Centaurea maculosa

Spotted Knapweed is a herbaceous biennial or perennial weed in the Asteraceae family native to Europe and western Asia and is considered invasive in many states within the United States. The plant grows on stream banks, pond shorelines, sand prairies, old fields and pastures, roadsides, and along railroads, and many open, disturbed areas. It crowds out native species and forage for livestock. The species prefers to grow in dry, sterile, gravelly or sandy soils and full sunlight.  

Spotted Knapweed usually has a stout taproot and/or pubescent stems when young. A basal rosette of deeply lobed petiolate leaves covered in fine, short hairs approximately 8 inches in size is produced the first year.  Pubescent stems form when young with highly dissected leaves that are alternate and may be slightly lobed or linear. Leaves become smaller and less lobed toward the top. The stem is erect or ascending, slender, hairy and branching, and can grow up to three feet tall. As many as 6 stems emerge from the same root crown during a single growing season. The flowers are a vibrant lavender with black margins of the flower bract tips which give the flower heads a spotted look, which is the origin of its common name. The fruit is an achene (about a quarter-inch long) finely hairy, straw-colored at maturity with a tuft of short, stiff, light brown bristles at the tip.  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 controlled 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 control 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.