79. Helianthus L. (sunflower)

(Heiser et al., 1969)

Plants annual or perennial herbs, the annuals with taproots, the perennials usually with rhizomes. Stems erect or ascending, unbranched or more commonly several-branched, mostly above the midpoint, with several longitudinal lines or ridges, variously glabrous to roughened or hairy. Leaves opposite and/or alternate, in a few species mostly basal, sessile or petiolate, the bases sometimes somewhat expanded and wrapping around the stem. Leaf blades simple, narrowly linear to nearly circular, tapered to shallowly cordate at the base, mostly tapered or less commonly broadly angled to a sharply pointed tip (rarely rounded), the margins entire to variously sharply toothed, the surfaces variously glabrous to roughened or hairy, sometimes also with scattered, sessile, spherical, yellow glands. Inflorescences most commonly panicles (sometimes few-flowered and appearing racemose) or in some species loose axillary clusters or of solitary axillary or terminal heads, the heads with short to more commonly relatively long, usually bractless stalks. Heads radiate. Involucre cup-shaped to broadly cup-shaped or somewhat bell-shaped, the bracts in mostly 2–4 unequal to subequal, overlapping series. Involucral bracts 12–40, variously narrowly lanceolate to ovate, ascending or with the tips loosely spreading, green with often hairy margins, the outer surface glabrous or more commonly variously roughened or hairy, often also with scattered, sessile, spherical, yellow glands, with 1 to several fine nerves. Receptacle flat to convex or slightly conical, usually not elongating as the fruits mature (occasionally broadening somewhat as the fruits mature), with chaffy bracts subtending the ray and disc florets, these somewhat concave and wrapped around the florets. Ray florets sterile (lacking stamens and style at flowering and with an ovary that is shorter and thinner than those of the disc florets, not developing into a fruit), the corolla showy, relatively broad, yellow (often pale yellow in H. decapetalus), not persistent at fruiting. Disc florets numerous, perfect, the corolla yellow or purple to reddish brown, not expanded at the base or persistent at fruiting (but often with a swollen portion toward the midpoint of the tube). Style branches with the sterile tip somewhat elongate and tapered. Pappus of the ray and disc florets of 2 short awns 1–5 mm long, sometimes also with 1–6 additional inconspicuous scales or awns 0.2–2.0 mm long, not persistent at fruiting. Fruits obovate to somewhat wedge-shaped in outline, somewhat flattened and often somewhat 4-angled in cross-section, the surface smooth or sometimes with several fine lines or nerves, glabrous or hairy, purplish black, sometimes with lighter mottling, usually not shiny. About 51 species, U.S., Canada, Mexico.

The sunflowers are a variable group, but most species are relatively easily recognized as members of the genus. Sometimes specimens of Helianthus are confused with Heliopsis (see the treatment of that genus for further discussion) or with Silphium (whose species tend to be more resinous and to have larger, broader involucral bracts). Within Helianthus, delimitation of particularly the perennial species is complicated by the presence of polyploidy and hybridization. Determination of specimens is complicated by the widespread incidence of hybridization among several of the common species, but in Missouri the hybrids generally are encountered as solitary or few individuals within populations of one or both putative parents. The exception to this rule is H. ×laetiflorus, which occasionally occurs as small populations separate from either parent. In the Ozarks, the main problems of species-level recognition appear to center around H. divaricatus, H. hirsutus, and H. strumosus, although smaller plants of H. tuberosus also can be problematic to identify. See the treatments of those species for further discussion.