Annuals or perennials, (5–)20–300(–500) cm. Stems erect or ascending to decumbent or procumbent, usually branched distally. Leaves basal and/or cauline; opposite, or opposite (proximal) and alternate, or alternate; petiolate or sessile; blades usually 3-nerved (1-nerved in H. eggertii, H. smithii, and H. maximiliani), mostly deltate, lance-linear, lanceolate, lance-ovate, linear, or ovate, bases cordate to narrowly cuneate, margins usually entire or serrate, rarely lobed, faces glabrous or hairy, often gland-dotted. Heads usually radiate (sometimes discoid in H. radula), borne singly or in ± corymbiform, paniculiform, or spiciform arrays. Involucres usually ± hemispheric, sometimes campanulate or cylindric, 5–40+(–200+ in cultivars) mm diam. Phyllaries persistent, 11–40(–100+ in cultivars) in 2–3+ series (subequal to unequal). Receptacles flat to slightly convex (conic in H. porteri), paleate (paleae ± conduplicate, usually rectangular-oblong, usually ± 3-toothed, sometimes entire, apices sometimes reddish or purplish). Ray florets usually 5–30+(–100+ in cultivars), rarely 0, neuter; corollas usually yellow. Disc florets (15–)30–150+(–1000+ in cultivars), bisexual, fertile; corollas yellow or reddish (at least distally), tubes shorter than campanulate throats, lobes 5, triangular (style branches slender, appendages ± attenuate). Cypselae (usually purplish black, sometimes mottled) ± obpyramidal, ± compressed (glabrous, glabrate, or ± hairy); pappi 0 (H. porteri), or readily falling, of 2(–3) usually lanceolate, aristate, or erose scales (at the 2 principal angles, 1–5 mm) plus 0–8 usually shorter scales (0.2–2 mm). x = 17.

Distribution

North America, Mexico, introduced in the Old World.

Discussion

Species 52, including 1 hybrid (52 in the flora).

The identification of sunflower species has long been problematic. C. B. Heiser et al. (1969) felt that the greatest contribution of their sustained efforts to understand sunflower taxonomy was not providing an easy way to identify sunflowers but rather an explanation for why they are so difficult. Taxonomic difficulties are based on a combination of factors, notably developmental and ecologic plasticity, the frequency of interspecific hybridization, and the presence of polyploidy. L. H. Rieseberg (1991) and Rieseberg et al. (1988, 1990) have subsequently documented the presence of species of homoploid hybrid origin to add to the complexity. The keys in this treatment have been modified somewhat with observations of features that require microscopic observation, such as the distribution of glands (sometimes referred to as “resin dots”) and the color of anther appendages; they still are based heavily on those of Heiser et al. There will still be specimens, of hybrid origin or growing in unusual conditions or incompletely collected, that defy certain placement into a single species. The taxonomic rank of certain taxa is also problematic, and for the most part the treatment of Heiser et al. has been followed, with relatively few exceptions. With the exception of Helianthus ×laetiflorus, which is widespread and commonly encountered away from either parental species, hybrids are not treated as separate entities. Helianthus verticillatus has been verified as a distinct entity, unlikely to be simply a hybrid. Whether H. praetermissus E. Watson, which continues to be known only from a single fragmentary specimen, represents a distinct species or an anomalous collection is still uncertain; it has not been included here.

Selected References

Heiser, C. B., D. M. Smith, S. Clevenger, and W. C. Martin. 1969. The North American sunflowers (Helianthus). Mem. Torrey Bot. Club 22(3).

Heiser, C. B. 1976. The Sunflower. Norman.

Rogers, C. E., T. E. Thompson, and G. J. Seiler. 1982. Sunflower Species of the United States. Fargo.

Storbeck, T. A. 1984. A Taxonomic Study of Helianthus Series Microcephali and Allied Species. Ph.D. dissertation. Indiana University.

Lower Taxa

Helianthus agrestis, Helianthus angustifolius, Helianthus annuus, Helianthus anomalus, Helianthus argophyllus, Helianthus arizonensis, Helianthus atrorubens, Helianthus bolanderi, Helianthus californicus, Helianthus carnosus, Helianthus ciliaris, Helianthus cusickii, Helianthus debilis, Helianthus decapetalus, Helianthus deserticola, Helianthus divaricatus, Helianthus eggertii, Helianthus exilis, Helianthus floridanus, Helianthus giganteus, Helianthus glaucophyllus, Helianthus gracilentus, Helianthus grosseserratus, Helianthus heterophyllus, Helianthus hirsutus, Helianthus laciniatus, Helianthus laevigatus, Helianthus longifolius, Helianthus maximiliani, Helianthus microcephalus, Helianthus mollis, Helianthus neglectus, Helianthus niveus, Helianthus nuttallii, Helianthus occidentalis, Helianthus paradoxus, Helianthus pauciflorus, Helianthus petiolaris, Helianthus porteri, Helianthus praecox, Helianthus pumilus, Helianthus radula, Helianthus resinosus, Helianthus salicifolius, Helianthus schweinitzii, Helianthus silphioides, Helianthus simulans, Helianthus smithii, Helianthus strumosus, Helianthus tuberosus, Helianthus verticillatus, Helianthus ×laetiflorus

Key

1	Annuals; paleae 3.5–4.6 mm, apices entire (1-toothed); disc corollas 2.8–3.5 mm; pappi 0	Helianthus porteri
1	Annuals or perennials; paleae 5+ mm, apices usually 3-toothed, sometimes subentire or entire; disc corollas (3–)3.5–8.5 mm; pappi (readily falling) of 2(–3+) scales	> 2
2	Annuals (taprooted; disc corolla lobes reddish and style branches yellow); paleae entire or weakly 3-toothed (apices purplish, abaxial faces glabrous); cypselae glabrous (usually ± tuberculate)	Helianthus agrestis
2	Annuals (taprooted; if disc corolla lobes reddish, style branches also reddish), or perennials (often rhizomatous or with crown buds, sometimes taprooted); paleae entire or ± 3-toothed (apices purplish, greenish, or yellow-brown, abaxial faces glabrous or hairy); cypselae glabrous or hairy (not tuberculate)	> 3
3	Annuals or perennials (taprooted); leaves mostly alternate, petiolate (petiole lengths at least 1/5 blades); paleae (at least central ones) either bearded (with apical tufts of whitish hairs) or prominently 3-toothed (middle teeth relatively narrow, lengths 4 or more times width); disc corolla lobes and style branches usually reddish (rarely yellow in H. annuus and H. debilis)	> 4
3	Perennials (rhizomatous or with crown buds); leaves opposite or alternate, petiolate or sessile; paleae (at least central) glabrous or ± hispid to puberulent (not bearded) and entire or relatively weakly 3-toothed (if 3-toothed, lengths of middle teeth usually less than 4 times widths); disc corolla lobes yellow or reddish (if reddish, style branches yellow)	> 17
4	Plants 100–300 cm; leaf blades (at least larger) 10–40 cm wide, abaxial faces gland-dotted; phyllaries ovate to lance-ovate (larger usually 5–8 mm wide), apices narrowed abruptly (acute to acuminate)	> 5
4	Plants mostly 25–200 cm; leaf blades (larger) usually less than 12 cm wide (bases cuneate, truncate, or cordate), abaxial faces sometimes gland-dotted; phyllaries usually lanceolate to lance-ovate (usually less than 4 mm, sometimes to 5 mm, wide), apices narrowed gradually	> 6
5	Stems (leaves, phyllaries) hispid; leaf margins usually serrate	Helianthus annuus
5	Stems (leaves, phyllaries) densely silvery white tomentose or floccose; leaf margins usually entire, sometimes serrulate	Helianthus argophyllus
6	Palea apices (at least central paleae) bearded (with tufts of whitish hairs)	> 7
6	Paleae apices glabrous or puberulent to hispid (not bearded)	> 10
7	Stems (leaves, phyllaries) densely canescent	Helianthus petiolaris
7	Stems (leaves, phyllaries) strigillose to hispid	> 8
8	Leaf blades usually deltate-lanceolate to lanceolate (lengths of larger usually 2+ times widths), bases truncate to cuneate; phyllaryapices relatively short-attenuate	Helianthus petiolaris
8	Leaf blades usually lanceolate to deltate-ovate or ovate (lengths of larger usually less than 2 times widths), bases truncate to cordate; phyllary apices relatively short- to long-attenuate	> 9
9	Stems hispid; leaf margins subentire to serrulate; phyllary api-ces relatively long-attenuate (surpassing discs)	Helianthus neglectus
9	Stems hispid to hirsute; leaf margins serrate to serrulate; phyllary apices relatively short-attenuate (scarcely surpassing discs)	Helianthus praecox
10	Leaves abaxially gland-dotted; phyllaries hirsute (paleae: middle teeth notably surpassing, often arching over discs)	> 11
10	Leaves sometimes abaxially gland-dotted; phyllaries hairy or glabrous (paleae: middle teeth equaling or slightly surpassing discs)	> 12
11	Leaf blades ovate to lance-ovate, usually serrate (discs, at leastlarger heads, 2+ cm diam.); cypselae 3.5–4.5 mm	Helianthus bolanderi
11	Leaf blades lance-linear to lance-ovate, usually entire or subentire(discs usually less than 2 cm diam.); cypselae 2.7–3.5 mm	Helianthus exilis
12	Phyllaries densely white-canescent	Helianthus niveus
12	Phyllaries glabrous or sparsely hispid to hispidulous	> 13
13	Leaf blades (at least proximal) usually ovate or deltate, bases usually cordate to truncate (sometimes widely cuneate in H. debilis subsp. debilis); cypselae 2.5–3 mm, glabrous or sparsely hairy	> 14
13	Leaf blades lanceolate to lance-ovate, bases cuneate; cypselae 3–9 mm, glabrous or villous to pilose	> 15
14	Leaf blades deltate-ovate, lance-ovate, or ovate (not constricted near middles, gradually tapering to apices); phyllaries 8–17 × 1–3 mm, apices acute to relatively long-acuminate	Helianthus debilis
14	Leaf blades deltate to ovate (sometimes constricted near middles); phyllaries 9–15 × 2–4 mm, apices relatively short-acuminate to short-attenuate	Helianthus praecox
15	Phyllaries 17–33 mm (greatly surpassing discs, margins notably ciliate); cypselae 4.6–9 mm	Helianthus anomalus
15	Phyllaries 6–19 mm (equaling or slightly surpassing discs, margins not notably ciliate); cypselae 3–5 mm	> 16
16	Stems densely hispid; cypselae 4–5 mm, pilose; pappi usually of 2 linear scales 1.6–2.5 mm plus 2–4 linear or ovate scales 05.1–1 mm	Helianthus deserticola
16	Stems glabrate or ± hispid; cypselae 3–4 mm, glabrous; pappi usually of 2 lanceolate scales 2.5–2.9 mm	Helianthus paradoxus
17	Leaves (at flowering) mostly or all basal (cauline leaves abruptly smaller)	> 18
17	Leaves (at flowering) mostly cauline (not abruptly smaller distally)	> 25
18	Ray florets 0 or 2–8, laminae 1–2(–10) mm (inconspicuous, often tinged reddish)	Helianthus radula
18	Ray florets 8–13(–25), laminae (7–)10–40 mm (yellow)	> 19
19	Stems (and leaves) usually glabrous; disc corolla lobes yellow	> 20
19	Stems (and leaves) usually hispid or hirsute to scabrous; disc corolla lobes yellow or reddish	> 21
20	Heads usually borne singly; phyllaries 8–16 × 3–5 mm	Helianthus carnosus
20	Heads 3–12; phyllaries 5–11 × 1–2.5 mm	Helianthus longifolius
21	Phyllaries lanceolate (the larger 1.5–2.5 mm wide); abaxial faces of leaves (and usually ray laminae) notably gland-dotted; disc co rolla lobes yellow; cypselae 3–5 mm	Helianthus occidentalis
21	Phyllaries lanceolate to ovate (the larger 3+ mm wide); abaxial faces usually not gland-dotted (if gland-dotted, cypselae 5–6 mm); disc corolla lobes reddish or yellow	> 22
22	Leaves mostly basal (cauline relatively few, narrowly lanceolate to linear); phyllary apices acute to accuminate	Helianthus heterophyllus
22	Leaves mostly basal or basal and cauline (at least proximal 1–2 cauline pairs well developed, mostly lanceolate, rhombic-ovate, or ovate, similar to basal leaves); phyllary apices usually obtuse to acute, sometimes mucronate	> 23
23	Perennials (rhizomatous); leaf blades lance-linear, lance-ovate, oblong-lanceolate, or rhombic-ovate, bases cuneate (onto winged petioles); cypselae 5–6 mm	Helianthus pauciflorus
23	Perennials (with crown buds); leaf blades broadly lanceolate, ovate, or suborbiculate, bases cuneate, rounded, or truncate (usually abruptly contracted onto winged petioles); cypselae 2.8–4 mm	> 24
24	Leaves (at least basal) lanceolate to ovate (hairs on abaxial midribs 1+ mm); cauline leaves to 8 pairs proximal to heads; nonflowering stems usually absent	Helianthus atrorubens
24	Leaves (at least basal) ovate to suborbiculate (hairs on abaxial midribs to 1 mm); cauline leaves 9+ pairs proximal to heads; nonflowering stems usually present	Helianthus silphioides
25	Disc corolla lobes reddish(at least at tips)	> 26
25	Disc corolla lobes yellow	> 35
26	Leaf blades lanceolate, lance-linear, or linear (lengths usually 10+ times widths)	> 27
26	Leaf blades deltate, deltate-ovate, lanceolate, lance-ovate, or linear (lengths seldom more than 5 times widths)	> 29
27	Stems glabrous (glaucous); leaf margins entire (flat)	Helianthus salicifolius
27	Stems usually hairy; leaf margins entire (± revolute)	> 28
28	Plants 50–150 cm (rhizomes absent or poorly developed); leaves 8–15 × 0.15–0.5(–1) cm	Helianthus angustifolius
28	Plants 150–260 cm (rhizomes well developed); leaves 9–22 × 0.7–4 cm	Helianthus simulans
29	Leaves sessile or nearly so, margins often irregularly toothed or lobed; ray laminae 8–11 mm	> 30
29	Leaves petiolate to subsessile, margins entire or serrate; ray laminae 15–37 mm	> 31
30	Plants 50–120 cm; stems glabrate to strigose or hispid; leaves (green orgrayish, not bluish green) hispid	Helianthus laciniatus
30	Plants 40–70 cm; stems glabrous or glabrate (glaucous); leaves (often bluish green) glabrous or glabrate (glaucous)	Helianthus ciliaris
31	Phyllaries oblong, oblong-lanceolate, obovate, or ovate, 3–5 mm wide	> 32
31	Phyllaries lanceolate, 1.5–3 mm wide	> 34
32	Leaf blades usually broadly ovate to orbiculate, abaxial faces not gland-dotted	Helianthus silphioides
32	Leaf blades lance-linear, lanceolate, lance-ovate, oblong-lanceolate, or rhombic-ovate, abaxial faces usually gland-dotted	> 33
33	Phyllaries ovate, apices acute, abaxial faces glabrate to hispid	Helianthus pauciflorus
33	Phyllaries oblong-lanceolate, apices acuminate, abaxial faces usually hairy	Helianthus ×laetiflorus
34	Stems (usually reddish or purplish) hairy; leaves all or mostly opposite; disc corollas 5–6 mm (anthers reddish brown, appendages yellow ordull orange); cypselae 3–4 mm	Helianthus gracilentus
34	Stems (usually green) hairy; leaves usually opposite or alternate, rarely whorled; disc corollas 4–5 mm (anthers dark brown or black, append-ages dark); cypselae 2.5–3 mm	Helianthus floridanus
35	Phyllaries ovate to lanceolate, (3–)5–8 mm wide, apices abruptly attenuate (disc corollathroats notably bulbous at bases)	Helianthus annuus
35	Phyllaries linear to lanceolate or lance-ovate, usually 2–4 mm wide, apices gradually narrowed (disc corolla throats not notably bulbous at bases)	> 36
36	Stems glabrous or glabrate (at least proximal to arrays of heads, sometimes glaucous)	> 37
36	Stems hairy (± throughout, not glaucous)	> 53
37	Leaves usually whorled (3s–6s), sometimes opposite	Helianthus verticillatus
37	Leaves opposite or alternate (not whorled)	> 38
38	Leaves sessile (grayish green or bluish green), abaxial faces glabrous (glaucous)	> 39
38	Leaves sessile or petiolate (light to dark green, sometimes whitish abaxially, not grayish or bluish green); abaxial faces glabrous or hairy	> 41
39	Plants 20–30 cm; stems ascending to erect; ray laminae 7–9 mm;disc corollas 3–3.5 mm	Helianthus arizonensis
39	Plants (30–)100–220 cm; stems erect; ray laminae 15–20 mm; disc corollas 5–7 mm	> 40
40	Leaves: adaxial faces glabrous or glabrate (smooth or slightly rough to touch); phyllaries 2–3 mm wide; rays 5–10	Helianthus laevigatus
40	Leaves: adaxial faces strumose (rough to touch); phyllaries 3.5–4.5 mm wide; rays 10–18	Helianthus eggertii
41	Leaf blades (1-nerved) lance-linear to lanceolate	Helianthus smithii
41	Leaf blades (3-nerved) lance-linear, lanceolate, lance-ovate, or ovate	> 42
42	Involucres 5–7 or 8–9 mm diam; rays usually 5 or 8	> 43
42	Involucres (8–)9–28 mm diam.; rays (8–)10–21 (at least in larger heads)	> 44
43	Leaves: abaxial faces (whitish) glabrous, not gland-dotted (glaucous)	Helianthus glaucophyllus
43	Leaves: abaxial faces (greenish) usually tomentulose, sometimes glabrate, densely gland-dotted	Helianthus microcephalus
44	Plants (not rhizomatous); phyllaries (paleae and ray laminae) gland-dotted; anthers dull orange or orange-brown, appendages dull orange to yellowish	> 45
44	Plants (rhizomatous); phyllaries sometimes gland-dotted; anthers usually dark brown to black, rarely reddish brown, appendages yellow, or dark or reddish brown	> 46
45	Perennials (with crown buds); phyllaries 4–8 mm	Helianthus gracilentus
45	Perennials (taproots stout, parsniplike); phyllaries 11–18 mm	Helianthus cusickii
46	Leaves sessile (3-nerved from bases), bases rounded to cordate	Helianthus divaricatus
46	Leaves sessile or petiolate (3-nerved distal to bases), bases ± cuneate (narrowing gradually)	> 47
47	Anther appendages yellow	> 48
47	Anther appendages dark or reddish brown	> 50
48	Leaves petiolate (petioles 2.5–10 cm, lengths 1/2+ blades), blades oblong-lanceolate or elliptic to ovate; phyllaries usually appressed, strongly un-equal, not surpassing discs	Helianthus occidentalis
48	Leaves petiolate or sessile (petioles 05–1.5 or 1–5 cm, lengths usually less than 1/4 blades), blades lanceolate to lance-ovate; phyllaries usually loose, spreading, ± subequal	> 49
49	Leaves: petioles (1–)2–5 cm, blades 10–32 × (1.2–)4–9 cm, margins coarsely serrate	Helianthus grosseserratus
49	Leaves: petioles 0.5–1.5 cm, blades 4–20 × 0.8–4 cm, margins entire or shallowly serrate	Helianthus nuttallii
50	Peduncles (gland-dotted); phyllaries (strongly reflexed) 3–5 mm wide (notably surpassing discs), gland-dotted	Helianthus californicus
50	Peduncles (not gland-dotted); phyllaries (sometimes reflexed) to 3 mm wide (sometimes surpassing discs), sometimes gland-dotted	> 51
51	Plants 100–200(–300) cm (producing tubers); leaves sessile or subsessile (petioles 0–1 cm)	Helianthus schweinitzii
51	Plants 60–200 cm (not producing tubers); leaves petiolate (petioles 1–5 cm)	> 52
52	Leaves: petioles 1–3 cm, blades moderately serrate or entire, abaxial faces usually densely gland-dotted; phyllaries (equaling or slightly surpassing discs):apices acute	Helianthus strumosus
52	Leaves: petioles 2–5 cm, blades (at least larger leaves) moderately to notably serrate, abaxial faces usually sparsely gland-dotted; phyllaries (at least longer, usually surpassing discs, by 1/2+ their lengths): apices acuminate	Helianthus decapetalus
53	Leaves all or mostly opposite, sessile, bases cordate	Helianthus mollis
53	Leaves opposite or alternate, petiolate or sessile, bases mostly cuneate (not cordate)	> 54
54	Leaf blades lanceolate to ovate (bases gradually narrowed onto petioles); phyllaries (squarrose to reflexed, notably surpassing discs): apices attenuate, abaxial faces densely gland-dotted (at least toward apices)	Helianthus resinosus
54	Leaf blades lance-linear or lanceolate (and bases attenuate to truncate or rounded), or lance-ovate to ovate (and sessile or petioles narrowly winged); phyllaries (not reflexed): apices acute to attenuate, abaxial faces sometimes gland-dotted	> 55
55	Perennials (taproots parsniplike); anthers and appendages usually dull orange or orange-brown, sometimes yellowish	Helianthus cusickii
55	Perennials (rhizomatous or with crown buds or slightly thickened taproots); anthers dark brown or black, appendages dark brown or black, or yellow.	> 56
56	Leaf blades (usually 1-nerved, conduplicate) entire; heads (1–)3–15 (borne singly or inracemiform or spiciform arrays)	Helianthus maximiliani
56	Leaf blades (3-nerved, not conduplicate) entire or serrate; heads (1–)3–16 (borne singly or in ± corymbiform, not racemiform or spiciform arrays)	> 57
57	Perennials (taprooted); leaves all or mostly opposite, both faces gland-dotted (phyllar-ies, paleae, and ray laminae as well)	Helianthus pumilus
57	Perennials (rhizomatous or with crown buds); leaves opposite or alternate, abaxial faces sometimes (adaxial never) gland-dotted (abaxial faces of phyllaries, paleae, and ray laminae sometimes gland-dotted)	> 58
58	Phyllaries usually appressed, strongly unequal	> 59
58	Phyllaries usually loose or spreading, ± subequal	> 60
59	Petioles 2.5–10 cm (lengths usually 1/2+ blades); leaf blades oblong-lanceolate, ovate, or elliptic; anther appendages yellow; cypselae 3–4 mm	Helianthus occidentalis
59	Petioles 1–5 cm (lengths usually less than 1/2 blades); leaf blades lanceolate to lance-ovate; anther appendages dark brown or black; cypselae (seldom formed)4–5 mm	Helianthus ×laetiflorus
60	Leaves petiolate, petioles 2–8 cm; blades lanceolate to ovate, 7–15 cm wide; cypselae 5–7 mm (plants producing tubers, late in growing season)	Helianthus tuberosus
60	Leaves sessile or petiolate, petioles 0–2 cm; blades elliptic, lance-linear, lanceolate, lance-ovate, linear, or ovate, 0.15–4(–8) cm wide; cypselae 2–5 cm (plants sometimes producing tubers)	> 61
61	Leaves petiolate, blade bases truncate to rounded	Helianthus hirsutus
61	Leaves petiolate or sessile, blade bases cuneate (gradually narrowing)	> 62
62	Leaf margins entire or subentire to serrulate (± flat); ray laminae not gland-dotted	> 63
62	Leaf margins entire or subentire to serrulate (usually revolute); ray laminae gland-dotted abaxially	> 64
63	Stems (usually reddish) erect; leaves subsessile or petiolate (petioles 0–1.2 cm, ciliate), abaxial faces scabrous or ± hirsute; antherappendages dark brown or black	Helianthus giganteus
63	Stems (usually yellow-brown or greenish) erect; leaves petiolate (petioles 0.5–1.5 cm, not ciliate), abaxial faces hispid to villous or tomentose; anther appendages yellow	Helianthus nuttallii
64	Involucres 9–10(–16) mm diam. (plants producing tubers)	Helianthus schweinitzii
64	Involucres (7–)10–20 mm diam. (plants not producing tubers)	> 65
65	Leaves lanceolate to elliptic or lance-ovate (lengths rarely more than 5 times widths), margins entire or serrulate (revolute and/or undulate); phyllary apices (at least outer) usually obtuse	Helianthus floridanus
65	Leaves lanceolate to linear (lengths more than 5 times widths), margins entire or subentire (not notably undulate); phyllary apices (at least outer) acute to acuminate	> 66
66	Plants 50–150+ cm (rhizomes absent or poorly developed); leaves 0.15–0.5(–1) cm wide	Helianthus angustifolius
66	Plants 150–260 cm (rhizomes well developed); leaves 0.7–4 cm wide	Helianthus simulans