Worldwide there are approximately 80 species of Hydrangea which are deciduous or evergreen climbers, shrubs or small trees often found in moist, montane forests. For example, Hydrangea arguta is endemic to the tropical wet forests of Hawaii. The native range of the genus extends from China, the Kuril Islands and tropical Asia through central and eastern North America to Central and South America. Whilst some species are not hardy, others will tolerate temperatures as low as -20 degrees Celsius. The name Hydrangea is derived from the Greek hydor meaning water and aggeion meaning vessel, an allusion to the cup-shaped fruit of the genus.
Hydrangeas are mainly cultivated for their flowers, although many species have attractive foliage, whilst others have flaky or peeling bark, such as in Hydrangea quercifolia. Many Hydrangea species are dioecious, that is they have separate male and female plants. Asian hydrangeas tend to be most widely cultivated in European gardens, including Hydrangea macrophylla and Hydrangea aspera. Hydrangea macrophylla, introduced into eighteenth-century Europe from Japan, is very widely cultivated around the world. Moreover, there are now many cultivars available, some of which are hybrids with Hydrangea serrata.
Hydrangea macrophylla is split into two groups, mopheads (also called hortensias) and lacecaps. In the former group, shrubs up to two metres tall have rounded flowerheads that range in colour from white to pink through to blue. The lacecaps have large, flat, slightly scented flowerheads. Flower colour is often associated with soil type. If grown below pH 5.5 the flowers produced will be blue, whilst above pH 7, flowers tend to be pink. Aluminium sulphate added to pot-grown specimens, or to plants growing in near neutral pH soils, tend to produce blue flowers. Forcing, the practice of bringing plants into flower before their usual flowering season, is often done for pot-grown plants. In Europe, this practise has been used with Hydrangea macrophylla since the nineteenth century to produce large flowerheads on small plants early in the season.
Climbing hydrangeas, such as the South Korean and Japanese Hydrangea petiolaris and the northern Mexican Hydrangea seemannii must not be forgotten. Most of these species are subtropical and grow in cloud forests, a few are found in tropical rainforests. Climbing hydrangeas use rocks and surrounding vegetation as supports for their adventitious roots. In the garden, climbing species are used to cover walls in moist shady areas, but are not as hardy as some hydrangeas.
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