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 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 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.
These are usually tall annuals, growing to a height of 50-390 or more cm.
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.