6. Physocarpus alternans (M. E. Jones) J. T. Howell, Proc. Calif. Acad. Sci., ser. 4. 20: 130. 1931.
Nevada or dwarf nine-bark
Neillia monogyna (Torrey) Greene var. alternans M. E. Jones, Zoe 4: 42. 1893; Physocarpus alternans subsp. annulatus J. T. Howell; P. alternans subsp. panamintensis J. T. Howell
Shrubs, 15 dm, ˂densely branched˃. Stems orientation not recorded, ˂brown becoming gray˃, stellate-hairy, glabrescent. Leaves: stipules oblong to narrowly elliptic or rhombic, 2–4 × 1.5–2 mm; petiole 2–10 mm; blade broadly ovate to orbiculate, 0.5–2 × 0.5–2 cm, slightly longer than wide, base rounded to truncate or slightly cordate, (0)3- or 5(7)-lobed, margins crenate to doubly crenate, apex rounded, abaxial surface more densely stellate-hairy (especially on veins), adaxial sparsely so. Inflorescences 5–10(–15)-flowered, open, umbel-like racemes, 3 cm diam.; bracts similar to stipules. Pedicels 0.6–1.5 cm, densely stellate-hairy. Flowers 7 mm diam.; hypanthium cup-shaped to campanulate, 1.5–3 mm, stellate-hairy; sepals ˂pale green to white, darker in center˃, triangular to narrowly ovate, 2.5–3 mm, apex gland-tipped, surfaces stellate-hairy; petals white to pale pink, broadly obovate to orbiculate, 3 × 2.5 mm; stamens ˂20–30˃, unequal (alternately long and short), shorter than petals, ˂anthers pink to dark brown˃; carpels 1(2), if 2 then connate at least 1/2 their lengths, indumentum not recorded. Follicles 1(2), ˂pale buff˃, shape not recorded, 3–6 mm (lengths equal to or slightly exceeding sepals), densely stellate-hairy; styles 2.5 mm ˂sometimes distinctly bent˃. Seeds 1 or 2, ˂2 mm˃.
Flowering Jun–Jul; fruiting Aug. Dry rocky canyon slopes, among pinyon pine, juniper, and Cercocarpus woodlands; 1500–3100 m; Calif., Colo., Idaho, Nev., Utah, Wyo.
The two subspecies described from California by Howell appear to be local variants with relatively small differences in leaf and hypanthium indumentum that probably do not merit formal recognition. Being drought tolerant, Physocarpus alternans should be considered more often for planting in dry habitats.