42. Helianthus grosseserratus M. Martens, Index Seminum (Louvain). 1839: unpaged. 1839.
Sawtooth sunflower, hélianthe à grosses dents
Helianthus instabilis E. Watson
Perennials, 100–400(–500) cm (rhi-zomatous). Stems erect, glabrous proximally, distally glabrate, scabrellous, or strigillose (glaucous). Leaves cauline; opposite (proximal) or alternate; petioles (1–)2–5 cm; blades (light to dark green, 3-nerved distal to bases) lanceolate to lance-ovate, 10–32 × (1.2–)4–9 cm, bases cuneate, margins usually coarsely to shallowly serrate, rarely subentire (flat), abaxial faces puberulent to tomentulose, gland-dotted. Heads 3–15+. Peduncles 0.3–10 cm. Invo-lucres broadly hemispheric, 15–25 mm diam. Phyllaries 25–30 (loose, spreading ), lance-linear, 10–14 × 1.5–2.5 mm (subequal), (margins ± ciliate) apices attenuate, abaxial faces glabrous or puberulent, not gland-dotted. Paleae 7–8 mm, entire or 3-toothed (apices acuminate, ± hairy). Ray florets 14–20; laminae 23–40 mm. Disc florets 100+; corollas 5–6 mm, lobes yellow; anthers dark brown to black, appendages yellow. Cypselae 3–4 mm, glabrate; pappi of 2 aristate scales 1.9–2.5 mm. 2n = 34.
Flowering late summer–fall. Dry to wet prairies, other open sites; 10–300+ m; Ont., Que.; Ark., Conn., Del., D.C., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Pa., S.Dak., Tenn., Tex., Va., Wash., W.Va., Wis.
Helianthus grosseserratus is native to midwestern North America and has spread as a roadside weed into other areas, such as New England and the southeastern United States. It is introduced in Canada. Hybrids between H. maximiliani and H. grosseserratus are known as H. ×intermedius R. W. Long (R. W. Long 1954; Long 1966). Hybrids of H. grosseserratus with H. salicifolius have been described as H. kellermannii Britton (Long 1955), and those with H. mollis as H. brevifolius E. Watson (R. C. Jackson and A. T. Guard 1957b).