1. Physocarpus opulifolius (Linnaeus) Maximowicz, Trudy Imp. S.-Petersburgsk. Bot. Sada. 6: 220. 1879.
[(as opulifolia), name conserved]
Common or Atlantic nine-bark, physocarpe à feuilles d'obier
Spiraea opulifolia Linnaeus, Sp. Pl. 1: 489. 1753; Opulaster alabamensis Rydberg; O. australis Rydberg; O. opulifolius (Linnaeus) Kuntze; O. stellatus Rydberg ex Small
Shrubs, to 30 dm. Stems spreading to ascending, ± glabrous. Leaves: stipules narrowly ovate, 6–10 × 1.5–2.5 mm, ˂apex acute˃; petiole 1–3 cm; blade ovate to obovate, 6–8.5(–10) × 4–7(–10) cm, usually longer than wide, base broadly cuneate to truncate, 3(–5)-lobed, margins irregularly serrate, apex obtuse to rounded, surfaces mostly glabrous, sometimes sparsely hairy abaxially. Inflorescences 30–50-flowered, open, hemispheric racemes, 5 cm diam.; bracts elliptic to spatulate or rhombic, 5 × 2.5 mm, apex acute to 3-fid or coarsely erose, ˂faces glandular˃. Pedicels 1–2 cm, usually stellate-hairy. Flowers 7–10 mm diam.; hypanthium cup-shaped, 1.5–2 mm, glabrous or sparsely stellate-hairy; sepals ˂pale green to white, darker in center˃, triangular, 1.5–2.5 mm, apex ˂mucronate˃, gland-tipped, surfaces usually stellate-hairy; petals white to pale pink, broadly elliptic to orbiculate, 4–5 × 4–5 mm; stamens equal to or exceeding petals, ˂anthers purplish˃; carpels 3–5, connate basally, sparsely stellate-hairy, glabrescent. Follicles 3–5, ˂connate basally, bright red to brownish red˃, ovoid, 5–10 mm (lengths ca. 2 times sepals), sparsely stellate-hairy, ± glabrescent; styles 4 mm. Seeds 2(–5), ˂pyriform, 2 mm˃. 2n = 18.
Flowering May–Jun; fruiting Jul–Sep. Rocky stream banks, lake shores, moist woods, swampy ground; 0–1300 m; N.B., N.S., Ont., Que.; Ala., Ariz., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., Maine, Md., Mass., Mich., Minn., Miss., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Pa., R.I., S.C., S.Dak., Tenn., Vt., Va., W.Va., Wis.; introduced in Europe.
Physocarpus opulifolius is the most widespread species in the wild and is commonly cultivated in North America and Europe; it occasionally escapes from cultivation and has become established in parts of Europe.