Subshrubs, shrubs, or small trees, erect or climbing, deciduous or evergreen. Branchlets, leaves, and inflorescences variously hairy. Leaves opposite, rarely verticillate, exstipulate; leaf blade simple, margin entire, serrate, or rarely pinnately lobed. Inflorescence terminal, occasionally axillary, a corymbose cyme, umbellate cyme, or thyrse; bracts deciduous. Flowers fertile or sterile. Sterile flowers few (more numerous in cultivated varieties), sometimes absent, borne at margin of inflorescence, with sepals 2 or 5, petaloid, and enlarged. Fertile flowers usually very numerous, bisexual, small; pedicel short. Calyx tube adnate to ovary, 4- or 5-dentate, persistent. Corolla lobes 4 or 5, free, rarely connate and forming a calyptra, ovate or spatulate, valvate. Stamens (8 or)10(or 25), inserted on disc; filaments linear; anthers oblong to subglobose, apex subrounded to obtuse. Ovary inferior to 2/3 superior, imperfectly or perfectly 2-4(or 5)-loculed; placentation parietal or axile; ovules numerous. Styles 2-4(or 5), free or basally connate, persistent; stigmas terminal or decurrent along style adaxially. Fruit a capsule, dehiscing apically among styles, hemispheric to turbinate, small, apex projected or truncate. Seeds numerous, small, winged or wingless; seed coat thin, with reticulate or striate veins.

The Japanese species Hydrangea macrophylla (Thunberg) Seringe is cultivated as an ornamental. Records of H. macrophylla var. macrophylla from China (e.g., in FRPS) are probably all from cultivated plants. One of the present authors (Bartholomew) believes that records of H. macrophylla var. normalis E. H. Wilson from China (e.g., in FRPS) are probably referable to related taxa such as H. caudatifolia, H. chungii, or H. stylosa. However, Wei believes that wild-growing H. macrophylla occurs in Guangdong, Guizhou, and Sichuan, with var. normalis in Zhejiang.

One of the present authors (Bartholomew) feels that the three species complexes in Hydrangea would be better treated as the widespread, variable species H. chinensis (to include H. davidii, H. linkweiensis, and H. mangshanensis), H. heteromalla (to include H. bretschneideri, H. dumicola, H. hypoglauca, H. macrocarpa, H. sungpanensis, and H. xanthoneura), and H. aspera (to include H. coacta, H. kawakamii, H. longipes var. fulvescens, and H. longipes var. lanceolata). Bartholomew believes that, with the recognition of these three species in the broader sense, as well as a number of other reductions (H. coenobialis to include H. candida and H. gracilis, H. chungii to include H. caudatifolia, and H. robusta to include H. longipes var. longipes), there are probably about 18 recognizable species of Hydrangea (nine endemic) in China; Wu Zhengyi (editor's note) agrees.

About 73 species: mainly in E Asia, a few species in SE Asia and the New World; 33 species (25 endemic) in China.

(Authors: Wei Zhaofen (Wei Chao-fen); Bruce Bartholomew)