Annuals, (5-)10-30(-50+) cm (taprooted). Herbage glabrous or sparsely tomentose. Stems usually single (erect), sometimes 2-6 (branching from bases, arching upward). Leaves (sometimes subsucculent) evenly distributed; ± petiolate (proximal); blades linear-lanceolate to lanceolate 2-5(-7+) × 0.5-2(-3+) cm, bases tapered, margins subpinnatifid to dentate or subentire (mid and distal leaves sessile, bases clasping). Heads usually 3-10(-20+) in open, cymiform arrays, sometimes borne singly (depauperate plants). Calyculi of 5-8+ lanceolate to lance-linear or filiform bractlets (0.5-3 mm, usually inconspicuous). Phyllaries ± 21 (sometimes fewer), 5-7(-8) mm, tips black. Ray florets ± 13; corolla laminae 8-11 mm. Cypselae hirtellous-strigose. 2n = 40.

Flowering late winter-spring. Sandy, dry or drying sites, especially near coast; 0-1200 m; Calif.; Mexico.

Dwarfish, subsucculent plants of Senecio californicus from near the coast have been recognized as S. ammophilus.