Perennials, 30–60 cm, colonial; long-rhizomatous. Stems 1–6, ascending to erect, glabrous or puberulent, especially distally. Leaves thin, margins entire, apices acute, faces glabrous or scabridulous; basal withering by flowering, petiolate, blades narrowly oblanceolate to linear, 10–60 × 2–10 mm, attenuate, margins entire; proximalmost cauline sometimes withering by flowering, sessile, blades linear to narrowly oblanceolate, 30–80 × 2–8 mm, bases cuneate, apices acute; distal sessile, blades linear, 10–50 × 2–8 mm, bases ± cuneate, truncate. Heads in racemiforn to paniculiform arrays, branches usually less than 10(–20) cm. Peduncles sparsely to densely hairy, bracts 3–10, linear to narrowly elliptic, much smaller than distal leaves. Involucres campanulate, 4–7 mm. Phyllaries in 5–6 series, appressed, narrowly oblanceolate (outer) to linear (inner), unequal, bases (outer) herbaceous or scarious less than 1/2, margins entire, green zones ovate to linear (inner), apices obtuse, faces glabrous. Ray florets 14–35; corollas usually white, sometimes pale violet, laminae 7–12 × 1–2 mm. Disc florets 20–35; corollas yellow, 4.5–6.5 mm, lobes triangular, 0.7–1 mm, glabrous. Cypselae brown, cylindric to obovoid, not compressed, 2–2.5 mm, 2–5-nerved, faces hairy; pappi white, 5–7 mm. 2n = 16, 32, 48.
Phenology: Flowering Jul–Aug.
Habitat: Summer-dry grasslands and meadows
Elevation: 0–500 m
Symphyotrichum hallii is restricted to open habitats of the Puget Trough of western Washington and the Willamette Valley of western Oregon, with outlying stations in the Columbia Gorge and central Washington. Some of the polyploid races appear to be alloploids involving the sympatric S. subspicatum, with larger leaves and fewer, larger heads with violet rays.