Shrubs, to 1.2–2.5 m, dioecious. Stems erect, grayish white, branched from base, woody-succulent, canescent; short shoots common; latex watery, cloudy-whitish. Leaves deciduous, mostly ± evenly distributed on long shoots, few on short shoots; stipules absent; petiole 1.3–3 cm, not stipitate-glandular; blade cordate to broadly ovate, 2.2–3.5 × 1.6–3 cm, unlobed or shallowly 3-lobed, base truncate-cordate, margins entire, apex rounded, ± coriaceous, abaxial surface canescent, adaxial surface sparingly hairy; venation pinnate (palmate if lobed). Inflorescences terminal on branches or on short shoots, staminate cymes, pistillate fascicles, or flowers solitary; peduncle 1–2.6 cm; bracts 1–2.5 mm, margins entire, sparsely hairy. Pedicels 2–3 mm. Staminate flowers: sepals connate to 1/4 length, lanceolate, 2–2.5 × 0.5–0.8 mm, margins entire, apex acute, canescent abaxially, glabrous or sparingly hairy adaxially; corolla grayish white, sometimes pinkish abaxially, subglobose-urceolate, petals connate 3/4–4/5 length, 5–8 × 1–2 mm, surfaces sparingly hairy; stamens 10 in 2 whorls (5 + 5); filaments of outer whorl connate 1/2 length, of inner whorl connate 1/4 length, outer whorl 2–3 mm, whorl series 4–5 mm. Pistillate flowers resembling staminate, but sepals connate only at base, 3–3.5 × 1.5–2 mm; petals connate 1/2–3/4 length, 8–11 × 3–5 mm; carpels 2[–3]; styles connate 3/4 their lengths, 2–5 mm. Capsules compressed ellipsoidal, 1.2–1.5 × 2–2.5 cm, 2-lobed [ellipsoidal, 3-lobed], tardily dehiscent. Seeds solid brown, subspheric, 9–12 mm; caruncle absent. 2n = 22 (Mexico).
Phenology: Flowering and fruiting late spring(–summer).
Habitat: Sandy washes, sand dunes.
Elevation: 0–500 m.
Ariz., Mexico (Baja California, Baja California Sur, Sinaloa, Sonora).
In Arizona, Jatropha canescens is found only in Pima County, primarily in Organ Pipe Cactus National Monument.
Jatropha canescens is part of a hybrid complex that includes the Mexican species J. cinerea (Ortega) Müller Arg. and, probably, J. giffordiana Dehgan & G. L. Webster. These can be difficult to distinguish. R. McVaugh (1945) suggested considering J. canescens as a synonym of J. cinerea, and F. Shreve and I. L. Wiggins (1964), as well as others, have done so. Jatropha canescens may be distinguished from J. cinerea and J. giffordiana most reliably by its crowded inflorescences of staminate flowers with smaller subglobose (as opposed to urceolate) whitish gray corollas (sometimes with some red or pink on the adaxial surface) as opposed to larger darker red corollas of J. cinerea and J. giffordiana. In addition, J. canescens generally has more numerous, longer and darker colored short shoots, and smaller and less often shallowly 3-lobed leaves. Although J. cinerea (in the strict sense) does not occur in northern Sonora, it is one of the more common plants in Baja California, Baja California Sur, and western mainland Mexico.