Spotted knapweed is a weedy annual or biennial with one to several erect, stiff stems that generally become branched towards the upper end of the stems. The stems range from 30-150 cm tall. The stems arise from a stout taproot. The leaves are pinnatifid, with narrow lobes. The basal leaves are up to 15 cm long, and may be densely numerous. The stem leaves become reduced in size and entire margined on the upper stems.
The flower heads tip the ends of the numerous branches. The involucre is 10-13 mm tall with the middle and outer bracts tipped with darkened comb-like fringes. The flowers are usually pink-purple, and occasionally white.
Spotted knapweed was introduced from Eurasia as a contaminant of alfalfa and clover seed. It is a major weed of several western pests. Due to its early spring growth, it may out compete native plants for both soil nutrients and moisture. Knapweeds may also release chemical substances which inhibit the growth of surrounding vegetation.
Spotted knapweed may be found in disturbed places such as fields, waste places, and roadsides.
Native to the Mediterranean region, spotted knapweed is found over much of the United States.
