Genus of about 80 species of deciduous and evergreen shrubs, trees and climbers from woodland in E. Asia and North and South America. Hydrangea has large, showy flowerheads, and attractive foliage. The flat, domed, or conical, terminal flowerheads consisat of corymbs or panicles of both tiny fertile flowers and larger sterile flowers with showy, petal-like sepals. Flower colour is affected by the relative availability of aluminium ions in the soil. Acidic soils with a pH of less than 5.5 produce blue flowers; Soils with pH greater than 5.5 produce pink flowers. White flowers are not affected by pH.