17. Solidago ptarmicoides (Torr. & A. Gray) B. Boivin (white upland aster, sneezewort aster)

Aster ptarmicoides Torr. & A. Gray

Oligoneuron album (Nutt.) G.L. Nesom

S. asteroides Semple

Pl. 237 c, d; Map 1006

Plants with the rootstock short and branched, not producing creeping rhizomes. Stems 1 to several, 10–50 cm long, erect or ascending, with several fine, longitudinal ridges or lines, moderately roughened with stiff, stout, broad-based, upward-curved hairs toward the tip, sparsely roughened to nearly glabrous toward the base, not shiny. Leaves basally disposed, often persistent at flowering (additional rosettes sometimes present adjacent to the flowering stem). Basal and lowermost stem leaves with the blade 6–20 cm long, 0.5–1.0 cm wide, more than 10 times as long as wide, narrowly oblanceolate to nearly linear, relatively thick and stiff, long-tapered to a sessile or short-petiolate base, angled to a sharply pointed tip, the margins entire or with a few shallow, sharp teeth and minutely roughened, the surfaces glabrous or sparsely roughened, the undersurface usually with 3 main veins. Median and upper stem leaves 1.5–8.0 cm long, linear to very narrowly elliptic-oblanceolate, the undersurface with 1 main vein or with a faint pair of lateral main veins, otherwise similar to the lower stem leaves. Inflorescences terminal panicles, appearing flat-topped or shallowly rounded in overall outline, the heads solitary or in small clusters at the branch tips. Involucre 4–7 mm long, the bracts in 4–6 unequal series. Involucral bracts linear to oblong-lanceolate, all but the outer series rounded to bluntly pointed at the appressed-ascending tip, the pale margins slightly irregular to irregularly hairy (but sometimes curled under and not observed), the outer surface glabrous, pale yellow to straw-colored at the base, with an oblong to elliptic or somewhat diamond-shaped green area above the base, the midvein usually thickened or keeled, the 1 or 2 pairs of additional veins usually faint. Receptacle naked. Ray florets 10–25, the corollas 6–9 mm long, white or less commonly pale cream-colored. Disc florets 30–38, the corollas 3.5–4.0 mm long, the lobes 0.5–0.7 mm long, white or less commonly pale cream-colored (usually appearing yellow because of the exserted yellow anthers). Pappus 3.5–4.0 mm long, most of the bristles slightly thickened toward the tip. Fruits 1.0–1.5 mm long, narrowly obovoid, glabrous. 2n=18. July–September.

Scattered in the Ozark and Ozark Border Divisions (eastern U.S. west to Montana, Colorado, and Oklahoma; Canada). Glades, tops of bluffs, and upland prairies; also pastures, railroads, and roadsides, often on calcareous substrates.

Long classified as an unusual aster (Steyermark, 1963), this taxon was first transferred to Solidago by Boivin (1971–1972), based on the observation that the taxon hybridizes with other goldenrods but not with asters. Brouillet and Semple (1981) performed a morphological analysis that supported this reclassification. Zhang’s (1996) molecular studies also supported a close relationship between S. ptarmicoides and members of the Oligoneuron group of Solidago. The species nomenclature has been controversial as well, with various opinions on the correct authorship of the epithet ptarmicoides and whether that name was legitimately published. However, the name seems to have become stabilized as cited above in the recent botanical literature.

This species differs from all other asters and goldenrods in the state in having both ray and disc florets with the corollas white. In Missouri, no putative hybrids involving S. ptarmicoides have been discovered yet, but elsewhere in its range it hybridizes with S. canadensis, S. riddellii, and S. rigida, as well as some non-Missouri species.