War on Weeds — Weeds are Everybody’s Problem

Spotted knapweed

Spotted Knapweed (Centaurea stoebe) is a biennial or short-lived perennial native to Eastern Europe. It was introduced into the U.S. in the 1800’s as a contaminate in seed. It is an extremely invasive weed that can spread quickly. It invades roadsides, rangeland and other dry disturbed sites.

Spotted knapweed seeds can germinate throughout the year and then they spend their first year as a rosette. During the second year they typically bolt, sending up stems that can grow up to 3 feet tall. Rosette leaves are deeply lobed and covered in short coarse hair. Upper leaves have lance shaped lobes, are grayish green in color and decrease in size towards the top of the plant. Flowers heads are urn-shaped and are topped with pink to purple flowers. Spotted knapweed is a prolific seed producer, a single plant can produce 1-100 flower heads and each flower head can produce up to 50 seeds. Seeds can remain viable in the soil for up to seven years. One key characteristic that can be used to tell the difference between spotted knapweed and other knapweeds is the bracts, found on the flower heads. Spotted knapweed bracts are triangular, and the upper tip is dark, giving it a spotted appearance.

Integrated Pest Management (IPM) options

• Prevention — Learn to identify this plant. Never transport unknown plant material. Clean clothing, pets and vehicles of debris when leaving infested areas.
• Mechanical — Mowing is not an effective form of control; plants will send up new shoots or flower below the height of the mower. Tillage can effectively control young plants but may not be effective for more established plants.
• Cultural — Plant beneficial perennial plants that will compete with weed seedlings. Minimize soil disturbance and use proper grazing techniques to benefit beneficial plants.
• 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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• Spotted knapweed, 2-22