Annuals, 40-200 cm. Stems erect, usually densely canescent, hispid, or strigillose, rarely ± hirsute or glabrate. Leaves mostly cauline; mostly alternate; petioles 2-4 cm; blades (often bluish green) lanceolate to deltate-ovate or ovate, 4-15 × 1-8 cm, bases subcordate or truncate to cuneate, margins entire or ± serrate, abaxial faces strigose, sparsely to densely, or not at all, gland-dotted. Heads 1-5. Peduncles 4-15(-40) cm. Involucres ± hemispheric, 10-24 mm diam. Phyllaries 14-25, lance-linear to lanceolate to lance-ovate, 10-14 × 1-4(-5) mm, (margins sometimes ciliate) apices short-attenuate, abaxial faces usually hispidulous, rarely sparsely hirsute to glabrate. Paleae 4.5-7.5 mm, 3-toothed, middle teeth ± ciliate or bearded, hairs whitish, 0.5-0.7 mm. Ray florets 10-30; laminae 15-20 mm. Disc florets 50-100+; corollas 4.5-6 mm, lobes usually reddish, rarely yellow; anthers reddish to purplish, appendages purplish (style branches reddish). Cypselae 3-4.5 mm, ± villous; pappi of 2 aristate scales 1.5-3 mm plus 0-2 erose scales 0.3-0.5 mm. 2n = 34. Helianthus petiolaris is adventive beyond western North America.

A third subspecies has yet to be named at that rank in Helianthus petiolaris; it has been called H. petiolaris var. canescens A. Gray. It differs in having stems, leaves, and phyllaries densely canescent and abaxial faces of leaves densely gland-dotted. It is additionally characterized by peduncles usually ebracteate, phyllaries 1-2 mm wide, disc corolla throats gradually narrowed distal to slight, not densely hairy basal bulges, and 2n = 34. It flowers late spring through late summer and grows on sandy soils in open areas at (10-)1000-2300 m in Arizona, California, Nevada, New Mexico, and Texas and in Mexico. It was treated as H. niveus (Bentham) Brandegee subsp. canescens (A. Gray) Heiser by C. B. Heiser et al. (1969); molecular and morphologic data appear to favor a placement within H. petiolaris.

Plant: Robust annual forb to 2 m in moist areas Leaves: leaves opposite below, alternate above, scabrous, lanceolate, petiolate