Perennials, 50-300 cm (with crown buds; nonflowering stems usually present). Stems erect, hispid to strigoso-hispid proximally, glabrate distally. Leaves basal and cauline; mostly opposite; petioles 0.1-5.5 cm; blades ovate to broadly ovate or suborbiculate, 7-15 × 4.5-15 cm, bases rounded to truncate or ± cuneate, margins entire or crenate to serrate, abaxial faces usually scabrous, sometimes ± strigose (hairs on midribs to 1 mm), not gland-dotted (cauline 9+ pairs proximal to heads, smaller ). Heads 3-15+. Peduncles 0.5-10 cm. Involucres hemispheric, 10-20 mm diam. Phyllaries 16-23, oblong to obovate, 8-10 × 3-5 mm, (margins ciliolate) apices obtuse to acute, sometimes mucronate, abaxial faces glabrous or glabrate. Paleae 9-10 mm, entire or ± 3-toothed. Ray florets 8-13; laminae 15-20 mm (abaxial faces not gland-dotted). Disc florets 75+; corollas 6-7 mm, lobes reddish; anthers dark, appendages dark (style branches yellow ). Cypselae 3-4 mm, glabrous or distally puberulent; pappi of 2 aristate scales 2.5-2.7 mm. 2n = 34. Flowering late summer-early fall. Open sites; 100-300 m; Ala., Ark., Ill., Ky., La., Miss., Mo., Okla., Tenn. Similar to Helianthus atrorubens, H. silphioides is distinguished by shorter hairs (less than 2 mm ) on stems proximally and on abaxial leaf midveins, and by petioles of basal leaves less than 1/2 lengths of blades and winged less than 1/2 their lengths. As befits the name, specimens of H. silphioides (and also H. atrorubens) are not infrequently misidentified as species of Silphium.
Similar to H. atrorubens, and passing into it where their ranges overlap; stems commonly several, to 3 m, relatively more leafy, often to above the middle; blades relatively broad, generally only 1-1.7(-2) times as long as wide, on short, generally inconspicuously hairy petioles that are seldom more than 1/3 as long as the blade and seldom conspicuously wing-flared upward; invol bracts not always ciliolate; 2n=34. Basically Ozarkian, from Ark. and s. Mo. to c. La., e. to s. Ill., w. Ky., and Tenn. July-Oct. Perhaps better called H. atrorubens var. pubescens Kuntze.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.