Subshrubs or shrubs, (30–)40–120+ cm (taproots forming woody crowns). Herbage glabrous or ± lanate-tomentose. Stems usually multiple (little branched, arching-erect). Leaves ± evenly distributed; sessile or obscurely petiolate; blades narrowly linear to filiform (or pinnatifid, the segments linear to filiform), 3–10(–12) cm, bases ± linear, ultimate margins entire or remotely toothed (fascicles of smaller leaves sometimes borne in axils of larger leaves). Heads (1–)3–10(–20+) in corymbiform or subcorymbiform clusters, often gathered in larger, showy arrays (involucres campanulate or cylindric). Calyculi usually of 3–5+ bractlets (lengths often to ¹ / 2 + phyllaries), sometimes 0. Phyllaries ± 13 or ± 21+, 5–10(–12) mm, tips green or minutely black. Ray florets usually ± 8 or ± 13, sometimes ± 21; corolla laminae 10–15(–20) mm (mostly yellow, sometimes ochroleucous). Cypselae hairy. 2n = 40.

Varieties 3 (3 in the flora): w United States, Mexico.

The varieties of Senecio flaccidus are distinguished by morphologic tendencies and geography. The species occurs southward to central Mexico; a sound under­standing of the infraspecific taxonomy must await revisionary studies that include the Mexican plants.