Hydrangea (Hydrangea) is a genus of roughly 100 species of flowering plants native to southern and eastern Asia (from Japan to China, the Himalaya and Indonesia) and North and South America. Most are shrubs 1-3 m tall, but some are small trees, and others vines reaching up to 30 m by climbing up trees. They can be either deciduous or evergreen, though the widely cultivated temperate species are all deciduous.

Hydrangeas produce flowers from early spring to late autumn; these are carried in bunches, at the ends of the stems. Each individual hydrangea flower is relatively small; however, the display of color is enhanced by a ring of modified bracts around each flower.

In most species the flowers are white, but in some species, flowers can be blue, red, pink, or purple; in these species the exact color often depending upon the acidity or alkalinity of the soil. Acidic soils produce blue flowers; neutral soils produce very pale cream petals, and alkaline soils results in pink or purple. Hydrangeas are one of very few plants that accumulate aluminum. Aluminum is released from acidic soils, and forms complexes in the hydrangea flower giving them their blue color.

Cultivation and uses

Hydrangeas are popular ornamental plants, grown for their large flower heads, with H. macrophylla being by far the most widely grown with over 600 named cultivars. Some are best pruned on an annual basis when the new leaf buds begin to appear. If not pruned regularly, the bush will become very ‘leggy’, growing upwards until the weight of the stems is greater than their strength, at which point the stems will sag down to the ground and possibly break.

Other varieties only flower on ‘old wood’. Thus new wood resulting from pruning will not produce flowers the following season.

Gardeners can often control the flower color by adding lime or potash to alter the alkalinity level around the plant.