1. Bachelor's Button, Cornflower, Garden Cornflower: Centaurea cyanus - An introduced species, with showy flower heads. The flowers are bright blue, pink, purple, or white.

2. Diffuse Knapweed, Tumble Knapweed, White Knapweed: Centaurea 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.

3. Spotted Knapweed: Centaurea stoebe ssp. micranthos (Synonyms: Centaurea biebersteinii, Centaurea maculosa) - 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.

4. 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.