Erigeron acris subsp. debilis (A. Gray) Piper; E. acris Linnaeus var. debilis A. Gray; E. angulosus Gaudin subsp. debilis (A. Gray) Piper; E. debilis (A. Gray) Rydberg; E. elatus (Hooker) Greene var. bakeri Greene; E. jucundus Greene; E. scotteri B. Boivin; Trimorpha acris (Linnaeus) Gray var. debilis (A. Gray) G. L. Nesom

Biennials or short-lived perennials, 5–25(–35) cm; usually fibrous-rooted, sometimes taprooted, caudices simple or branched, sometimes weakly short-rhizomatous. Stems erect to basally ascending, sometimes sparsely hirsuto-villous, minutely glandular. Leaves basal (persistent) and cauline (petiole margins coarsely ciliate); basal blades oblanceolate to spatulate, 20–60 × 2–6(–10) mm, cauline gradually reduced distally, margins entire or rarely with 1–2 pairs of shallow teeth, faces sparsely hirsuto-strigose, eglandular. Heads 1–6(–8) in corymbiform arrays (on curved-ascending peduncles). Involucres 5–6 × 8–11 mm. Phyllaries in 2–3(–4) series (inner apices attenuate to caudate), sparsely hirsuto-villous or glabrous, minutely glandular. Ray (pistillate) florets in 2 series; outer 40–70, corollas white to pinkish 5.5–7 mm, laminae (filiform) erect, not coiling or reflexing; inner many fewer than outer series, tubular, elaminate. Disc corollas 4.4–5.5 mm. Cypselae 2–2.3 mm, 2-nerved, faces sparsely strigose; pappi: outer of setae, inner of (12–)14–21 (accrescent) bristles. 2n = 18.

Flowering May–Aug. Rocky sites, gravel bars and banks, roadsides, meadows, open woods, up to subalpine areas with spruce in Rocky Mountains; 1200–3700 m; Alta., B.C., N.W.T., Yukon; Alaska, Calif., Colo., Idaho, Mont., N.Mex., Oreg., Utah, Wash., Wyo.

Erigeron nivalis usually has been treated as an infra-specific taxon within E. acris; the two are broadly sympatric in the northwestern United States and Canada without obvious intergrades. Both occur over a wide range of elevations and in similar habitats. Erigeron nivalis probably occurs in Nevada; it has not been taxonomically distinguished there. Erigeron scotteri was regarded by E. H. Moss and J. G. Packer (1983) as a synonym of E. acris (presumably var. debilis = E. nivalis; the heads are relatively small and borne singly).