Hydrangea arborescens - Smooth Hydrangea
Hydrangea cinerea - Ashy Hydrangea
Hydrangea macrophylla - French Hydrangea
Hydrangea paniculata - Panicled Hydrangea
Hydrangea quercifolia - Oakleaf Hydrangea
Hydrangea radiata - Silverleaf Hydrangea

Hydrangeas (scientific name: Hydrangea) are relatively large and soft-stemmed "leafy" deciduous shrubs.

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.

The flowers can be blue, red, pink, purple, or white, depending upon the acidity or alkalinity of the soil, as well as the type of Hydrangea. Gardeners can control the display by adding lime or potash to alter the alkalinity level around the plant. Acid soils produce blue flowers, neutral soils produce very pale cream petals, and alkalinity creates pink or purple. Hydrangeas are one of very few plants that accumulate aluminium. Aluminium is released from acidic soils, and forms complexes in the hydrangea flower giving them their blue color.

Hydrangeas are deciduous. Some varieties should be 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 blooms the following season.

Some popular Hydrangea cultivars include:

• 'Blue Bonnet'
• 'Blue Wave'
• 'Bluebird'
• 'Forever Pink'
• 'Lilicana'
• 'Nikko Blue'
• 'Veitchii'

A Hydrangea painted by the botanical artist Redouté