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.
Flowering (Jun-)Jul-Sep. Opening and margins, roadsides, often shaded, ponderosa pine, pine-fir, mixed conifer, aspen; 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.