Perennials, 60-200 cm (rhizomatous). Stems erect, glabrous. Leaves cauline; opposite (proximal or all) or alternate (distal); petioles (1-)2-5 cm; blades (green, 3-nerved distal to bases) lanceolate to ovate, 7-21 × 4-10 cm, bases rounded to cuneate (often shortly decurrent onto petioles, margins usually serrate (moderately to notably in larger leaves), abaxial faces ± scabro-hispidulous, relatively sparsely gland-dotted. Heads 3-6(-10). Peduncles 2-12 cm (not gland-dotted). Invo-lucres hemispheric, 12-25 mm diam. Phyllaries 20-25 (often reflexed), lance-linear to lanceolate, 11-16 × 2-3 mm (sometimes leaflike, longest surpassing discs by 1/2+ their lengths), (margins ciliate) apices attenuate, abaxial faces strigillose to glabrate, not gland-dotted. Paleae 8-10 mm, 3-toothed. Ray florets 8-12; laminae 20-25 mm. Disc florets 40+; corollas 6.5-7.2 mm, lobes yellow; anthers usually dark brown to black (rarely reddish brown ), appendages dark or reddish brown. Cypselae 3.5-5 mm; pappi of 2 aristate scales 3-4 mm. 2n = 34, 68. Flowering summer-fall. Mesic to wet woodland edges; 10-1200 m; N.B., Ont., Que.; Ala., Ark., Conn., Del., D.C., Ga., Ill., Ind., Iowa, Ky., La., Maine, Md., Mass., Mich., Miss., Mo., N.H., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Vt., Va., W.Va., Wis. Helianthus decapetalus is sometimes confused with Heliopsis helianthoides because of shared habitats and superficial similarities. The tetraploid cytotype of H. decapetalus intergrades (and apparently hybridizes) with H. strumosus, particularly in the southern Appalachians; individual specimens can be difficult to place in one or the other species. In addition to morphologic differences, H. decapetalus usually occurs in more mesic habitats, particularly along watercourses; H. strumosus is found in drier sites such as roadside slopes. Helianthus ×multiflorus Linnaeus is a sterile hybrid, often with 'doubled' heads (in which disc florets are replaced by ray florets); it is cultivated and is sometimes included within H. decapetalus, e.g., H. decapetalus var. multiflorus (Linnaeus) A. Gray; its parents are H. decapetalus and H. annuus.
From Flora of Indiana (1940) by Charles C. Deam
This sunflower is usually found in dry woods with oaks and less frequently with sugar maple. It is rarely found in the open or in moist locations. Frequent to infrequent throughout the state.
Perennial from slender rhizomes; stem 0.5-1.5(-2) m, glabrous below the short-hairy infl; lvs thin, pale beneath, moderately scabrous to subglabrous, broadly lanceolate to ovate, 8-20 נ3-8 cm, long-acuminate, serrate (generally sharply so), ±abruptly contracted near the base and decurrent onto the 1.5-6 cm petiole; upper lvs usually alternate; disk yellow, 1-2 cm wide; invol bracts very loose, thin, green, conspicuously ciliate, occasionally hispidulous on the back, attenuate- acuminate, at least some of them usually conspicuously surpassing the disk; rays 8-15, 1.5-3.5 cm; 2n=34, 68. Woods and along streams; Me. and Que. to Wis. and Io., s. to Ga. and Mo. Aug.-Oct. (H. trachelifolius)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.