Helianthus decapetalus
Helianthus giganteus
hélianthe à dix rayons, thin-leaf sunflower
giant sunflower, hélianthe géant, tall sunflower
erect, glabrous.
(usually reddish) erect, hairy.
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.
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.
2–12 cm (not gland-dotted).
8–12 mm.
hemispheric, 10–24 mm diam.
8–12;
laminae 20–25 mm.
12–20;
laminae (often pale yellow) 15–25 mm (abaxial faces not gland-dotted).
40+;
corollas 6.5–7.2 mm, lobes yellow;
anthers usually dark brown to black (rarely reddish brown), appendages dark or reddish brown.
60+;
corollas 5–6 mm, lobes yellow;
anthers dark brown or black, appendages dark.
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.
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.
3–6(–10).
1–12.
3.5–5 mm;
pappi of 2 aristate scales 3–4 mm.
3–4 mm, glabrate;
pappi of 2 aristate scales 2.4–3 mm.
-lucres hemispheric, 12–25 mm diam.
8–10 mm, 3-toothed.
7–9 mm, entire or ± 3-toothed (apices acute, hairy).
= 34, 68.
= 34.
Helianthus decapetalus
Helianthus giganteus
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.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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