4. Physocarpus malvaceus (Greene) Kuntze, Revis. Gen. Pl. 1: 219. 1891.
Mallow-leaved or mallow nine-bark
Neillia malvacea Greene, Pittonia 2: 30. 1889; Opulaster pauciflorus (Torrey & A. Gray) A. Heller; Physocarpus pauciflorus (Torrey & A. Gray) Piper
Shrubs, 20 dm. Stems spreading, ˂brown becoming grayish black˃, glabrous or finely stellate-hairy. Leaves: stipules oblong to elliptic or obovate, 6 × 2.5 mm, ˂base broad, apex rounded and erose to apiculate or acute˃; petiole 1–2.5(–3) cm; blade broadly ovate to orbiculate, 2–6 × 2.5–6 cm, sometimes wider than long, base truncate to slightly cordate, 3- or 5-lobed, ˂clefts between lobes may be shallow to deep˃, margins doubly crenate to doubly serrate, apex rounded, obtuse, or acute, surfaces glabrous or sparsely stellate-hairy (more so on abaxial veins). Inflorescences 15–20-flowered, fairly dense, hemispheric racemes, 2.5–4 cm diam.; bracts narrowly obovate to spatulate, 4.5 mm, apex erose or acute. Pedicels 8–15 mm, densely stellate-hairy. Flowers 5–8 mm diam.; hypanthium campanulate, 1.5–2 mm, densely stellate-hairy; sepals triangular to ovate, 2–3 mm, apex gland-tipped, surfaces densely stellate-hairy; petals white, broadly elliptic to obovate or orbiculate, 4.5 × 4.5 mm; stamens ˂ca. 30˃, equal to or slightly exceeding petals; carpels 2(3), connate at least 1/2 their lengths, densely stellate-hairy. Follicles 2(3), ovoid, flattened, 2.5 mm (lengths not exceeding sepals), ˂keeled apically˃, densely stellate-hairy, ˂hairs white˃; styles 2.5–3 mm. Seeds 1 or 2, ˂pyriform, 1.5 mm˃.
Flowering May–Jul; fruiting Aug. Rocky canyon slopes and cliffs, open grassy slopes, dry open forests among Pinus, Juniperus, Pseudotsuga, Picea, and Populus; 500–2800 m; Alta., B.C.; Colo., Idaho, Mont., Nev., Oreg., Utah, Wash., Wyo.
Physocarpus malvaceus deserves to be more widely cultivated.