Bot. Surv. Olympic Penins., 168. 1936,.
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.
Phenology: Flowering summer.
Habitat: Temperate rainforest, margins of lakes, ponds, and watercourses
Elevation: 10-700 m
Of conservation concern.
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.