26. Symphyotrichum Nees (aster)

Plants perennial or rarely annual herbs, usually with short and stout rhizomes (elongate in a few species), less commonly with a short, woody rootstock, rarely taprooted. Stems 1 to few, erect or ascending, unbranched or branched, with fine or less commonly coarse, longitudinal lines or ribs, glabrous or variously hairy. Basal leaves present or absent at flowering. Stem leaves usually gradually reduced toward the tip, variously shaped, glabrous or hairy. Inflorescences usually relatively elongate (occasionally short and broad in poorly developed or young plants), not appearing flat-topped or hemispherical, the branches sometimes appearing racemose, the heads short- to long-stalked, the bracts often (but not always) relatively numerous and conspicuously more slender than the leaves. Heads radiate (appearing discoid in S. ciliatum), not sticky or resinous. Involucre 3–10 mm long, cup-shaped to obconical or slightly bell-shaped. Involucral bracts in 3–9 subequal or unequal, overlapping series, 0.2–0.8 mm wide (to 1.2 mm in S. turbinellum) and mostly 4–10 times as long as wide, linear to narrowly lanceolate or narrowly oblanceolate, the tip ascending or less commonly spreading to reflexed, with a usually white to pale straw-colored, relatively thin base having a very slender, green midvein, this broadened to an ovate to broadly diamond-shaped green portion toward the tip, sometimes also with narrow purple margins. Receptacle flat or shallowly convex, often with minute, irregular ridges around the concave attachment points of the florets. Ray florets 6–75(–100) (or appearing absent in S. ciliatum, in which the marginal pistillate florets have inconspicuous, tubular corollas shorter than the pappus and disc corollas), pistillate, the corolla white, pink, lavender to purple, or purplish blue. Disc florets 6–100, perfect, the corolla shallowly to relatively deeply lobed, yellow, usually turning reddish purple to brownish purple after the pollen has been shed, not persistent at fruiting. Pappus of the ray and disc florets similar (the outermost bristles sometimes appearing slightly shorter), of 1(2) series of numerous (mostly 60–90) finely barbed bristles, these all tapered toward the tip, white to grayish white, straw-colored, light tan, or pale orangish brown. Style branches with the sterile tip (beyond the stigmatic lines) (0.2–)0.3–0.5 mm long, linear to lanceolate. Fruits narrowly obovoid, often somewhat flattened, with 2–6 ribs, glabrous or sparsely hairy, variously tan or brown. About 90 species, North America to South America.

Symphyotrichum is the largest segregate of Aster in the New World and includes most of the Missouri species formerly placed in that genus. It is less closely related to the Old World species of true asters (see the treatment of Aster for further discussion) than it is to the American Eurybia and the group of genera related to Machaeranthera Nees (Semple et al., 2002). Generic limits and relationships are still somewhat unsettled within this group. Symphyotrichum is morphologically variable, and the classification and taxonomic relationships within some groups of closely related taxa also remain controversial.

The change in disc corolla color from yellow to reddish purple in most species of Symphyotrichum and in a few related genera of tribe Astereae has been remarked upon by many botanists. Niesenbaum et al. (1999) investigated this color change in S. racemosum (as Aster vimineus) and determined that the development of reddish pigmentation in disc corollas was correlated with a reduction in the quantity of pollen available in a given floret, but it apparently resulted in increased overall visitation of heads by insects that are potential pollinators.

Steyermark (1963) noted that several asters provide browse for deer (especially the basal rosettes during the winter months) and that the fertile portions are eaten by turkeys and other wildlife. A number of species of Symphyotrichum are cultivated as garden ornamentals in the United States and Canada. Some of the species with larger heads and purple rays are choice ornamentals for the garden. However, care should be taken in planting the species with smaller heads and white ray corollas, as most of these can spread aggressively both by fruits and rhizomes. For situations where large displays of white-rayed heads are desired, species and cultivars of Boltonia appear to be a more easily managed choice.