Ann. Nat. 1: 14. 1820.
Perennials, 100–200 cm (rhizomatous). Stems erect, hirsute. Leaves cauline; mostly opposite; petioles 0.4–2 cm; blades (3-nerved from bases) lanceolate to ovate, 6.5–18 × 1–8 cm, bases truncate to broadly rounded or cuneate, margins subentire to serrate (flat), abaxial faces ± hirsute, gland-dotted (adaxial not gland-dotted). Heads 1–7. Peduncles 1–5 cm. Involucres hemispheric, 10–25 mm diam. Phyllaries 18–25 (usually loose, spreading, not reflexed), lanceolate, 7–12 × 2.5–3.5 mm, (margins ciliate) apices acute to short-acuminate, abaxial faces not gland-dotted. Paleae 7–10 mm, 3-toothed (apices yellowish, hairy). Ray florets 10–15; laminae 15–20 mm. Disc florets 40+; corollas 5.5–6.5 mm, lobes yellow; anthers dark brown or black, appendages dark or yellowish. Cypselae 4–4.5 mm, glabrate or distally puberulent; pappi of 2 aristate scales 2.5–3.2 mm. 2n = 68.
Phenology: Flowering late summer–fall.
Habitat: Dry, open sites, woodland edges, roadsides
Elevation: 10–900+ m
Ont., Ala., Ark., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Mich., Minn., Miss., Mo., Nebr., N.Y., N.C., Ohio, Okla., Pa., S.C., Tenn., Tex., Va., W.Va., Wis., Mexico (Coahuila, Nuevo León).
Helianthus hirsutus is distinguished from H. strumosus by hairy stems and usually yellow (as opposed to dark) anther appendages, and from H. divaricatus by petioles and leaf blades 3-nerved distal to bases. Mexican plants of H. hirsutus are sometimes labeled with the synonymous H. leptocaulis (S. Watson) S. F. Blake, and plants from Mexico and the southwestern United States often have leaf bases cuneate rather than truncate.