Subshrubs, 20-120+ cm (taproots forming woody crowns). Herbage usually glabrous, sometimes sparsely, unevenly hairy. Stems usually multiple (branching and arching upward). Leaves evenly distributed (proximal often smaller); sessile or obscurely petiolate; blades narrowly linear to filiform (or parted into linear-filiform lobes), 5-10 cm × 1-6 mm, bases ± linear, ultimate margins entire. Heads 10-20(-60) in compound corymbiform arrays (involucres cylindric or narrowly campanulate, 3-6 mm diam.). Calyculi 0 or of 1-3+ (minute, inconspicuous) bractlets. Phyllaries usually ± 8, sometimes ± 13, (5-)6-9(-10) mm, tips green or minutely black. Ray florets ± 5 (± 13); corolla laminae 8-12 mm. Cypselae usually hirtellous, sometimes glabrous. 2n = 40. Flowering summer-fall. Open, dry disturbed sites, especially stream banks and hillsides; 1000-3500 m; Ariz., Calif., Colo., Nebr., Nev., N.Mex., S.Dak., Tex., Utah, Wyo. Plants with the leaves parted into lobes that are seldom more than 1 mm wide have been recognized as Senecio spartioides var. multicapitatus (or as S. multicapitatus); expression of the character is inconsistent throughout the range and recognition of the two entities is all but impossible to maintain.
Senecio spartioides apparently hybridizes with S. eremophilus in Garfield County, Utah, and perhaps elsewhere (cf., Holmgren, Reveal, and LaFrance 3463 , BRY, KSC, NY). The name Senecio toiyabensis rests upon materials that suggest introgression with S. fremontii. Some specimens usually referred to 41. S. pattersonensis suggest introgression with S. spartioides.
Plant: Subshrub 2-8 dm, arched upward from taprooted crown; stems generally 1-few per rosette Leaves: alternate, 5-10 cm, linear, entire, ± toothed, or 2-4-lobed, lower generally petioled; middle generally weakly clasping; uppermost generally bract-like INFLORESCENCE: primary inflorescence a head, each resembling a flower; heads radiate, 10-20+; involucre cylindric to hemispheric; main phyllaries ± 8, sometimes 13, 6-9 mm, tips green Flowers: Ray flowers 5(8); ligules 10+ mm, ± yellow; Disk flowers < 40; corolla generally ± yellow; style tips truncate to obtuse, generally hair-tufted Fruit: achenes, soft-hairy, cylindric; ribs shallow, often stiff-hairy; pappus of thin, minutely barbed deciduous bristles ± = fruit body Misc: Dry, open rocky places; 1800-3200 m.; Jul-Aug