Description from Flora of China
Spiraea japonica Linnaeus f. var. formosana (Hayata) Masamune.
Shrubs. Branchlets yellow-brown to dark brown, terete, densely pubescent when young, later glabrescent; buds ovoid, ca. 5 mm, with several scales, apex acute. Petiole 2–3 mm, pubescent; leaf blade abaxially pale, oblong to oblong-ovate, 4–6(–8) × 2–2.5 cm, abaxially pubescent especially on veins, adaxially glabrous, base broadly cuneate, margin doubly serrate, apex acute. Corymbs terminal on erect, long, leafy shoots of current year, compound, 6–10 × 8–12 cm, many flowered; rachis and pedicels pubescent; pedicels 5–7 mm; bracts linear to subulate, pubescent. Hypanthium broadly campanulate, pubescent abaxially. Sepals triangular, 1.5–2 mm, reflexed in fruit. Petals orbicular, 2–3 mm in diam., apex obtuse or slightly emarginate. Stamens ca. 20, longer than petals. Disk annular, 10-lobed; lobes subobtuse at apex, rarely emarginate. Carpels adaxially pubescent; styles shorter than stamens. Follicles slightly spreading, subglabrous or puberulous on adaxial suture; styles persistent, spreading. Fl. May–Jun, fr. Aug–Sep.
Forests, slopes; 2100--3000 m. Taiwan.