Knapweeds are annual, biennial, or perennial herbs with alternate or all basal, entire to pinnately lobed leaves, and solitary to numerous, small to large flowerheads. The flowerheads have disk flowers only, the flowers sometimes being all trumpet-shaped and with both stamens and pistils, or more commonly the marginal ones are sterile, with enlarged, irregular corollas looking like rays. The involucral bracts partly overlap in several series. They are either spine-tipped or more often some of them have enlarged, membranous or thin appendages, which are dry, translucent, irregularly edged to irregularly cut or comb-like. The receptacle is nearly flat, and densely bristly. The corollas are purple or blue to yellow or white, with slender tube and long narrow lobes. The anthers are shortly to strongly tail-tipped. The style has a thickened, often hairy ring and an abrupt change of texture at the base of the flat, mostly more or less joined branches. The stigma-bearing lines are marginal, extending nearly or quite to the tip. The achenes are unequally-sided or sideways attached to the receptacle, seldom evidently nerved. The pappus consists of several series of graduated bristles or narrow scales, often much reduced, or lacking. The genus consists of about 400 species worldwide, centering in the Mediterranean region, but a few species being native to N. America, S. America, and Australia. All of our species are introduced, and some are troublesome weeds. The name comes from the Greek kentaurion, plant of the Centaurs.
Guide to Identify Presented Species of Genus Centaurea
FLOWERS PINK OR PURPLE
C. maculosa – Spotted Knapweed Rough-hairy, branched biennial. Noxious weed in fields and roadsides.
Flowerheads pink-purple, 2-2.5 cm wide. Involucral bracts with dark, fringed tips.
Leaves alternate, pinnate with thin, narrow lobes, or reduced upper ones entire.
