CENTAUREA, in botany, a genus of the family Compositae, containing about 600 species, almost all natives of the Old World, with the principal centre in the Mediterranean region. The plants are herbs with entire or cut, often spiny-toothed, leaves, and ovoid or globose involucres surrounding a number of tubular, oblique or two-lipped florets, the outer of which are usually large and neuter, the inner bisexual. Four species are native in Great Britain: C. nigra is the knapweed, common in meadows and pastureland; C. Cyanus is the bluebottle or cornflower, a well-known cornfield weed; C. Calcitrapa is the star-thistle, a rare plant, found in dry waste places in the south of England, and characterized by the rose-purple flower-heads enveloped by involucral bracts which end in a long, stiff spine.
In eastern North America, in addition to the British species mentioned, the brown knapweed (C. Jacea), the scabious knap weed (C. Scabiosa), the Tirol knapweed (C. Vochinensis), the spotted knapweed (C. maculosa), the yellow star-thistle (C. solsti tialis) and the rayless centaury (C. melitensis) are more or less ex tensively naturalized. On the Pacific coast C. melitensis, there known as Napa thistle, is a troublesome weed, especially in Califor nia, as is also C. solstitialis; less pernicious are the star-thistle, the blue-bottle and the Turkistan thistle (C. repens). The only native North American species, the basket-flower or American star thistle (C. americana) is a robust annual 2 f t. to 6 ft. high, with very showy rose-coloured or purplish flower-heads, 3 in. to 5 in. across. It is found in dry plains from Missouri to Louisiana, and westward to Arizona and Mexico, and is also grown as an orna mental plant.
Numerous species are in cultivation as garden plants ; among these the dusty miller (C. Cineraria), the bluebottle (C. Cyanus) and the sweet sultan (C. moschata).