Madroño 2: 19. 1931.
Herbs, perennial, with thickened, woody taproot. Stems prostrate to ascending, often mat-forming, 10–45 cm, pilose to villous. Leaves opposite; stipules distinct or connate, subulate to filiform, 0.4–1.1 mm, densely tomentose; petiole 0.4–1 mm, pilose to villous; blade suborbiculate to oblong-ovate, 3–8 × 2–6 mm, base slightly asymmetric, rounded, margins entire, apex rounded to obtuse, surfaces pilose to villous; 3-veined at base, midvein conspicuous, venation often obscured by pubescence. Cyathia solitary at distal nodes; peduncle 0.5–1.8 mm. Involucre obconic-campanulate, 1.2–2.3 × 1–1.8 mm, densely pilose to villous; glands 4, yellow to red, subcircular to oblong, 0.2–0.5 × 0.4–0.9 mm; appendages white, flabellate to oblong, 0.1–0.7 × 0.5–1.9 mm, distal margin entire or crenulate, adaxial surface ciliate-puberulent. Staminate flowers 15–22(–50). Pistillate flowers: ovary densely pilose; styles 0.4–0.8 mm, 2-fid 1/2 length. Capsules ovoid, 1.5–2.2 × 1.8–2.2 mm, tomentose; columella 1.2–1.6 mm. Seeds white, gray, or light brown, ovoid, sharply 4-angled in cross section, abaxial faces slightly convex, adaxial faces concave, 1.2–1.7 × 0.6–0.9 mm, smooth.
Phenology: Flowering and fruiting late spring–fall.
Habitat: Roadsides, desert scrub, streamsides, sandy washes.
Elevation: 700–2000 m.
The specific epithet of Euphorbia vallis-mortae is a misnomer because the species does not occur in Death Valley; instead, it is found at the transition of the northern edge of the Mojave Desert and the foothills of the southern Sierra Nevada in Inyo, Kern, and San Bernardino counties.