3. Helianthus annuus Linnaeus, Sp. Pl. 2: 904. 1753.
Helianthus annuus subsp. jaegeri (Heiser) Heiser; H. annuus subsp. lenticularis (Douglas ex Lindley) Cockerell; H. annuus var. lenticularis (Douglas ex Lindley) Steyermark; H. annuus var. macrocarpus (de Candolle) Cockerell; H. annuus subsp. texanus Heiser; H. aridus Rydberg; H. jaegeri Heiser; H. lenticularis Douglas ex Lindley; H. macrocarpus de Candolle
Annuals, 100–300 cm. Stems erect, usually hispid. Leaves mostly cauline; mostly alternate; petioles 2–20 cm; blades lance-ovate to ovate, 10–40 × 5–40 cm, bases cuneate to subcordate or cordate, margins serrate, abaxial faces usually ± hispid, sometimes gland-dotted . Heads 1–9. Peduncles 2–20 cm. Involucres hemispheric or broader, 15–40(–200+) mm diam. Phyllaries 20–30(–100+), ovate to lance-ovate, 13–25 × (3–)5–8 mm, (margins usually ciliate) apices abruptly narrowed, long-acuminate, abaxial faces usually hirsute to hispid, rarely glabrate or glabrous, usually gland-dotted. Paleae 9–11 mm, 3-toothed (middle teeth long-acuminate, glabrous or hispid). Ray florets (13–)17–30(–100+); laminae 25–50 mm. Disc florets 150+(–1000+); corollas 5–8 mm (throats ± bulbous at bases), lobes usually reddish, sometimes yellow ; anthers brownish to black, appendages yellow or dark (style branches yellow) . Cypselae (3–)4–5(–15) mm, glabrate ; pappi of 2 lanceolate scales 2–3.5 mm plus 0–4 obtuse scales 0.5–1 mm. 2n = 34.
Flowering summer–fall. Open areas; 0–3000 m; St. Pierre and Miquelon; Alta., B.C., Man., N.B., N.W.T., N.S., Ont ., P.E.I., Que., Sask.; Ala., Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Mexico; intoduced nearly worldwide.
Helianthus annuus is widely distributed, including weedy, cultivated, and escaped plants. It is the only native North American species to become a major agronomic crop. Despite its considerable variability, attempts have failed to produce a widely adopted infraspecific system of classification. Forms with red-colored ray laminae, known from cultivation and occasionally seen escaped, trace their ancestry to a single original mutant plant. It hybridizes with many of the other annual species.