Spotted knapweed is a short-lived, noncreeping perennial that reproduces from seed and forms a new shoot each year from a taproot. Diffuse and spotted knapweed are readily controlled with herbicides. Unless cultural techniques are used, however, the weeds will reinvade. It reproduces and spreads from seed. Rosette and lower shoot leaves are finely divided.
Leaves become smaller toward the top of the shoot and have smooth margins.
Many solitary flowering heads occur on shoot tips. Flowers usually are white but may be purplish. Involucre bracts are divided like teeth on a comb and tipped with a slender spine that makes them sharp to the touch. Sometimes the bracts are dark-tipped or spotted like spotted knapweed. The long terminal spine differentiates diffuse from spotted knapweed.