffuse knapweed (Centaurea diffusa) looks similar, but it has teeth on the edges of the bracts and the bracts are not curved outward like squarrose knapweed.
Integrated Pest Management (IPM) Options:
• Prevention— Learn to identify this plant. Never transport unknown plant material. Always plant clean seed!
• Mechanical— Hand digging or pulling can be effective on small infestations. Remove as much of the crown as possible. Cultivation can be effective if roots are cut deep below the soil surface.
• Cultural— Establish a healthy stand of beneficial plants. Grazing plants when they are young and palatable may help suppress populations.
• Chemical—Aminopyralid and Picloram are two herbicide active ingredients that can effectively control this weed. Refer to https://pnwhandbooks.org/weed then search “knapweeds”. Always read and follow herbicide label directions.
Justin Hatch, University of Idaho Extension Agriculture Educator in Caribou and Bear Lake Counties. 208-547-3205 [email protected]
