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Ecology and Management of Diffuse Knapweed

Friday, August 5th, 2011

By Jim Jacobs, Invasive Species Specialist, NRCS, Bozeman, Montana and Sharlene Sing, Assistant Research Professor, Montana State University, Bozeman, Montana


A close relative of spotted knapweed in the Asteraceae taxonomic family, diffuse knapweed is typically biennial, reproducing exclusively by seed. It forms a rosette with a central crown and tap root in the juvenile stage and a single upright stem one to three feet (0.3 to 0.9 m) tall with numerous spreading branches at maturity. White (occasionally purple) flowers are borne in heads with spiny bracts (see Figure 1). It is generally found on more arid sites than spotted knapweed. Native to the Mediterranean region, diffuse knapweed was first recorded in Montana from Mineral County in 1951 and by 2008 had been reported from 39 of Montana’s 56 counties. Its dense, spiny overstory reduces the availability of desirable forage plants to livestock and wildlife, and grass production can be reduced by over 90% in heavy infestations.

Declines in diffuse knapweed populations in western North America have been attributed to the lesser knapweed flower weevil (Larinus minutus), one of 14 approved biological control insects it is host to. Herbicidal management can best be achieved by application of any of the following chemicals at the rosette to bolt stages: aminopyralid, clopyralid, dicamba, picloram, and 2,4-D. Grazing management using sheep or goats should be applied during the rosette and bolt stages before the formation of spiny flowerheads, and animals should be removed when 50% of grass forage has been utilized. Tillage can be used to remove plants and release the seed bank. Persistent hand pulling and grubbing may be practical on small scale infestations. Diffuse knapweed re-sprouts following fire, including intense wildfire. Cultivating and re-vegetating competitive plants will improve the longevity of control applications and reduce re-establishment from the seed bank.

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