Annuals; puberulent or gla-brous (trichomes simple and subappressed, and 2-rayed). Stems erect to ascending (often flexuous, weak, often tangled with desert shrubs), usually branched distally, 1-8 dm, glabrous or puberulent. Basal leaves rosulate; petiole 0.3-2.5 cm; blade oblanceolate to spatulate, 0.7-6 cm × 2-27 mm, margins usually coarsely dentate or somewhat pinnatifid, rarely entire, (surfaces glabrous). Cauline leaves (median) sessile; blade lanceolate or oblong, 1.5-7.5 cm × 5-20 mm (smaller distally, base amplexicaul to sagittate), margins dentate or entire, (surfaces glabrous). Racemes (lax), without a terminal cluster of sterile flowers. Fruiting pedicels reflexed, 1-4.5 mm, usually glabrous, rarely puberulent. Flowers: sepals erect, (purplish or yellow-green), narrowly lanceolate, 3-6.5 × 0.8-1.5 mm (equal); petals yellow-green to purplish (often with purple veins), 4.5-9 mm, blade 2-3 × 0.7-1.5 mm, not crisped, claw narrowly oblong-oblanceolate, 2.5-7 × 1-1.5 mm; filaments slightly tetradynamous, median pairs 2-4.5 mm, lateral pair 1.5-3.5 mm; anthers oblong, equal, 1.5-2 mm. Fruits usually reflexed, rarely divaricate (often subfalcate), terete, 2-6 cm × 1.5-2.5 mm; valves each with prominent midvein, (glabrous or puberulent); ovules 24-48 per ovary; style 0.2-2.7 mm; stigma slightly 2-lobed. Seeds 1-2 × 1-1.2 mm. 2n = 28.

Flowering (Jan-)Feb-Mar. Desert shrubs, woodlands; 600-2300 m; Ariz., Calif., Nev., Utah; Mexico (Baja California).

Caulanthus cooperi is distributed in the Colorado and Mojave deserts in western Arizona, central and southern California, southern Nevada, and southern Utah.