24.
Helianthus decapetalus
Linnaeus, Sp. Pl. 2: 905. 1753.
Thinleaf sunflower, hélianthe à dix rayons
Helianthus trachelifolius
Miller
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.
2
n
= 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
.
