Annuals or perennials, 30-200 cm (taprooted). Stems decumbent to erect, glabrous, hirsute, or puberulent. Leaves mostly cauline; mostly alternate; petioles 1-7 cm; blades deltate-ovate, lance-ovate, or ovate, 2.5-14 × 1.8-13 cm, bases cordate to truncate or broadly cuneate, margins subentire to serrate, abaxial faces glabrate to hispid, not gland-dotted. Heads 1-3. Peduncles 9-50 cm. Involucres hemispheric, 10-22 mm diam. Phyllaries 20-30, lanceolate, 8-17 × 1-3 mm, apices acute to long-attenuate, abaxial faces glabrous or ± hispid, not gland-dotted. Paleae 7.5-8 mm, apices 3-toothed (middle teeth acuminate, usually glabrous or hispid, sometimes ± villous or bearded). Ray florets 11-20; laminae 12-23 mm. Disc florets 30+; corollas 4.5-5 mm, lobes usually reddish, sometimes yellow; anthers dark, appendages dark (style branches usually reddish, rarely yellow). Cypselae 2.5-3.2 mm, glabrous or sparsely hairy; pappi of 2 lanceolate or lance-linear scales 1.2-2.5 mm. C. B. Heiser (1956) placed 8 subspecies in Helianthus debilis; he noted that alternative taxonomic treatments might recognize these in as many as three species, or expand the single species to include H. petiolaris. Later, Heiser et al. (1969) separated three of the subspecies as H. praecox. Isozyme data (R. P. Wain 1982, 1983; L. H. Rieseberg and M. F. Doyle 1989) show that all are closely related. Documented hybridization with H. annuus further complicates the situation. The treatment by Heiser et al. is followed here.
Helianthus debilis is adventive beyond the Atlantic and Gulf coasts of the United States.