Initial growth is in the form of a rosette. Lower leaves are early deciduous. Cauline leaves are alternate, smaller, and pinnatifid, while the upper reduced leaves are entire. Achenes are 2-3 mm long, light brown to black, with the pappus generally absent or consisting of only a fringe of bristles less than 1 mm long. The bracts of spotted knapweed have dark tips with no spine.
Russian knapweed has rounded bracts with pointed papery tips.
Diffuse knapweed is a highly competitive, noxious weed native to Eurasia. The earliest record of diffuse knapweed in western North America is from an alfalfa field in Washington in 1907. It may have been introduced with Turkestan alfalfa from the Caspian Sea region. This plant is also capable of invading relatively undisturbed native plant communities. Diffuse knapweed generally forms large, dense infestations, but solitary plants and small patches are common in recently invaded areas.