Spotted Knapweed Centaurea maculosa

Spotted knapweed is an herbaceous biennial or perennial weed in the Asteraceae (aster) family and native to Europe and western Asia. It is considered invasive in many states within the United States, mostly the northwest and north central states. The plant grows on stream banks, pond shorelines, sand prairies, old fields and pastures, roadsides, along railroads, and many open, disturbed areas. It crowds out native species and forage for livestock. The flowers are a vibrant lavender with black margins of the flower bract tips, which give the flower heads a spotted look and is the origin of its common name.   

Spotted knapweed prefers to grow in dry, sterile, gravelly or sandy soils, and full sunlight. It usually has a stout taproot below a basal rosette of deeply lobed leaves covered in fine, short hairs. As many as 6 erect stems emerge from the same root crown during a single growing season and grow up to 3 feet tall. The seeds are primarily distributed by the wind. An individual plant may produce as many as 40,000 seeds. 

Light infestations of this invasive weed can be managed by pulling the weeds by hand or by mowing early when the plant is just beginning to flower. Plants are less likely to regrow if allowed to form flower stalks before mowing. All management methods must be repeated for several years until seeds stored in the soil become depleted. Spotted knapweed is resistant to some herbicides, likely due to wooly hairs on its stems and leaves limiting retention or penetration of the herbicide.

Insects, Diseases, and Other Plant Problems: Invasive tendencies.