War on Weeds — Weeds are Everybody’s Problem

Meadow knapweed

Meadow knapweed (Centaurea debauxii ssp. thuillieri), a perennial forb native to Europe, is thought to be a fertile hybrid between black knapweed (Centaurea nigra) and brown knapweed (Centaurea jacea). It is not palatable to livestock, allowing it to outcompete palatable plants in grazing systems. It invades roadsides, pastures, rangeland and riverbanks. It prefers moist sites.

Meadow knapweed grows 20-40 inches tall. Leaves can be up to 6 inches long and 1.5 inches wide at the base of the plant, leaves higher on the stem are progressively smaller. Leaves at the base of the plant can be smooth, lobed or have toothed edges. Leaves towards the top of the plant have smooth edges. Flowers can be pink, purplish-red or white; heads can be up to 1 inch wide. Flowers are produced from midsummer to fall. Seeds are brownish gray and have plumes on the tips that fall off when mature.

Look-a-Likes: Meadow knapweed is the offspring of black knapweed and brown knapweed; all three species look very similar. They can be differentiated by bracts under the flower. Black knapweed has dark bristly bracts, brown knapweed has bracts that are rounded and papery and meadow knapweed shares characteristics of both.

Integrated Pest Management (IPM) options

• Prevention — Learn to identify this plant. Never transport unknown plant material. Always plant clean seed.
• Mechanical — Cultivating infested areas frequently can control meadow knapweed populations.
• Cultural — Using fallow in a rotation is an effective way to control this weed.
• Biological — Blunt knapweed flower weevil (Larinus obtusus), has been found to cause damage to flower heads.
• Chemical — There are several herbicide options available to use on this weed species. For more information on products and rates visit: https://pnwhandbooks.org/weed. Then search “Knapweeds.” Always read and follow herbicide label directions.

Idaho is a wonderful place with awesome natural resources, landscapes, agriculture and recreational opportunities. Weeds threaten all the things that we love about Idaho. To detect, contain and control weeds in Idaho we have declared “War on Weeds.” These short articles are designed to give a brief description of each species, an explanation about why it’s a problem, insights into identification and Integrated Pest Management control options. — Justin Hatch

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• Meadow knapweed, 4-20