Helianthus

Sunflowers
Sunflower (Helianthus annuus)
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Asterids
Order:	Asterales
Family:	Asteraceae
Subfamily:	Helianthodeae
Tribe:	Heliantheae
Genus:	Helianthus L.[1]
Synonyms[1]
Harpalium (Cass.) Cass.

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Helianthus or sunflowers (from the Greek: ήλιος, Hēlios, "sun" and ανθός, anthos, "flower") L. /ˌhiːliˈænθəs/^[2] is a genus of plants comprising about 70 species^[3] in the family Asteraceae, all of which are native to North America except three species in South America. The common name, "sunflower," also applies to the popular annual species Helianthus annuus, the common sunflower.^[4] This and other species, notably Jerusalem artichoke (H. tuberosus), are cultivated in temperate regions as food crops and ornamental plants.^[5]

The genus is one of many in the Asteraceae that are known as sunflowers. It is distinguished technically by the fact that the ray flowers, when present, are sterile, and by the presence on the disk flowers of a pappus that is of two awn-like scales that are caducous (that is, easily detached and falling at maturity). Some species also have additional shorter scales in the pappus, and there is one species that lacks a pappus entirely. Another technical feature that distinguishes the genus more reliably, but requires a microscope to see, is the presence of a prominent, multicellular appendage at the apex of the style.

There is quite a bit of variability among the perennial species that make up the bulk of the species in the genus. Some have most or all of the large leaves in a rosette at the base of the plant and produce a flowering stem that has leaves that are reduced in size. Most of the perennials have disk flowers that are entirely yellow, but a few have disk flowers with reddish lobes. One species, H. radula, lacks ray flowers altogether.

The domesticated sunflower, H. annuus, is the most familiar species. Perennial sunflower species are not as popular for gardens due to their tendency to spread rapidly and become invasive. Whorled sunflowers, H. verticillatus, were listed as an endangered species in 2014 when the U.S. Fish and Wildlife Service issued a final rule protecting it under the Endangered Species Act. The primary threats are industrial forestry and pine plantations in Alabama, Georgia, and Tennessee. They grow to six feet tall and are primarily found in woodlands, adjacent to creeks and moist, prairie-like areas.^[6]

Description

Sunflowers are usually tall annuals, that grow to a height of 50–390 centimetres (20–154 in).

The rough and hairy stem is branched in the upper part in wild plants but is usually unbranched in domesticated cultivars.

The petiolate leaves are dentate and often sticky. The lower leaves are opposite, ovate or often heart-shaped. The upper leaves are alternate and narrower.

They bear one or several to many wide, terminal capitula (flower heads), with bright yellow ray florets at the outside and yellow or maroon (also known as a brown/red) disc florets inside. Several ornamental cultivars have red-colored ray florets; all of them stem from a single original mutant.^[7] During growth, sunflowers tilt during the day to face the sun, but stop once they begin blooming. This tracking of the sun in young sunflower heads is called heliotropism. By the time they are mature, sunflowers generally face east.^[8]

Helianthus species are used as food plants by the larvae of many lepidopterans.

Diversity

Accepted species^[9][10]

1. Helianthus agrestis Pollard – southeastern sunflower – FL GA
2. Helianthus ambiguus Britt. – WI MI OH NY
3. Helianthus angustifolius L. – swamp sunflower – TX + FL north to southern IL + Long Island
4. Helianthus annuus L. – common sunflower, girasol (Spanish) – most of USA + Canada
5. Helianthus anomalus S.F.Blake – western sunflower – NV UT AZ NM
6. Helianthus argophyllus Torr. & A.Gray – silverleaf sunflower – TX NC FL
7. Helianthus arizonensis R.C.Jacks. – Arizona sunflower – AZ NM
8. Helianthus atrorubens L. – purpledisk sunflower – LA AL GA FL SC NC TN KY VA
9. Helianthus bolanderi A.Gray – serpentine sunflower – CA OR
10. Helianthus × brevifolius E.Watson – TX IN OH
11. Helianthus californicus DC. – California sunflower – CA
12. Helianthus carnosus Small – lakeside sunflower – FL
13. Helianthus ciliaris DC. – Texas blueweed – WA CA AZ NM NV UT TX OK CO KS IL Tam Coah Chih Son
14. Helianthus cinereus Small – MO KY IN OH
15. Helianthus coloradensis Cockerell – prairie sunflower – CO NM
16. Helianthus cusickii A.Gray – Cusick's sunflower – WA OR CA ID NV
17. Helianthus debilis Nutt. – cucumberleaf Sunflower – TX to ME; MI
18. Helianthus decapetalus L. – thinleaf sunflower – eastern USA; Ont Que
19. Helianthus deserticola Heiser – desert sunflower – AZ NV UT
20. †Helianthus diffusus Sims – MO†
21. Helianthus dissectifolius R.C.Jacks.
22. Helianthus divaricatus L. – woodland sunflower or rough woodland sunflower – eastern USA; Ont Que
23. Helianthus × divariserratus R.W.Long MI IN OH CT
24. Helianthus × doronicoides Lam. – TX OK AR MO IA MN IL KY IN OH PA MI NJ VA
25. Helianthus eggertii Small - Alabama, Kentucky, and Tennessee
26. Helianthus exilis A.Gray – CA
27. Helianthus floridanus A.Gray ex Chapm. – Florida sunflower – LA AL GA FL SC NC
28. Helianthus giganteus L. – giant sunflower – eastern USA; most of Canada
29. Helianthus glaucophyllus D.M.Sm – whiteleaf sunflower – TN SC NC
30. Helianthus × glaucus Small – scattered locales in southeastern USA
31. Helianthus gracilentus A.Gray – slender sunflower – CA
32. Helianthus grosseserratus M.Martens – sawtooth sunflower – Great Plains, Great Lakes, Ont Que
33. Helianthus heterophyllus Nutt. – variableleaf sunflower – Coastal Plain TX to NC
34. Helianthus hirsutus Raf. – hairy sunflower – central + Eastern USA, Ont
35. Helianthus × intermedius R.W.Long – intermediate sunflower – scattered locales in USA
36. Helianthus laciniatus A.Gray – alkali sunflower – AZ NM TX Coah NLeón
37. Helianthus × laetiflorus Pers. – cheerful sunflower, mountain sunflower – scattered in eastern + central USA + Canada
38. Helianthus laevigatus Torr. & A.Gray – smooth sunflower – GA SC NC VA MD WV
39. Helianthus lenticularis Douglas ex Lindl. CA TX
40. Helianthus longifolius Pursh – longleaf sunflower – AL GANC
41. Helianthus × luxurians (E.Watson) E.Watson – Great Lakes region
42. Helianthus maximiliani Schrad. – Maximillian sunflower – much of USA + Canada
43. Helianthus membranifolius Poir. – French Guiana
44. Helianthus mollis Lam. – downy sunflower, ashy sunflower – Ont, eastern + central USA
45. Helianthus multiflorus L. – manyflower sunflower – OH
46. Helianthus navarri Phil. – Chile
47. Helianthus neglectus Heiser – neglected sunflower – NM TX
48. Helianthus niveus (Benth.) Brandegee – showy sunflower – CA AZ; Baja California, Baja California Sur
49. Helianthus nuttallii Torr. & A.Gray – western + central USA + Canada
50. Helianthus occidentalis Riddell – fewleaf sunflower, western sunflower – Great Lakes region, scattered in southeastern USA
51. Helianthus × orgyaloides Cockerell – CO KS
52. Helianthus paradoxus Heiser – paradox sunflower – UT NM TX
53. Helianthus pauciflorus Nutt. – stiff sunflower – central USA + Canada
54. Helianthus petiolaris Nutt. – prairie sunflower, lesser sunflower – much of USA + Canada
55. Helianthus porteri (A.Gray) Pruski – Porter's sunflower – AL GA SC NC
56. Helianthus praecox Engelm. & A.Gray Texas sunflower – TX
57. †Helianthus praetermissus  – New Mexico sunflower – NM†
58. Helianthus pumilus Nutt. – little sunflower – CO WY MT UT ID
59. Helianthus radula (Pursh) Torr. & A.Gray – rayless sunflower – LA MS AL GA SC FL
60. Helianthus resinosus Small – rescindot sunflower – MS AL GA SC NC FL
61. Helianthus salicifolius A.Dietr. – willowleaf sunflower – TX OK KS MO IL WI OH PA NY
62. Helianthus sarmentosus Rich. – French Guiana
63. Helianthus scaberrimus Elliott – SC
64. Helianthus schweinitzii Torr. & A.Gray – Schweinitz's sunflower – SC NC
65. Helianthus silphioides Nutt. – rosinweed sunflower – Lower Mississippi Valley
66. Helianthus simulans E.Watson – muck sunflower – southeastern USA
67. Helianthus smithii Heiser – Smith's sunflower – AL GA TN
68. Helianthus speciosus Hook. – Michoacán
69. Helianthus subcanescens (A.Gray) E.Watson – Manitoba, north-central USA
70. Helianthus subtuberosus Bourg.
71. Helianthus tuberosus L. – Jerusalem artichoke, sunchoke, earth-apple, topinambur – much of USA + Canada
72. Helianthus × verticillatus Small – whorled sunflower – AL GA TN

Formerly included^[9]

• Flourensia thurifera (Molina) DC. (as H. thurifer Molina)
• Helianthella quinquenervis (Hook.) A.Gray (as H. quinquenervis Hook.)
• Helianthella uniflora var. uniflora (as H. uniflorus Nutt.)
• Pappobolus imbaburensis (Hieron.) Panero (as H. imbaburensis Hieron.)
• Viguiera procumbens (Pers.) S.F.Blake (as H. procumbens Pers.)

Gallery

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  Prairie sunflower (H. petiolaris)

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  Giant sunflower (H. giganteus)

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  Red Sunflower

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  Helianthus 'Strawberry Blonde'

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  Willowleaf sunflower (H. salicifolius)

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  Flower bud of Sunflower, not yet bloomed into a full-size flower

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  Jerusalem artichoke (H. tuberosus)

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  Leaves of Sunflower plant

• 

  Rear view of Sunflower head

References

External links

• Helianthus. On-line version of Flora North America, with key.