Perennials, 50-150+ cm (rhizoma-tous). Stems erect, hirsute to villous. Leaves mostly cauline; mostly opposite (sometimes alternate among heads); sessile; blades (ashy or gray-green, 3-nerved distal to bases) lanceolate to broadly ovate, 5.5-14.5 × 1.8-6.5 cm, bases rounded to cordate, margins entire or serrulate, abaxial faces hispid to tomentose, gland-dotted. Heads 1-15. Peduncles 0.1-15 cm. Involucres broadly hemispheric, 12-25 mm diam. Phyllaries 30-40, lanceolate, (5-)10-16 × 2-3.5 mm, apices usually acute, sometimes acuminate, abaxial faces densely hispid to villous or tomentose, densely gland-dotted. Paleae (oblanceolate) 9-11 mm, entire (1-toothed, densely hairy, densely gland-dotted). Ray florets 17-22; laminae 25-30 mm (abaxial faces densely gland-dotted). Disc florets 75+; corollas 6-7.5 mm, lobes yellow; anthers dark, appendages dark. Cypselae 3.5-4 mm, distally villous; pappi of 2 aristate scales 2.8-3.2 mm. 2n = 34. Flowering summer-early fall. Prairies, roadsides; 10-600+ m; Ont.; Ala., Ark., Conn., Del., D.C., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Miss., Mo., Nebr., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Va., W.Va., Wis. Helianthus mollis is introduced in Ontario and adventive in the eastern United States (e.g., Maine), where it is continuing to spread, particularly along roads. Natural hybrids between H. mollis and H. occidentalis have been named H.cinereus Torrey & A. Gray (R. C. Jackson and A. T. Guard 1957); they differ from H. mollis by having smaller heads with fewer ray florets and narrower leaves with cuneate bases. Hybrids of H. mollis with H. giganteus have been called H.doronicoides Lamarck (Jackson 1956).
From Flora of Indiana (1940) by Charles C. Deam
This sunflower is generally found in black sandy soil in prairie habitats but is sometimes found in moist, hard, white clay soil in the Illinoian drift area. It is rather frequent in the northwestern counties and along the western part of the state, becoming very rare or absent in the central counties and local in the southern counties. Doubtless it prefers a slightly acid soil because it soon dies out when transferred to a neutral soil.
Perennial from stout rhizomes, usually colonial, 0.5-1(-1.2) m, densely and softly hairy throughout; lvs sessile, subcordate, ascending, broadly lanceolate to broadly ovate or oblong, 6-15 נ2-8 cm, 1.4-3(-4) times as long as wide, serrulate or entire; uppermost lvs sometimes alternate; heads few or solitary; disk yellow, 2-3 cm wide; invol bracts slightly imbricate, lanceolate, acuminate, often finally glandular as well as densely white-hairy, the upper part loose or spreading; rays mostly 16-35, 1.5-3.5 cm; 2n=34. Prairies and other dry places; chiefly Ozarkian and midwestern, from O. to Wis., s. Io., and e. Kans. s. to Ga. and Tex., and occasionally intr. e. to the Atlantic. July-Sept. A probable hybrid with H. giganteus has been named H. doronicoides Lam.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.