Quite possibly the fastest spreading weed in the western U.S. at the end of the 1900`s. This species can act as an annual, biennial, or perennial. The plants are adapted to spread as a tumbleweed but are more rapidly spread by vehicles. In general this species prefers a drier site than spotted knapweed, but the two are often found together. Diffuse knapweed can have dark tipped bracts like spotted knapweed (attributed by some to hybridization between the two species) but can always be distinguished by the fact that the terminal spine of the bract is always longer than the marginal spines.