War on Weeds—weeds are everybody’s problem
Photo courtesy of Cindy Roche, Bugwood.org
THE ISSUE: Diffuse knapweed
Diffuse knapweed (Centaurea diffusa) is an annual, biennial, or short-lived perennial forb native to southeast Europe and western Asia. It was introduced into the U.S. as a contaminate in seed. It is not palatable to livestock, allowing it to outcompete palatable plants in grazing systems, subsequently allowing it to replace native vegetation. It invades roadsides, pastures, rangeland, and sandy riverbanks.
After diffuse knapweed germinates, it develops into a rosette with greyish green leaves that are finely divided. As the plant bolts and matures leaves develop short dense hair. Leaves located towards the bottom of the plant are long and lobed. Leaves become decreasingly smaller towards the top of the plant. It typically grows single stems with many branches, that can grow up to 3 feet tall. It has a stout taproot that allows it to thrive in dry conditions. Flowers are usually white but may also be pink to lavender in color. As it matures it develops into a tumble weed, spreading its seed across the landscape as it is pushed along by the wind.
Integrated Pest Management (IPM) Options:
• Prevention— Learn to identify this plant. Never transport unknown plant material. Always plant clean seed!
• Mechanical— Mechanical control methods are not usually recommended as it can cause further invasion. Under some circumstances tillage can be effective if used with other control practices. Mowing is not recommended either because it can cause plants to produce more flowers than normal.
• Cultural— Plant beneficial perennial plants that will compete with weed seedlings. Manage plant populations in a way that will benefit beneficial plants. Burning is not recommended, as it will stimulate germination.
• 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!
Justin Hatch, University of Idaho Extension Agriculture Educator in Caribou County. 208-547-3205 [email protected]
