Mem. Amer. Acad. Arts, n. s. 4: 68. 1849.
Biennuals or short-lived perennials, 3–15 cm; usually fibrous-rooted, sometimes taprooted, caudices lignescent, rarely branched. Stems first erect (greenish proximally; usually single, simple), then producing herbaceous, leafy, prostrate runners (usually with rooting plantlets at tips, populations often becoming clonal mats), strigose (often sparsely; hairs antrorsely appressed, consistent in orientation), sometimes slightly glandular distally. Leaves basal (often persistent) and cauline; basal blades broadly oblanceolate to elliptic, 20–55 × 3–9 mm, cauline abruptly reduced distally, margins entire or dentate, faces strigose, eglandular. Heads 1(–3, on proximal branches). Involucres 3–5 × 6–13 mm. Phyllaries in 2–3 series, strigose to loosely hirsute, minutely glandular. Ray florets 40–125; corollas white, often with an abaxial midstripe, often drying lilac, 4–10 mm, laminae not coiling or reflexing. Disc corollas 2–3.5 mm. Cypselae 0.8–1.3 mm, 2-nerved, faces sparsely strigose; pappi: outer of setae, inner of 10–17 bristles. 2n = 18, 27, 36, 45, 54.
Phenology: Flowering May–Aug(–Sep).
Habitat: Meadows and grassy slopes, often moist, open areas in grasslands, pinyon pine, oak-pine, pine, aspen, and spruce-fir
Elevation: (1700–)2100–3600 m
Alta., B.C., Ariz., Calif., Colo., Idaho, Kans., Mont., Nebr., Nev., N.Mex., N.Dak., Okla., Oreg., S.Dak., Tex., Utah, Wash., Wyo., Mexico.
Early season forms of Erigeron flagellaris may consist of a basal rosette and a single, erect, scapiform, monocephalous stem; leafy runners usually develop quickly. Many polyploids of this species are indistinguishable from diploids; some polyploids have features suggestive of genetic influence of E. tracyi.