17. Symphyotrichum pilosum (Willd.) G.L. Nesom (white heath aster)

Aster pilosus Willd.

A. pilosus var. demotus S.F. Blake

A. pilosus var. platyphyllus (Torr. & A. Gray) S.F. Blake

A. pilosus var. pringlei (A. Gray) S.F. Blake

S. pilosum var. pringlei (A. Gray) G.L. Nesom

Pl. 247 h, i; Map 1029

Plants perennial herbs, with a short, somewhat woody, relatively stout rootstock, sometimes also with stout, somewhat succulent rhizomes. Stems 1 to several, 20–150 cm long, unbranched or with few to many ascending to spreading branches mostly above the midpoint, glabrous or sparsely to densely pubescent with short, spreading and/or curved hairs, more densely so toward the tip, the hairs usually relatively evenly distributed, occasionally in longitudinal lines or bands. Basal and/or lower stem leaves usually absent from the flowering stems (but new rosettes often produced from the rootstock by flowering time), sessile or with a short, poorly differentiated petiole, the blade 4–8 cm long, 1.0–2.5 cm wide, oblanceolate to obovate, tapered at the base, rounded or angled to a usually bluntly pointed tip, the margins usually with spreading hairs and entire or with sparse, blunt, shallow teeth, the surfaces glabrous or the upper surface sparsely to moderately roughened with short, stiff hairs, the secondary veins on the leaf undersurface often faint and difficult to distinguish from the veinlets, these forming an irregular network of relatively short areoles. Median and upper stem leaves progressively smaller, the larger ones often withered by flowering time, sessile, the base sometimes slightly expanded but not clasping the stem, the blades 1–10(–15) cm long, linear to less commonly elliptic-lanceolate or oblanceolate, the margins entire or shallowly toothed (sometimes somewhat curled under), tapered at the base, angled or tapered to a sharply pointed tip (often with a short, hard point at the tip), the veinlets usually relatively easily observed, otherwise similar to the lower stem leaves. Inflorescences usually appearing as panicles with short to more commonly relatively long, loosely ascending to spreading, racemose branches, sometimes reduced to a solitary raceme, the heads appearing mostly long-stalked and oriented upward, the bracts along the ultimate branches 0.2–1.0 cm long, relatively numerous, more or less leaflike, linear, noticeably shorter and often narrower than the adjacent foliage leaves. Heads mostly 1.4–2.0(–2.5) cm in diameter (including the extended ray corollas) at flowering. Involucre 4–8 mm long, urn-shaped to more or less cup-shaped (becoming bell-shaped to broadly obconical when pressed), the bracts in 4–6 unequal, overlapping series. Involucral bracts narrowly oblong-oblanceolate (sometimes slightly broadened at the base), at least the outer few series somewhat inrolled, thickened, and tapered at the sharply pointed, usually awl-shaped tip, with a white to yellowish- or purplish-tinged, bristlelike, minute spinelike, or rarely softer hairlike point at the very tip, the tip ascending or somewhat outward- then upward-curved, the outer series with the narrowly elliptic green portion reaching nearly to the base, the other series with the slender midvein broadened gradually in the apical 1/4–1/2 into an elliptic or diamond-shaped (3–7 times as long as wide) green tip, the surfaces glabrous or the outer surface sparsely hairy, the margins minutely toothed or somewhat irregular and often sparsely short-hairy. Ray florets 15–35 in usually 1 or 2 series, the corollas well developed, 5–10 mm long, white (rarely pink). Disc florets 20–40, the corollas 2.5–5.0 mm long, the slender portion of the tube noticeably shorter than the slightly expanded apical portion, the lobes 0.5–0.9 mm long, 22–30 percent of the total length of the expanded portion. Pappus bristles 2.5–5.0 mm long, white. Fruits 1.0–1.5 mm long, with 3–4 longitudinal ribs, pale gray to tan, moderately hairy, the hairs lacking swollen bases. 2n=32, 40, 48. August–November.

Common throughout the state (eastern U.S. west to South Dakota and Texas; Canada; introduced in the Pacific Northwest). Bottomland and upland prairies, glades, bottomland forests, mesic to dry upland forests, savannas, banks of streams and rivers, margins of ponds, lakes, and sinkhole ponds; also fallow fields, pastures, old fields, fencerows, gardens, railroads, roadsides, and open, disturbed areas.

Symphyotrichum pilosum is among the most widespread and weediest of our native asters. Steyermark (1963) knew it primarily as an upland species, but now it is encountered with some frequency at bottomland sites as well. He recommended a form with pink ray corollas (Aster pilosus var. pilosus f. pulchellus Benke) for cultivation as an ornamental, but it can spread aggressively by seed in a garden situation. The species comprises a polyploid complex and also is quite variable in its vegetative morphology. Semple and Chmielewski (1985) studied the cytology of the group across its range, finding both tetraploid (2n=32) and hexaploid (2n=48) populations in Missouri, with a reported pentaploid (2n=40) that may have resulted from hybridization between the other two cytotypes. Steyermark (1963) reported putative hybrids with S. parviceps, S. praealtum, and S. racemosum (as Aster vimineus).

Because of the complex cytological and morphological variation within the species, opinions have differed on the merits of providing formal taxonomic recognition to any of the variants (Jones, 1989). Steyermark (1963) treated three varieties, but most subsequent students of the group have treated his A. pilosus var. platyphyllus as broad-leaved plants showing an environmental response to particular soil and moisture conditions (Jones, 1989). Nesom (1994) and Semple et al. (2002) accepted the other two of Steyermark’s varieties, with glabrous-stemmed plants called var. pringlei (Aster pilosus var. demotus) and hairy-stemmed plants called var. pilosum. Earlier, Semple (1978) had stated that glabrous-stemmed plants can produce hairy stems in greenhouse culture, suggesting that this feature is at least partially under environmental control. According to Steyermark (1963), both types are equally widespread in the state. In practice, however, a large number of specimens have sparse hairs in lines extending downward from each node and are difficult to categorize into either variant. Semple and Chmielewski (1985) found that plants of var. pringlei that they studied were exclusively hexaploids, whereas plants assignable to var. pilosum were either tetraploids or hexaploids, but they also suggested that hybridization occurred between the two varieties. Subsequently, Semple et al. (1993) studied tetraploid (2n=32) and hexaploid (2n=48) plants in the S. parviceps/pilosum complex from dolomite glades in Washington County, stating that morphologically some of the plants were unusually glabrescent and thus vegetatively very similar to hexaploid plants that they had determined previously as S. pilosum var. pringlei but also noting similarities to S. priceae (Britton) G.L. Nesom, an unusual octoploid that is endemic to a small region from Kentucky to Alabama and Georgia. In Missouri, plants referable to var. pringlei tend to have more slender stems and narrower main stem leaves than do most specimens of var. pilosus. In light of the confusion surrounding the infraspecific classification within this species, no varieties are recognized formally in the present treatment.