Perennials, 50-200 cm (with crown buds; nonflowering stems usually absent). Stems erect, proximally villous to strigoso-hispid, distally hispid or glabrate. Leaves mostly basal; opposite; petioles 4-25 cm (usually winged at least 1/2 their lengths); blades lanceolate to ovate, 7-26 × 3-10 cm, bases broadly cuneate to nearly truncate (often decurrent onto petioles), margins serrate to crenate, abaxial faces strigoso-hispid (hairs of midribs1+ mm), not gland-dotted (cauline to 8 pairs proximal to heads, ovate to elliptic, smaller). Heads (1-)3-15+. Peduncles 0.3-17 cm. Involucres broadly hemispheric, 9-16 mm diam. Phyllaries 15-22, broadly ovate to oblong, 7-9 × 4-5 mm, (margins ciliolate) apices obtuse to acute, sometimes mucronate, abaxial faces usually glabrous. Paleae 4-5.5 mm, ± 3-toothed to entire. Ray florets 10-15; laminae 15-22 mm (abaxial faces not gland-dotted). Disc florets 75+; corollas 4-6 mm, lobes reddish; anthers dark, appendages dark (style branches yellow ). Cypselae 2.8-3 mm, glabrous or distally puberulent; pappi of 2 aristate scales 2.5-2.8 mm. 2n = 34. Flowering late summer-fall. Open mixed woods, roadsides; 0-900 m; Ala., Fla., Ga., Ky., La., N.J., N.C., S.C., Tenn., Va. Helianthus atrorubens and H. silphioides form a morphologically similar pair of species that share the distinctive feature of relatively broad, tightly appressed phyllaries with apices obtuse to acute. Helianthus atrorubens has a more easterly geographic distribution along the Piedmont and the Atlantic coastal plain and the southern Appalachian Mountains; distribution of H. silphioides is centered in the Ozark region. In general, H. silphioides differs in its usually well developed cauline leaves, and basal leaves with winged petioles that are less than half the total lengths. Involucres with phyllaries tightly appressed are also observed in H. occidentalis and H. pauciflorus subsp. pauciflorus, which differs in having the phyllary apices acute to acuminate as well as in usually having abaxial faces of leaves and ray laminae densely gland-dotted.
Fibrous-rooted perennial from a very short, stout rhizome or crown; stems mostly solitary, 0.5-2 m, usually conspicuously spreading-hairy, at least below; lvs nearly all opposite, 3-8 pairs below the infl, commonly hairy on both sides, especially on the main veins beneath, lance-ovate to sometimes broadly ovate, mostly (1.3-)1.7-2.5(-3) times as long as wide, abruptly contracted to the petiole, the largest ones near the base, these commonly 6-20(-25) cm, toothed; petioles tending to be conspicuously wing-flared upward, often conspicuously spreading-hairy, the lower generally 1/3 to fully as long as the blade; heads several on long, naked peduncles in a corymbiform infl; disk red-purple, 1-1.5(-2) cm wide; invol bracts evidently imbricate, broad, firm, appressed, mostly oblong or elliptic, rounded to acutish, sometimes with an abrupt, very short acumination, 2.5-4(-5) mm wide, ciliolate, otherwise glabrous or nearly so; rays 10-15, 1-3 cm; pappus without accessory scales; 2n=34. Dry, open woods; basically Appalachian and Atlantic, from Va. and e. Ky. to c. Ga. and Ala., w. to w. Tenn. and se. La. July-Oct.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.