Symphyotrichum hendersonii (Fern.) G. L. Nesom

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.

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.

Aster hendersonii Fernald, Bull. Torrey Bot. Club 22: 273. 1895 (as hendersoni); A. foliaceus Lindley subsp. lyallii (A. Gray) Cronquist

Symphyotrichum hendersonii (formerly Aster hendersonii) is a species of flowering plant in the family Asteraceae native to the northwestern United States. Commonly known as Henderson's aster, it is a perennial, herbaceous plant that may reach 70 to 150 centimeters (2 to 5 feet) tall. Its flowers have purple ray florets and yellow disk florets.

Isotype of Aster hendersonii stored at the Harvard University Herbarium. Collected 5 August 1894 at the Saint Maries River, Kootenai County, Idaho.