Phytologia 77: 283. 1995.
Perennials, 70–150 cm, colonial; long-rhizomatous. Stems 1–5+, ascending to erect, glabrous or puberulent, especially distally. Leaves thin, margins entire, apices acute, faces glabrous or scabridulous; basal persistent or withering by flowering, petiolate, blades oblanceolate, 50–150 × 10–20 mm, bases attenuate or cuneate; proximal cauline sessile, blades elliptic to oblanceolate, 80–200 × 10–25 mm, bases cuneate, auriculate; distal sessile, 40–80 × 7–20 mm, reduced distally. Heads in paniculiform arrays, branches numerous, 10–45 cm. Peduncles sparsely to densely hairy, bracts 3–10, acute, lanceolate to linear, bases auriculate. Involucres campanulate, 5–10 mm. Phyllaries in 4–5 series, narrowly oblong or linear (outer) to linear (inner), unequal, bases scarious, margins entire, sometimes proximally ciliate, green zones linear, apices acute, faces glabrous. Ray-florets 25–45; corollas violet, laminae 9–15 × 1–2.5 mm. Disc-florets (50–) 70–80; corollas yellow, 5–6 mm, lobes triangular, 0.5–0.8 mm. Cypselae brown, cylindric to obovoid, not compressed, 3–3.5 mm, 3–4-nerved, faces hairy; pappi white, 4.5–5 mm. 2n = 16, 32.
Phenology: Flowering Jul–Aug.
Habitat: Meadows, stream banks, forest openings
Elevation: 1000–1500+ m
Calif., Idaho, Mont., Oreg., Wash.
Symphyotrichum hendersonii is concentrated in north-central Idaho and adjacent Montana and Washington, with possibly outlying populations in the Siskiyou Mountains of southwestern Oregon and adjacent California. It is in some respects intermediate between S. spathulatum and S. cusickii; forms transitional to both of these species occur.
"thin" is not a number.