The flowers of this species are all discoid, but I have placed them here because the outer ring of corollas may be greatly enlarged, falsely giving them the appearance of having ray flowers to those who don't look closely.
Blessed Thistle: Centaurea benedicta (Synonym: Cnicus benedictus) -
Bachelor's Button, Cornflower, Garden Cornflower: Centaurea cyanus - An introduced species, with showy flower heads. The flowers are bright blue, pink, purple, or white.
Diffuse Knapweed, Tumble Knapweed, White Knapweed: Centaurea diffusa (Synonym: Acosta diffusa) - An introduced, weedy species. The stems are stiff, erect, and many branched. Flower heads are narrow, one at the end of each branch, with a few white flowers.
Hybrid Knapweed, Meadow Knapweed, Protean Knapweed: Centaurea xgerstlaueri (Synonyms: Centaurea debeauxii ssp. thuillier, Centaurea xmoncktonii, Centaurea nigra x Centaurea jacea, Centaurea xpratensis, Centaurea pratensis) -
Brown Knapweed, Brownray Knapweed: Centaurea jacea (Synnoyms: Centaurea jacea nothossp. pratensis, Centaurea jacea ssp. jacea, Centaurea jacea ssp. nigra) -
Maltese Starthistle, Maltese Star-thistle, Tocalote: Centaurea melitensis -
Black Knapweed, Hardheads, Lesser Knapweed: Centaurea nigra -
Short-fringed Knapweed, Tyrol Knapweed: Centaurea nigrescens (Synonyms: Centaurea dubia, Centaurea dubia ssp. nigrescens, Centaurea dubia ssp. vochinensis, Centaurea jacea ssp. nigrescens, Centaurea transalpina, Centaurea vochinensis) -
St. Barnaby's Thistle, Yellow Star Thistle, Yellow Star-thistle: Centaurea solstitialis - An introduced, weedy species. The stems are stiff, erect, and many branched. The flower heads have yellow flowers, and distinct, long, straw-colored spines arising from the involucral bracts.
Spotted Knapweed: Centaurea stoebe ssp. micranthos (Synonyms: Centaurea biebersteinii, Centaurea maculosa, Centaurea stoebe ssp. australis) - An introduced, weedy species. The stems are stiff, erect, and many branched. The flower heads are larger than the previous species, with purple-pink flowers. A good diagnostic feature is the dark comb-like bristles at the tips of the involucral bracts.
Hybrid Diffuse Knapweed, Sand Knapweed: Centaurea xvarnensis (Synonym: Centaurea xpsammogena) -