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. Flowering May-Aug(-Sep). Meadows and grassy slopes, often moist, open areas in grasslands, pinyon pine, oak-pine, pine, aspen, and spruce-fir; (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.