Plants stoloniferous; rhizomes proliferating above ground. Flowering stems 25-70 cm. Basal leaves: petiole 3-15 cm; blade reniform, 3-7-lobed to 1/2-3/4 its length, (3-)5-11 × (5-)8-14 cm, margins 2-3-times dentate. Cauline leaves: stipules (2-)4-9 mm, usually foliaceous, smaller ones fringed with brown hairs. Inflorescences ± pyramidal, (5-)9-13-flowered per primary branch. Pedicels densely stipitate-glandular. Flowers: hypanthium campanulate to turbinate, free portion 0.8-1.3 mm, densely stipitate-glandular; sepals erect, lanceolate, (1.5-)2-3 mm, apex acute; petals spatulate to ovate, 3-7 × 1-3 mm, base tapered to claw, margins plane; nectary greenish yellow; stamens 1.3-1.8 mm; ovary subinferior. Capsules ovoid, turbinate, or urceolate. Seeds usually black, tuberculate. 2n = 28.

Flowering summer. Temperate rainforest, margins of lakes, ponds, and watercourses; of conservation concern; 10-700 m; Oreg., Wash.

Specimens considered to represent Boykinia intermedia from northern Idaho (R. Bacigalupi 1952; F. D. Johnson and R. Steele 1978) were regarded as B. major by R. J. Gornall and B. A. Bohm (1985); they require further study. Boykinia intermedia differs from that species in its stoloniferous habit, smaller stipules, a campanulate rather than saucer-shaped hypanthium, a shorter free-hypanthium, petals tapered rather than contracted abruptly to a claw and with plane rather than undulate margins, a more pyramidal inflorescence, and an absence of polymethylated flavonols.