Fl. N.W. Amer., 185. 1898.
Plants weakly rhizomatous, 7–15 dm. Rhizomes horizontal, stout, 10–15 mm wide, internodes 1–4 cm; root-tubers absent. Stems finely puberulent distally. Leaves: basal 1 or 2, deciduous by flowering; stipules round, 1–1.5 cm diam., base not auriculate; lateral leaflets in 1–3 pairs or lacking, remote, ovate to elliptic, to 1 cm, margins serrate; terminal leaflets round, 6–16 cm diam., palmately 5–7-lobed, lobes ovate to oblong-lanceolate, margins doubly serrate, apex acute to acuminate, surfaces short-appressed hairy at least on veins. Inflorescences 100+-flowered; branches and pedicels densely short-appressed hairy, hairs straight. Flowers: hypanthium nearly flat, saucer-shaped; sepals (4–) 5 (–6), green, narrowly triangular, 4–6 mm, margins often serrate, usually with midrib, abaxially puberulent, adaxially glabrous; petals (4–) 5 (–6), white, in buds sometimes marginally pink, oblanceolate, 10–15 mm, not clawed, base broad, margins entire; stamens white, about equal to petals. Achenes 7–12, flattened, lanceolate, straight, 6–7.5 mm, sutures densely ciliate, faces sparsely hairy; stipes 1–2 mm; styles 1.5–2 mm.
Phenology: Flowering summer (Jun–Aug).
Habitat: Wet, mossy rock along forest streams, at or slightly above water level, wet rock on mountain slopes, riverbanks, rocky summits
Elevation: 30–1000 m
Of conservation concern.
Filipendula occidentalis is known only from the valleys of several small rivers in the coast ranges of northwestern Oregon and southwestern Washington. The species is threatened by logging and associated forest-management activities. The species is most probably related to F. camtschatica (Pallas) Maximowicz from the Pacific coast of Asia.