Fl. Bor.-Amer. 1: 333. 1834.
Perennials, 40–100(–250) cm (caudices ligneous, branched). Herbage glabrous or lightly floccose-tomentose proximally when young. Stems single or loosely clustered. Leaves evenly distributed (proximal often withering before flowering); petiolate or subsessile; blades lanceolate or narrowly lanceolate to sublinear, 5–15(–20+) × (1–)1.5–4 cm, bases tapered, margins dentate to subentire (distal leaves smaller, bractlike). Heads 30–90+ in corymbiform to subpaniculiform arrays. Calyculi of 2–6 linear to filiform bractlets (0.5–5 mm). Phyllaries ± 8 or ca. 13, 4–9 mm, tips usually green, sometimes black. Ray florets ± 5 or ± 8; corolla laminae 5–10 mm. Cypselae glabrous or glabrate. 2n = 40.
Calif., Colo., Idaho, Mont., Nev., Oreg., Utah, Wash., Wyo.
Varieties 2 (2 in the flora).
Varieties of Senecio serra are distinguished by head size and distribution.