20. Helianthus silphioides Nuttall, Trans. Amer. Philos. Soc., n. s. 7: 366. 1841.
Helianthus kentuckiensis F. T. McFarland & W. A. Anderson
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.