Japan Flora: Shrub; young branches angled, glabrous or sparingly short-pubescent; leaves membranous, ovate to nar­rowly so, 3-6 cm. long, 1.5-3.5 cm. wide, acute, acutely dupli­cate-toothed except on lower margin, thinly puberulent with appressed short hairs above while young, more or less pubes­cent beneath, the petioles 3-8 mm. long, slightly pubescent; inflorescence a loosely pubescent umbellate raceme, with short axis; pedicels 10-15 mm. long; flowers many, white, about 10 mm. across; calyx-teeth reflexed, subacute; petals orbicular, shorter than the stamens; fruiting carpels connivent, about 3 mm. long, pubescent, the styles suberect. May.

Hokkaido, Honshu (centr. distr. and northw.). Sakhalin, n. Korea, Manchuria, Amur, Ussuri, and Siberia. Europe to Siberia.

Korea Flora: Deciduous shrub, 1m tall. Winter buds ovoid, apex acute, with 6-8 basal scales. Branchlets distinctly ridged, glabrous. Leaf scars triangular or semicircular. Leaves alternate, ovate, acuminate apex, rounded base, glabrous on the upper surface; 4-7cm × 2-3cm. Lower surface whitish-gray, glabrous in vein axils. Margins doubly serrate, resembling leaves of Sorbus alnifolia . Petioles 4-6mm long. Flowers bisexual, in terminal racemose or umbellate inflorescences on new shoots; pedicels slender, glabrous, inflorescence width 3cm. Flowers 8-10mm in diameter; petals white; styles borne at the apex of the ovary, 2.5mm long. Fruits large, with a dorsal apical projection, 3mm long, densely pubescent, fusiform. Seeds 1.2-1.5mm long.

Chromosome number: 32.
Flowering: Late April to late May (some populations in Gangwon ).
Fruiting: July-September.
Distribution: Northeastern China, Xinjiang, southern Far East Russia, Hokkaido to Kyushu (Japan); South and Hamgyeongbukdo s, Gangwon , and Ulleungdo Island (Korea).

Taxonomic note: Taxa such as Spiraea flexuosa Fisch. ex Cambess and S. ussuriensis Pojark. are recognized as distinct species in Russian literature, but they remain controversial groups along with Physocarpus insularis . Leaf morphology shows significant variation in size and shape, and specimen examination suggests that S. flexuosa tends to have slightly smaller fruits. While var. chamaedryfolia differs in leaf morphology, it is considered part of a highly variable group widely distributed across the Northern Hemisphere. Rehder treated S. flexuosa (long-leaved ninebark) as a synonym of S. chamaedryfolia. Recent Japanese floras treat var. ulmofolia as a separate variety, using the name var. pilosa (Nakai) H. Hara, but these are likely synonymous (Ikeda, 2001). Physocarpus insularis (Nakai) Nakai is regarded as a synonym of S. chamaedryfolia (Kim et al., 2000).