9. Opuntia chlorotica Engelmann & J. M. Bigelow, Proc. Amer. Acad. Arts. 3: 291. 1856.
Trees or shrubs, erect, 2-2.5 m, with spiny, well-defined trunk to 30 cm diam. Stem segments not disarticulating, blue-green, flattened, obovate to circular, 13-21 × 11.5-19 cm, nearly smooth, glabrous; areoles 7-10 per diagonal row across midstem segment, subcircular to elliptic, with basal ridge, 3-6 × 2.5-4 mm; wool tan, aging grayish white. Spines absent or usually in distal areoles to evenly distributed over entire stem segment, yellow, aging red-brown to blackish, straight or weakly curved, ± acicular, deflexed, or some erect in marginal areoles; larger spines 0-7 per areole, terete or basally flattened, 25-45 mm, usually accompanied by few straight to wavy bristle-spines. Glochids crowded in narrow crescent along adaxial margins, longer toward base of areole margins, subapical tuft absent or poorly developed, yellow, aging reddish brown, to 14 mm. Flowers: inner tepals yellow (sometimes with reddish blush near base), abaxially reddish streaked along midveins, broadly spatulate-apiculate, 18-30 mm; filaments white to yellow; anthers; style and stigma lobes white, yellowish, or pale green. Fruits red, barrel-shaped, 30-60 × 18-40 mm, fleshy (often mummifying), glabrous, spineless; umbilicus 6-9 mm deep; areoles 40-68. Seeds yellowish, 3.5-4 × 3-3.5 mm, 1.5-1.8 mm thick, reniform to subcircular, flattened, often warped; girdle protruding 0.1-0.5 mm. 2n = 22.
Flowering spring-summer (Apr-Jul). Desert grasslands, woodlands, chaparral, desert flats, rocky ledges, hills, canyons; 600-2400 m; Ariz., Calif., Nev., N.Mex., Utah; Mexico (Baja California, Baja California Sur, Sonora).
Opuntia chlorotica hybridizes with O. santa-rita in southeastern Arizona and with the hexaploid O. phaeacantha forming the tetraploid O. ×curvispina in areas of Arizona, California, and Nevada.