3. Physocarpus capitatus (Pursh) Kuntze, Revis. Gen. Pl. 1: 219. 1891.
Spiraea capitata Pursh, Fl. Amer. Sept. 1: 342. 1813; Physocarpus opulifolius (Linnaeus) Maximowicz var. tomentellus (Seringe) B. Boivin
Shrubs, to 45(–60) dm. Stems erect, sometimes suckering, ˂angled˃, glabrous or finely stellate-hairy. Leaves: stipules linear to narrowly elliptic, 4 × 0.5–2 mm; petiole 1–2(–3) cm; blade broadly ovate to obovate, (3–)4–8 cm, usually as wide as long, base rounded to truncate or slightly cordate, 3- or 5-lobed, margins irregularly crenate to doubly serrate, apex obtuse to acute, abaxial surface ˂paler˃, glabrous or more densely stellate-hairy, adaxial glabrous or sparsely stellate-hairy. Inflorescences 30–50-flowered, dense, hemispheric racemes, 3 cm diam., ˂sometimes compound with some proximal pedicels becoming secondary peduncles˃; bracts narrowly elliptic to spatulate, 4 × 2 mm, apex acute or erose-dentate, ˂faces glandular˃. Pedicels 1–1.5 cm, densely stellate-hairy. Flowers 5–8 mm diam.; hypanthium cup-shaped, 2 mm, densely stellate-hairy; sepals ˂pale green to white, darker in center˃, triangular, 2–3 mm, apex gland-tipped, surfaces densely stellate-hairy; petals white, broadly elliptic to orbiculate, 3–4 × 3–4 mm; stamens equal to or exceeding petals; carpels 3–5, connate basally, mostly glabrous, sometimes hairy (on ventral suture). Follicles 3–5, ˂connate basally, shiny brown˃, ovoid, 5–7 mm (lengths slightly exceeding sepals), glabrous; styles 2.5–3 mm. Seeds 2(–5), ˂pyriform, 2.3–2.8 mm˃. 2n = 18.
Flowering May–Jul; fruiting Jul–Aug. Open sunny slopes on clay (higher elevations), stream and swamp banks, lake margins in moist woods (lower elevations); 20–1300 m; B.C.; Alaska, Calif., Idaho, Oreg., Wash.
Physocarpus capitatus is commonly cultivated.