18. Symphyotrichum praealtum (Poir.) G.L. Nesom (willow-leaved aster)

Aster praealtus Poir.

S. praealtum var. angustior (Wiegand) G.L. Nesom

A. praealtus var. angustior Wiegand

S. praealtum var. subasperum (Lindl. ex Hook.) G.L. Nesom

A. praealtus var. subasper (Lindl. ex Hook.) Wiegand

Pl. 247 a–c; Map 1030

Plants perennial herbs, often somewhat colonial from relatively long, slender, branched rhizomes. Stems usually solitary, 30–150 cm long, unbranched or with few to most commonly many ascending branches mostly above the midpoint, sparsely to moderately pubescent with short, spreading or curled hairs toward the tip, these usually in longitudinal lines or bands, usually glabrous toward the base or sometimes nearly throughout, often somewhat glaucous. Basal and/or lower stem leaves usually absent from the flowering stems, sessile or with a short, poorly differentiated petiole, the blade 3–7 cm long, 1.0–2.5 cm wide, oblanceolate, tapered at the base, rounded or angled to a bluntly or sharply pointed tip, the margins with minute, forward-pointing hairs and usually entire (rarely with a few shallow teeth), the upper surface glabrous or sparsely to moderately and minutely roughened, the undersurface glabrous or nearly so, the secondary veins on the leaf undersurface usually difficult to distinguish but the veinlets relatively prominent, forming a network with areoles that are about as long as wide or slightly longer than wide (these often containing finer veinlets). Median and upper stem leaves progressively smaller, sessile, the base sometimes slightly expanded but not clasping the stem, the blades 1–15 cm long, linear to narrowly elliptic or oblong-lanceolate, the margins entire or sparsely toothed, angled or tapered at the base, angled or tapered to a sharply pointed tip, otherwise similar to the lower stem leaves. Inflorescences usually appearing as panicles with short to relatively long, loosely ascending to spreading branches (these racemose or less commonly with the heads solitary or in small clusters toward the branch tips), the heads appearing short- to more commonly relatively long-stalked and usually oriented upward, the bracts along the ultimate branches 0.4–1.5 cm long, relatively numerous, more or less leaflike, linear or narrowly oblong-lanceolate, often noticeably shorter than the adjacent foliage leaves. Heads mostly 1.5–2.5 cm in diameter (including the extended ray corollas) at flowering. Involucre 4–7 mm long, cup-shaped to slightly bell-shaped when fresh (sometimes becoming obconical when pressed), the bracts in 4 or 5 unequal, overlapping series. Involucral bracts linear-lanceolate to narrowly oblong-oblanceolate, angled or tapered at the bluntly or sharply pointed tip, lacking a bristlelike or spinelike point at the ascending tip (sometimes with an abrupt, minute, reddish, sharp point at the tip), the slender midvein broadened gradually or more commonly relatively abruptly in the apical 1/4–1/2 into an elliptic, diamond-shaped, or oblanceolate (2–5 times as long as wide) green tip, the outer surface glabrous (the inner surface often sparsely hairy), the margins often slightly irregular and sparsely to moderately hairy especially toward the tip. Ray florets 20–35 in usually 1 or 2 series, the corollas well developed, 5–12 mm long, purple or bluish purple). Disc florets 20–35, the corollas 4–6 mm long, the slender portion of the tube noticeably shorter than the slightly expanded apical portion, the lobes 0.4–1.1 mm long, 18–25 percent of the total length of the expanded portion. Pappus bristles 4.0–6.5 mm long, white or off-white. Fruits 1.5–2.0 mm long, with 4 or 5 longitudinal ribs, purplish brown or tan with purple lines or streaks, sparsely to moderately hairy, the hairs lacking swollen bases. 2n=32, 48. August–October.

Scattered nearly throughout the state (eastern U.S. west to Minnesota, Colorado, and Texas; Canada). Bottomland prairies, moist depressions of upland prairies, banks of streams, rivers, and spring branches, fens, margins of ponds and lakes, bottomland forests, and mesic upland forests; also ditches, railroads, roadsides, and open, disturbed areas.

Steyermark (1963) recorded this species as nearly absent from the eastern half of the Ozark Division, but more recent collections have served to fill in the range. The infraspecific taxonomy requires more detailed study (Semple et al., 2002). Steyermark accepted three varieties as occurring in the state, but at least two additional varieties have been described from states to the west of Missouri. One of the varieties, var. subasperum, originally was described (as Aster subasper Lindl. ex Hook.) based on specimens collected by Thomas Drummond near St. Louis. Jones (1989) indicated that at least in Illinois the variants accepted by Steyermark (1963) intergrade too much to make formal taxonomic recognition feasible. In his treatment of S. praealtum (as Aster praealtus), Steyermark separated var. angustior as a rare, narrow-leaved variant that was present sporadically around the state. He treated rare, relatively broad-leaved plants (also with slightly broader involucral bracts) occurring sporadically in the state as var. subasperum. The bulk of the Missouri specimens with leaves of intermediate length-to-width ratios were classified in var. praealtum. Since Steyermark’s (1963) treatment, a number of collections have been made, but these serve to blur the lines between the variants rather than reinforcing them. Because of this, the present treatment does not provide formal taxonomic recognition for varieties within S. praealtum.

Steyermark (1963) noted that this species, with its relatively leafy branches and bright, showy flowering heads, does well in the wildflower garden. It is available commercially in some midwestern wildflower nurseries. Steyermark also indicated the presence of rare putative hybrids with S. pilosum and S. lateriflorum, and putative hybrids with S. lanceolatum also have been collected.