Perennials, 30-90 cm. Internodes (± mid stem) 5-11 cm. Leaves: petioles 0-1 mm; blades simple or 3-foliolate, simple blades or leaflets ± lance-elliptic to narrowly lanceolate, 25-55(-100+) × (4-)6-15(-30+) mm (seldom lobed). Peduncles 2-6+ cm. Calyculi of 7-8 oblong to linear bractlets 3-6 mm. Phyllaries 8, lance-oblong to lance-ovate, 6-8 mm. Ray laminae 12-30+ mm. Disc florets 40-80+; corollas yellow (often drying blackish to purplish), 6-7 mm. Cypselae obovate to oblong, 4-5 mm. 2n = 26, 78, 104. Flowering May-Aug. Oak barrens, road clearings, wooded bottoms; 300-800 m; Ala., Fla., Ga., Ind., Ky., La., Mass., Miss., N.Y., N.C., Ohio, Pa., S.C., Tenn., Va., W.Va.
Much like no. 5 [ Coreopsis tripteris L.], but smaller, 5-10 dm, vigorously rhizomatous but the stems commonly tufted, the herbage often short-hairy; lvs sessile, the lfls averaging a little shorter, 3-8 cm; outer invol bracts longer and often leafier, sometimes equaling the inner; 2n=26, 78, 104. Mostly in dry, open woods; s. Pa. and O. s. to Fla. and Tex. July-Sept. Most of our plants belong to the widespread var. major, with lfls 1-3 cm wide. The var. rigida (Nutt.) Boynton, with firmer lfls mostly 5-10 mm wide, often in more exposed or less favorable sites, occurs from Va. to S.C. and Ga., but not to the exclusion of var. major; it has 2n=104 and may reflect alloploid introgression from C. delphinifolia Lam., which is itself thought to be an alloploid of nos. 6 [Coreopsis major Walter] and 7 [Coreopsis verticillata L.].
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.