December’s Weed of the Month is not one, but three species of knapweed: brown knapweed (Centaurea jacea), meadow knapweed (C. x moncktonii), and diffuse knapweed (C. diffusa). All three species are native to Eurasia, and it’s thought they were introduced to western North America for forage. In Minnesota, infestations of meadow knapweed have been reported in St. Louis and Koochiching counties, brown knapweed has been reported in Koochiching County, and diffuse knapweed has been found in Duluth.
Several characteristics of these knapweed species have helped them thrive and become a threat to Minnesota. They outcompete pasture grasses and native plants, leading to large bare patches of soil that is more susceptible to erosion and water runoff. Once established, the knapweeds reduce hay quality and pasture productivity. The plants can also hybridize between species, which makes identification difficult and increases the risk for an aggressive plant that can invade many soil types and growing conditions.
Knapweeds can be biennial or perennial. They reproduce primarily by seed, which can be spread with infested hay, on equipment, or by wind and water movement. The flowers are tight clusters of individual flowers called florets. The flower color ranges from pink to white. The plants produce a rosette of leaves, which then sends up a flowering stalk in the summer.
To prevent the knapweeds from further spreading throughout Minnesota, several management strategies are available.
To report infestations of these species of knapweeds or any other noxious weeds on the eradicate list, please use the Early Detection and Distribution MAPping System.