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.
Varieties of Senecio serra are distinguished by head size and distribution.