Phytologia 77: 289. 1995.
Perennials, 15–35 (–45) cm, colonial or cespitose; rhizomatous. Stems 1–3+, ascending to erect (sometimes purple or purplish brown), glabrous or glabrate, sparsely hairy in leaf-axils [villous to sericeous]. Leaves thin, apices callous-pointed, faces glabrous or glabrate; basal usually withering by flowering, long-petiolate, petioles sheathing, sparsely ciliate, blades lanceolate, 40–110 × (3–) 5–7 mm, bases attenuate, margins entire to crenulate, rarely with 1–2 teeth, apices obtuse to rounded, callus-pointed; proximalmost cauline sometimes withering by flowering, winged-petiolate or sessile distally, blades lanceolate to linear-lanceolate, 50–120 (–180) × 1–6 mm, bases attenuate, margins entire, sparsely ciliate, apices acute; distal sessile, blades subulate to lanceolate, 10–50 × 1–2 mm, bases clasping, apices acuminate. Heads (1–) 3–20 (–40), in paniculiform arrays, branches patent. Peduncles 1–4 (–6) cm, ascending, glabrate, bracts 1–4 (–5), subulate to lanceolate, foliaceous. Involucres cylindric to hemispheric, (4.5–) 5–7.3 mm. Phyllaries in 2–3 (–5) series, subulate to lanceolate, bases indurate, margins hyaline, entire to erose, often ciliate, green zones linear-lanceolate (outer) or rhombic (inner), apices acute, faces glabrous [sparsely villous]. Ray-florets 14–27; corollas white, laminae (4.6–) 5.6–7 (–9.5) –10.3 × 1.3–2 mm. Disc-florets 18–35 (–42); corollas yellow, 3.5–5 mm, tubes shorter than funnelform throats (hairy), lobes± spreading, triangular, 0.5–0.7 mm. Cypselae light-brown to gray, narrowly obovoid to fusiform, sometimes ± compressed, 2–3 mm, 5-nerved, faces glabrate to sparsely strigillose; pappi white to cream, 3.5–5.5 mm. 2n = 10.
Phenology: Flowering (Jun–)Jul–Sep.
Habitat: Moist, muddy, sandy, or rocky soils along streams
Elevation: 1500–1900 m
Of conservation concern.
Symphyotrichum potosinum is known from Cochise County, Arizona, south into Mexico to northern Oaxaca.
"thin" is not a number.