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Helianthus
See list of 15 species in this genusIdentification of Helianthus is complicated by phenotypic plasticity, polyploidy, and occasional hybridization. Micromorphological characters are a great asset within this genus and are used extensively in the key (e.g., anther appendage and style branch colors, disk corolla indument, cypsela size and indument). Reference: Schilling (2006).
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1a. Plants annual, with fibrous roots; leaves alternate (except the lowermost); receptacle flat or nearly so; disk corollas red-purple (sometimes yellow in H. annuus)
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2a. Involucral bracts narrow-oblong to ovate, (3–) 5–8 mm wide, abruptly narrowed to the apex, conspicuously long-ciliate; cypselas usually glabrous except near the pubescent apex, 4–8 mm wide; style branches yellow; leaf blades 5–25 cm wide, dentate; plants (0.5–) 1–3 m tall
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2b. Involucral bracts lanceolate, 1–4 (–5) mm wide, gradually narrowed to the apex, either eciliate or ciliate with short hairs of similar length to the hairs of the abaxial surface; cypselas pubescent, 1.2–2.5 mm wide; style branches red (rarely yellow in H. debilis); leaf blades 1.5–9 cm wide, entire to undulate-dentate; plants 0.4–1 (–1.5) m tall
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3a. Chaff near the center of the disk inconspicuously short-pubescent; cypselas mottled, pubescent with spreading-ascending hairs; stems frequently mottled; leaf blades dark green and scabrous
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3b. Chaff near the center of the disk conspicuously pubescent near the apex with long, white hairs; cypselas not mottled, pubescent with appressed-ascending hairs; stems not mottled; leaf blades pale green and strigose
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1b. Plants perennial, with rhizomes, stolons, tuberous roots, and/or tough, overwintering bases; leaves opposite (except often the upper); receptacle usually convex to some degree; disk corollas yellow (red-purple in H. pauciflorus)
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4a. Reproductive stems with 3–5 (–8) nodes bearing highly reduced leaves (rarely the lower 2–4 nodes with well-formed blades), often the upper 50% of the stem lacking leaves; plants with a basal rosette of leaves
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4b. Reproductive stems with (5–) 6–15 or more leaf-bearing nodes, the blades gradually, if at all, descreasing in size upward, only the upper 25% or less of the stem lacking leaves; plants without a basal rosette of leaves, during anthesis the leaves chiefly cauline (except sometimes in H. pauciflorus)
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5a. Disk corollas red-purple (sometimes only the lobes red-purple and the remainder of the limb yellow); involucral bracts acute to obtuse at the apex, 3–5 mm wide, appressed, of several, conspicuously different lengths [Fig. 420]
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5b. Disk corollas yellow; involucral bracts acuminate to attenuate (rarely acute) at the apex, 1.2–3.5 mm wide, at least some merely ascending, of several, inconspicuously different lengths [Fig. 417]
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6a. Stem and axis of capitulescence conspicuously pubescent, not glaucous
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7a. Stem, abaxial surface of leaf blades, and involucral bracts densely pubescent with short, soft hairs; leaf blades sessile, subcordate to cordate at the base
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7b. Stem, abaxial surface of leaf blades, and involucral bracts with sparser and/or coarser pubescence; leaf blades short- to long-petioled (sometimes sessile in H. giganteus), tapering to rounded at the base (sometimes truncate in H. hirsutus)
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8a. Leaf blades (4–) 5–12 (–15) cm wide, with a petiole (15–) 20–80 mm long; roots forming tubers later in growing season; cypsela body 5–7 mm long
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8b. Leaf blades 1–4 (–8) cm wide, with a petiole up to 20 mm long; roots fibrous or fleshy, merely thickened; cypsela body 3–5 mm long
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9a. Stem strigose; leaf blades sometimes folded and falcate-arched, especially in drying, pinnately veined near the base [Fig. 419]
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9b. Stem spreading-hirsute; leaf blades flat, usually with 3 prominent, parallel nerves near the base
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10a. Leaf blades cuneate at base; petioles 0–12 mm long; roots fleshy and somewhat thickened; involucral bracts linear, 1.2–2 mm wide; anther appendages dark brown (in part)
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10b. Leaf blades rounded to truncate at base; petioles 4–20 mm long; roots fibrous; involucral bracts lanceolate, 2.5–3.5 mm wide; anther appendages usually yellow
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6b. Stem glabrous (or very nearly so) and often glaucous, only the axis of the capitulescence pubescent
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11a. Leaves sessile or with short petioles to 5 (–10) mm long [Fig. 418]
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12a. Leaf blades lanceolate to oblong-lanceolate, widest near middle, narrowly tapering to the base, usually alternately arranged on the upper portion of the stem; disk 15–25 mm wide; anther appendages dark brown (in part)
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12b. Leaf blades lanceolate to triangular-ovate, widest near the base, truncate to broadly rounded at the base [Fig. 418], usually oppositely arranged throughout the stem; disk 10–15 mm wide; anther appendages yellow
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11b. Well-developed leaves with petioles (5–) 10–60 mm long
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13a. Disk 4–10 mm wide; capitula with 5–8 ray flowers
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13b. Disk 15–35 mm wide; capitula with 8–20 ray flowers
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14a. Leaf blades at least somewhat triple-veined, those near the middle of the stem 25–100 mm wide, broad-lanceolate to ovate, usually less than 3 times as long as wide; capitula with 8–15 ray flowers
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15a. Involucral bracts spreading, evidently surpassing the disk [Fig. 417]; lobes of the disk corollas pubescent; petioles 15–60 mm long; leaves relatively thin, membranaceous to herbaceous, prominently serrate
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15b. Involucral bracts ascending, approximately equaling the height of the disk; lobes of the disk corollas glabrous; petioles 5–30 mm long; leaves thick and firm, subentire to shallowly serrate
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14b. Leaf blades pinnately veined, those near the middle of the stem 10–40 (–90) mm wide, lanceolate to narrow-ovate, usually 3–8 times as long as wide; capitula with 10–20 ray flowers
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16a. Lobes of the disk corollas glabrous or glabrate; anther appendages yellow; leaf blades slightly, if at all, scabrous on the adaxial surface and pubescent on the abaxial surface with hairs up to 0.5 mm long
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16b. Lobes of the disk corollas pubescent; anther appendages red-brown to dark brown; leaf blades strongly scabrous on the adaxial surface and pubescent on the abaxial surface with hairs 1 mm long or longer (in part)
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Each photo represents one species in this genus.