1. Hydrangea L. (hydrangea) (McClintock, 1957)

About 25 species, North America to South America, Asia to Borneo.

Hydrangea occurs in temperate regions of eastern Asia and eastern North America, and extends southward into the tropics in both hemispheres. A group of species with deciduous leaves and shrubby habit has diversified in temperate eastern Asia and also includes the two species native to the United States (sect. Hydrangea). These species are considered relicts of the extensive Arcto-Tertiary forest that once extended continuously across the Northern Hemisphere. Another group of species with evergreen, leathery leaves and a climbing habit has diversified in subtropical montane regions of Central and South America (sect. Cornidia (Ruiz & Pav.) Engl.). The species in cultivation are mostly members of the deciduous group (McClintock, 1957). Recent molecular and morphological studies indicate that the species of Hydrangea do not form a monophyletic unit, and that some of the exotic species are more closely related to other genera of Hydrangeaceae (Soltis et al., 1995).

Many species and varieties of Hydrangea have been brought into cultivation. Some were cultivated in China and Japan long before their introduction into Europe. Among the most popular are cultivars in the hortensia group of H. macrophylla (Thunb.) Ser., which have large inflorescences in which all of the flowers have been replaced by showy white, pink, or blue sterile flowers and are sold under the names bigleaf hydrangea and blue snowball bush. Selections of H. arborescens with an inflorescence consisting of all sterile flowers are also popular. Many of these selections initially were given formal scientific names, leading to an abundance of nomenclature in the literature on the genus. Hydrangea is a pollination generalist and is visited by many different insects. The compound cyme serves as a stable platform for insects to move about. Bagging studies suggest that self-pollination is possible and probably common (Pilatowski, 1982).