(Hydrangea), any of a genus of erect or climbing woody shrubs, in the family Hydrangeaceae, native to the Western Hemisphere and eastern Asia. About 23 species are known. Several species are grown in greenhouses and gardens for their showy, usually ball-like flower clusters.

Hills-of-snow, or wild hydrangea (H. arboresceus), a shrub slightly more than 1 metre (4 feet) tall, has rounded clusters of white flowers. The French hydrangea, or hortensia (H. macrophylla), is widely cultivated in many varieties for its large globular flower clusters in colours of rose, lavender, blue, and, rarely, white. These cultivated varieties are the florist's hydrangeas, forced for holiday use. The flower colour is variable, depending on the acidity of the growing medium: rose-pink under neutral to low soil acidity and blue under conditions of stronger acidity. Blue flowers can be produced by the early addition of chemicals to increase the acidity of the growing medium.

Peegee hydrangea (H. paniculata ‘Grandiflora'), growing to a height of 9 m, is a common landscape hydrangea, with tapering flower clusters, opening white and fading to pink, then to bronze. Oakleaf hydrangea (H. quercifolia), up to 2 m high, has white flower clusters and deep wine-red fall foliage. The climbing hydrangea (H. anomala petiolaris, or H. petiolaris), can reach up to 15 m, clinging to any solid support by means of aerial rootlets.