Black knapweed is considered an aggressive weed, invading disturbed areas, roadsides, and meadows and fields. A hybrid of Black knapweed and Brown knapweed is fertile and widespread in the northeastern United States.
Black knapweed is a perennial herb native to Europe. It was introduced to the United States as an ornamental plant and in ships' ballast and was first discovered in1895 in Pullman, Washington. The bracts of Black knapweed are black, distinguishing Black knapweed from Brown knapweed.
Purple flowers appear in heads from June to October. The flowers are rarely white.