Sunflower photography by Edward M. Fielding
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Helianthus L. /ˌhiːliˈænθəs/ (sunflower) is a genus of plants comprising about 52 species in the Asteraceae family, all of which are native to North America. The common name “sunflower” also applies to the popular annual species Helianthus annuus. This and other species, notably Jerusalem artichoke (H. tuberosus), are cultivated in temperate regions as food crops and ornamental plants.
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 cauducous (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 pap-pus entirely. Another technical feature that distinguishes the genus more reliably, but requires a microscope to see, is the presence of a prominent, multi-cellular 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, Helianthus 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, Helianthus 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.