Aster hesperius A. Gray in A. Gray et al., Syn. Fl. N. Amer. 1(2): 192. 1884; A. coerulescens de Candolle var. wootonii (Greene) Wiegand; A. durus Lunell; A. fluvialis Osterhout; A. hesperius var. wootonii Greene; A. lanceolatus Willdenow subsp. hesperius (A. Gray) Semple & Chmielewski; A. lautus Lunell; A. osterhoutii Rydberg; A. wootonii (Greene) Greene; Symphyotrichum hesperium (A. Gray) Á. Löve and D. Löve

Stems stout, glabrous or at most hairy in lines. Leaf blades linear-lanceolate or -oblanceolate, proximal cauline margins shallowly serrate, mid to distal entire. Heads 1–100+, borne singly or congested at ends of lateral branches, usually subtended by large foliaceous bracts. Involucres 4–7.2 mm. Phyllaries: outer 2.6–6.3 × 0.4–1 mm, inner 4–7.4 × 0.2–1.1 mm, outer ² / 3 + length of inner. Ray florets 18–45; corollas usually pale to dark purplish blue, sometimes white, laminae 4.2–10.1 mm. Disc florets 18–52; corollas 3.3–5.7 mm, limbs 2–3.8 mm, lobes 0.5–1.1 mm. Cypselae 0.7–2.7; pappi 3.7–6.4 mm. 2n = 64.

Flowering Jul–Oct. Along streams and banks in prairies, wet meadows, open conifer forests in montane zone, often calcareous soils, roadside ditches; 10–2700 m; Alta., B.C., Man., N.W.T., Ont., Que., Sask.; Ariz., Calif., Colo., Idaho, Iowa, Kans., Minn., Mont., Nebr., Nev., N.Mex., N.Dak., Okla., Oreg., S.Dak., Tex., Utah, Wash., Wis., Wyo.; Mexico (Baja California, Chihuahua, Sonora).

Variety hesperium is present at the northeastern limit of its range in Quebec only on the clays of the southern James Bay; a report of introduction in the Gatineau area is attributable to Symphyotrichum robynsianum. In north-central Ontario, it is found mainly in areas of clay deposits or in calcareous stream valleys, but not in the Clay Belt, reaching the southern James Bay. This variety has been treated mostly as a distinct species in floras. Character ranges overlap considerably with var. lanceolatum, and it is often difficult to distinguish the two entities where their distributions overlap. In areas of sympatry, the two taxa hybridize to form septaploid plants (2n = 56).