Spotted knapweed is a biennial or short-lived perennial with a stout taproot. It can have one or more branched stems and grows 1 to 3 feet ( 30-91 cm) tall.
The leaves are arranged alternately on the stem. Basal leaves grow up to 6 inches (15 cm) long, are narrowly elliptic to oblanceolate and are entire to pinnately parted. The leaves higher up the stem are pinnately divided.
Single flowering heads develop at the end of branches. The stiff bracts on the base of the flower head (involucre) are tipped with a dark comblike fringe. The ray flowers are pinkish-purple or rarely cream-colored. The flowering period extends from June to October.
The seeds are about 1/8 inch (3 mm) long and they are tipped with a tuft of persistent bristles.
Introduced from Eurasia as a contaminant of alfalfa and clover seed, spotted knapweed is an invasive weed displacing desired species and diverse plant communities. This species and other knapweeds readily establish themselves in disturbed soil. Their early spring growth makes them competitive for soil moisture and nutrients. The seed is readily dispersed by vehicles, by the sale of dried specimens for floral arrangements and movement of contaminated sand and gravel. There is also some evidence that knapweeds release chemical substances which inhibit germination and growth of surrounding vegetation.
Spotted knapweed rosettes appear in early spring as deeply lobed leaves radiating from a common point.
Flowers of spotted knapweed are usually pinkish-purple. Bracts under the flowers have dark spots tipped with fringe. Leaves of the mature plant are finely divided.