War on Weeds: Spotted knapweed

Jeffrey Pettingill


This weed may invade your land. Be ready to oppose it.

The Enemy: Spotted knapweed (Centaurea stoebe)

Strategy: This biennial and mostly perennial aster is native to Europe. It grows about 3 feet tall and has a single taproot that can go as deep as 18 inches. It has a pink flower (seldom with white) with brown spots on the flower head. The plant has heavily lobed leaves and can produce more than 25,000 seeds.

Attack: This is probably the most widely spread weed in North America. Just walk along the Shelley greenbelt to see all you desire. Due to it’s ability to be spread by the wind, it can quickly establish itself in dry and moist areas. It releases a chemical that prevents other plants from growing near it, and the seed can remain viable for eight years. It is rarely fed on by livestock and will replace grasses and forbs in pastures and the range.

Defense: This plant is not difficult to control; it just grows fast and everywhere, so people have a tendency to give up on it. Mechanical control is effective and is why we do not see it in cultivated crops, except hay. Numerous insects have been released to control the flowers, as well as its root growth. Sheep and goats will eat the invader. Herbicides such as Tordon 22k, Milestone, Chaparral/OpenSight and Curtail/WideMatch offer great control up to bud stage or even into late fall. Banvel and 2,4-D have been used with unsatisfactory results. Multiyear programs including replanting desirable species is a must. Many species of knapweed exist, so make sure you know your pest prior to controlling them. Call your county weed superintendent for positive identification and recommendations.

To learn more, call Bonneville County Weed Superintendent Jeffrey Pettingill at 208-529-1397 or email weeds@co.bonneville.id.us.