Aerial shoots (7.5-)10-30 cm, from rhizomes, rhizomes horizontal. Basal leaves 0-2, ternate; petiole 10-15 cm; terminal leaflet sessile, ovate to rhombic, (2.5-)3-5(-6) × (2-)2.5-3.5 cm, base slightly oblique to broadly cuneate, margins crenate to serrate on distal 2/3, apex acuminate, surfaces abaxially glabrous, rarely puberulous on veins, adaxially usually glabrous; lateral leaflets unlobed, rarely 1×-lobed; ultimate lobes (8-)10-20 mm wide. Inflorescences 1-flowered; peduncle sparsely hispid or glabrous; involucral bracts 3, 1-tiered, simple, ±similar to terminal leaflets of basal leaves, ovate, rarely rhombic, (2.5-)4-8 cm, bases distinct, broadly cuneate, margins crenate to serrate, or dentate on distal 2/3, apex acuminate, surfaces usually glabrous; lateral leaves unlobed or occasionally 1×-lobed; ultimate lobes 8-15 mm wide. Flowers: sepals usually 5, white, ovate to obovate, rarely oblanceolate, (12-)15-25 × (8-)10-15(-20) mm, glabrous; stamens 100-120. Heads of achenes spheric; pedicel (5-)7-12 cm. Achenes: body ovoid, swollen, 2.5-4 × 2-3 mm, not winged, proximally hispid, distally glabrous; beak straight to subulate, 0.5 mm or less, glabrous. 2n=14.
Phenology: Flowering spring–summer (Apr–Jul).
Habitat: Open to deep woods
Elevation: 200-2000 m
Calif., Oreg., Wash.
Reports of Anemone deltoidea in British Columbia are erroneous (H.J. Scoggan 1978-1979, vol. 3).
Rhizomes 7m long and about 2 mm in diameter have been observed in Anemone deltoidea.