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.
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 disc florets inside. Several ornamental cultivars have red-colored ray florets; all of them stem from a single original mutant. 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.
Annual (in ours) or perennial herbs. Leaves alternate (in ours). Capitula large, solitary and terminal or laxly corymbose, heterogamous, radiate; ray florets neuter, yellow; disk florets bisexual, fertile. Phyllaries 2-3-seriate. green and herbaceous. Receptacular scales folded, partly enclosing achenes at maturity. Achenes ± flattened, slightly angled. Pappus of 2(-4) deciduous bristles.
"Patterning seeds in spirals of Fibonacci numbers allows for the maximum number of seeds on a seed head, packed uniformly, with no crowding at the center and no 'bald patches' at the edges. In other words, the sunflower has found optimal space utilization for its seed head. The Fibonacci sequence works so well for the sunflower because of one key characteristic—growth. On a sunflower seed head, the individual seeds grow and the center of the seed head continues to add new seeds, pushing those at the periphery outwards. Following the Fibonacci sequence ensures growth on the same terms indefinitely. That is to say, as a seed head grows, seeds will always be packed uniformly, and with maximum compactness." (Courtesy of the Biomimicry Guild)
"The leaf rosettes of the carnivorous Pinguicula moranensis follow a spiral phyllotaxis approaching a Fibonacci pattern while the stalked flowers arise from extra-axillary sites between the leaves…The leaves of consecutive articles of such sympodially constructed rosettes are arranged along a spiral Fibonacci pattern (with divergence angles around 137°)…Sympodial construction of flowering shoots and leaf rosettes is also known from Aloe, Gunnera and Philodendron." (Grob et al. 2007:857) Learn more about this functional adaptation.
Grob V; Pfeifer E; Rutishauser R. 2007. Sympodial construction of Fibonacci-type leaf rosettes in Pinguicula moranensis (Lentibulariaceae). Annals of Botany. 100(4): 857-863.
Barcode of Life Data Systems (BOLD) Stats Specimen Records:305 Specimens with Sequences:331 Specimens with Barcodes:97 Species:63 Species With Barcodes:62 Public Records:216 Public Species:50 Public BINs:0