June’s Weed of the Month is spotted knapweed (Centaurea stoebe ssp. micranthos). It is a grassland perennial plant native to Eurasia. It was first recorded in North America in 1893 and in Minnesota in 1918, likely introduced as a contaminant of alfalfa seed.
Spotted knapweed is able to outcompete other plants by exuding a toxic chemical from its roots. This allows it to establish and spread quickly to colonize vast acreages. Infestations decrease forage, wildlife habitat, plant diversity and recreational quality yet increase soil erosion. A study determined that spotted knapweed caused annual losses of $42 million in Montana. In Minnesota, it is most abundant in the northwest and Twin Cities but is spreading throughout the state. Spotted knapweed is overtaking natural and disturbed habitats, roadsides, pastures, and field margins.
Spotted knapweed is a short-lived perennial that grows two to four feet tall. It forms a rosette its first year and flowers in subsequent years. The leaves are grayish-green, narrow near the top of the plant and lobed near the bottom. The small, thistle-like pink to purple flowers are distinctive for the stiff, dark bracts that give it a spotted appearance. It blooms from June to August and reproduces by seed. Spotted knapweed also forms a thick taproot with lateral shoots that produce new rosettes.
As a prohibited noxious weed on the control list, landowners must attempt to control the spread of spotted knapweed seed. To manage spotted knapweed, infestations need to be monitored and treated until the seedbanks are depleted.