Noxious bushy weeds with spiny or comb-like phyllaries and white, pink, or purple flowers. Plants exist as basal rosettes until erect, highly branched flowering stems with are produced late spring/summer. Centaurea species produce allelopathic effects and are highly competitive with other plants, often displacing desired vegetation. Centaurea is a large genus comprised of about 500 species, Typically biennial, occasionally annual or triennial, to 0.8 m tall. Usually forms large, dense infestations. Introduced from southeast Eurasia

SEEDLINGS :Cotyledons spatulate to oval. Rosette leaves pinnate-divided. Purple and Iberian starthistles develop straw-colored spines at the centers of rosettes

MATURE PLANT:Upper stem leaves not winged. Foliage variously covered with short to medium interwoven gray hairs. Leaves alternate. Lower stem leaves deeply 1- or 2-pinnate-lobed, ~ 10-20 cm long.

ROOTS and UNDERGROUND STRUCTURES:Taprooted long in diffuse knapweed

FLOWERS: Flower heads consist of few to many fertile disc flowers interspersed with long bristles on the receptacle. Phyllaries overlapping in several rows, with tips variously spiny or comb-like. Phyllary characteristics are important for species identification. Refer to the Flower Head Comparison of White, Pink, or Purple-flowered Centaurea and Related Species Table for a comparison.

• spotted knapweed: June-October. Flowers 30-40 per head. Corollas white, pink, or purple, 12-25 mm long. Involucre (unit of phyllaries) length 10-13 mm. Phyllaries pale green or pink-tinged, with parallel veins. Phyllary tips dark, comb-like, not spine-tipped. Self-fertile.
• diffuse knapweed: June-September. Flowers average 12-13 per head. Corollas white, pink, or pale purple, 12-13 mm long. Involucre length 10-13 mm. Main phyllaries pale green, spine-tipped; spines straw-colored. Central spine spreading, to 3 mm long.
• squarrose knapweed: June-August. Flowers 4-8(10) per head, outer sterile. Corollas pink to pale purple, 7-9 mm long. Involucre 7-8 mm long. Main phyllaries pale green to straw-colored, sometimes purple-tinged, spine-tipped. Central spine usually reflexed downwards, to 3 mm long.

HABITAT: Fields, roadsides, disturbed open sites, grasslands, overgrazed rangelands, and logged areas. Plants seldom persist in shaded places and colonize most soil types with a disturbed A horizon.