Mem. Torrey Bot. Club 5: 328. 1894.
Perennials, 15–90 cm; rhizomatous to subrhizomatous, fibrous-rooted, caudices usually branched, woody, thick. Stems erect, moderately to densely hirsute (hairs 0.5–0.8 mm), eglandular. Leaves basal (usually withering by flowering) and cauline; basal blades oblanceolate-spatulate, 30–80 × 6–20 (–27) mm, margins entire, faces evenly hirsute to strigoso-hirsute, usually eglandular; cauline blades lanceolate to oblong, oblong-ovate, or broadly ovate, nearly even-sized distally or sometimes mid largest (continuing to immediately proximal to heads, bases clasping to subclasping). Heads 1–6 (–21) in corymbiform arrays. Involucres 6–9 × 13–20 mm. Phyllaries in 2–3 (–4) series, moderately to densely hirsute, minutely glandular. Ray-florets 100–150; corollas blue to lavender, 7–18 mm (ca. 1 mm wide), laminae coiling at tips. Disc corollas 4–5 mm. Cypselae 1.6–2 mm, 2 (–4) -nerved, faces sparsely strigose; pappi: outer of setae, inner of 20–30 bristles.
Phenology: Flowering (Jun–)Jul–Sep.
Habitat: Opening and margins, roadsides, often shaded, ponderosa pine, pine-fir, mixed conifer, aspen
Elevation: 1800–3000(–3500) m
Colo., Idaho, Mont., Nebr., N.Mex., N.Dak., S.Dak., Utah, Wyo.
Erigeron subtrinervis is variable in vestiture, perhaps reflecting gene exchange with E. speciosus. Erigeron speciosus var. mollis (A. Gray) S. L. Welsh may be a recurrent hybrid; it is identified here within E. subtrinervis.
"thick" is not a number.