Subshrubs, 30-100 cm (taproots forming woody crowns). Herbage glabrous. Stems usually multiple (branching upward). Leaves ± evenly distributed (proximal often withering before flowering, pendulous); sessile or obscurely petiolate; blades linear-filiform (or irregularly pinnately divided into linear-filiform lobes), blades or lobes 4-9 cm × 1-5 mm, bases ± linear, ultimate margins entire. Heads 5-20+ in close, corymbiform arrays (involucres campanulate, 7-10 mm diam.). Calyculi usually of 3-8+ lance-linear to filiform bractlets (lengths 1 / 10 - 1 / 3 phyllaries). Phyllaries ± 13, 7-10(-12+) mm, tips green. Ray florets ± 8; corolla laminae (often falling early) 8-10 mm. Cypselae hirtellous. 2n = 40. Flowering mostly mid summer-fall, occasionally spring. Sandy or rocky open sites, especially drying, open, flood plains; 600-2500 m; Ariz., Colo., Kans., Nebr., N.Mex., Okla., S.Dak., Tex., Wyo. Senecio riddellii intergrades morphologically with S. spartioides. Typically, the former has larger heads with campanulate involucres 7-10 mm diam.; the latter has cylindric involucres rarely more than 6 mm diam.
Senecio riddellii is poisonous to livestock. It is now locally scarce because of efforts to eradicate it.