Othonna palustris Linnaeus, Sp. Pl. 2: 924. 1753; Senecio arcticus Ruprecht; S. congestus (R. Brown) de Candolle; S. congestus var. laceratus (Ledebour) Fernald; S. congestus var. palustris (Linnaeus) Fernald; S. congestus var. tonsus Fernald; S. tubicaulis Mansfeld; Tephroseris palustris subsp. congesta (R. Brown) Holub

Annuals or biennials (perhaps rarely perennials), 20–100 cm (loosely arachnose or villous, hairs white, light yellowish, or reddish brown, indument fugitive in some populations; caudices fibrous-rooted). Stems single. Leaves basal and cauline (basal and proximal sometimes withering before flowering, mid-stem leaves prominent at flowering); petioles weakly defined; blades oblanceolate to linear-oblanceolate or spatulate, 5–15 × 0.5–3(–5) cm, margins subentire to coarsely dentate or subpinnatifid (distal leaves bractlike). Heads (4–)6–20(–40+), in loose to crowded, corymbiform arrays. Involucres ± abruptly contracted to peduncles. Phyllaries usually 21, green or yellowish green (tips sometimes pinkish), 4–10 mm. Ray florets (13–)21+; corolla laminae 5–9+ mm (sometimes incompletely opened, appearing tubular). Disc florets 30–50; corollas yellow. Cypselae glabrous; pappi white or dirty white. 2n = 48.

Flowering May–Sep. Wet soils, shorelines, pond margins, brackish habitats; 0–1000 m; Alta., B.C., Man., Nfld. and Labr. (Labr.), N.W.T., Nunavut, Ont., Que., Sask., Yukon; Alaska, Iowa, Mich., Minn., N.Dak., S.Dak., Wis.; Eurasia.

Tephroseris palustris varies greatly in stature and in distribution and persistence of tomentum. The variations have been used to distinguish infraspecific taxa or two species; contemporary thought is that the complex is best treated as a single, polymorphic species.