Diffuse knapweed flowers
USDA APHIS PPQ Archive, Bugwood.org
Diffuse knapweed is native to Eurasia. It was accidentally introduced into the U.S. through contaminated seed.
Keywords: Asteraceae, annual, biennial, short-lived perennial, herbaceous, tumble weeds; Common names: white knapweed, spreading knapweed, tumble knapweed
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Diffuse knapweed distribution
Diffuse knapweed is an annual, biennial, or short-lived perennial herbaceous plant growing 4-24 in. tall. Plants form low rosettes and may remain in this form for one to several years. Stems are upright, highly branched, and angled, with short, stiff hairs on the angles. Plants have long, deciduous basal leaves forming the rosette and stem leaves that are alternate, smaller, less divided, stalkless, and bract-like near flower clusters. Urn-shaped flower heads contain ray flowers around the edges surrounding tubular disk flowers. Petals are white, rose-purple, to lavender. Small dry fruits contain light brown to black wind-dispersed seeds. A single plant can produce up to 18,000 seeds. Diffuse knapweed grows along rights-of-way, roads, dry forest edges, open forested areas, fringe forest, riverbanks, rangelands, and disturbed or overgrazed lands. Plants prefer open habitats to shaded areas, light, dry, porous soils, and semi-arid to arid conditions. Diffuse knapweed has the ability to travel and spread seeds over relatively long distances as a tumble weed. Once established, it can form monotypic stands, suppress other vegetation by competing for water, and crowd out native species.