Giant Sunflower

Helianthus giganteus was probably introduced in Newfoundland. It intergrades with H. nuttallii and H. grosseserratus; it is distinctive in its hairy, reddish-colored stems, dark anther appendages, and prominent setae on the petioles. The natural hybrid of H. giganteus with H. grosseserratus has been called H. luxurians E. Watson (R. W. Long 1954). Hybrids of H. giganteus with H. divaricatus have been referred to as H. ambiguus (Torrey & A. Gray) Britton and H. ×divariserratus R. W. Long (Long 1954).

Perennials, 100–400 cm (rhi-zomatous). Stems (usually reddish) erect, hairy. Leaves cauline; mostly alternate; petioles 0–1.2 cm (ciliate); blades (3-nerved from bases) lanceolate to lance-ovate, 7–20 × 1.2–3.5 cm, bases cuneate, margins serrulate to subentire (± flat), abaxial faces scabrous or ± hirsute, sometimes gland-dotted. Heads 1–12. Peduncles 8–12 mm. Involucres hemispheric, 10–24 mm diam. Phyllaries 20–25 (loose or spreading), linear, 8–15 × 1.2–2 mm, (margins usually ciliate) apices acute to attenuate , abaxial faces sparsely strigose, not gland-dotted. Paleae 7–9 mm, entire or ± 3-toothed (apices acute, hairy). Ray florets 12–20; laminae (often pale yellow) 15–25 mm (abaxial faces not gland-dotted). Disc florets 60+; corollas 5–6 mm, lobes yellow; anthers dark brown or black, appendages dark. Cypselae 3–4 mm, glabrate; pappi of 2 aristate scales 2.4–3 mm. 2n = 34.

Helianthus alienus E. Watson; H. validus E. Watson