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Hydrangeas are a genus of roughly 70-75 species of flowering plants that are native to southern and eastern Asia (from Japan to China, the Himalaya and Indonesia) and they are also native to North and South America. By a long shot, the greatest species diversity is found in eastern Asia, notably China and Japan. Most are shrubs around 1-3 m tall, but some are actually small trees, and others lianas reaching up to an amazing 30 m by climbing up trees. I bet you never saw those in your grandmothers backyard! Hydgrangeas can be either deciduous or evergreen, though the widely cultivated temperate species you find most often are all deciduous.
The flowers are produced from early spring to late autumn and they grow in flowerheads (corymbs or panicles) at the ends of the plant stems. In many species, the flowerheads actually contain two types of flowers, one lot of small fertile flowers in the middle of the flowerhead, and also large sterile bract-like flowers that form a ring around the edge of each flowerhead. Still, other species have all the flowers the same size and they are all fertile.
What makes them so appealing is obviously their long-blooming flowers which can often continue for weeks and even months. Another bonus with hydrangeas is that most types come into bloom in mid-summer - a time when after most other shrubs have finished their show. The reason for this is that they don't have petals. Their "flowers" are colorful showy sepals that are sterile. The flower heads of the big-leaved mophead or hortensia types (H. macrophylla) actually consist mostly of showy sepals with just a few small fertile flowers within. The sepals can not be pollinated and so they don't then go to seed - and that is what makes the shrubs' flower show go on for a long time.
Generally with most species the flowers are white, but in some (notably H. macrophylla), they can be blue, red, pink, or purple. This has nothing to do with the flower itself, but rather depends on the pH of the soil. If you ahve acidic soils it will produce blue flowers, whilst neutral soils produce very pale cream petals, and pink and purple flowers are resultant from alkaline soils . They are also one of very few plants that actually accumulate aluminium. This is released from the acidic soils and can form complexes in the hydrangea flower that give them their blue colour.
They are very popular ornamental plants and they are grown for their large flowerheads. Hydrangea macrophylla is by far the most widely grown vairety with more than 600 named cultivars. Many of these cultivars have been selected to have only large sterile flowers in the flowerheads. They tend to be best pruned on an annual basis just about when the new leaf buds begin to appear. If not pruned regularly, then the the bush will become very 'leggy' and they will grow vertically until the weight of the stems is greater than their strength and the stems will then sag down to the ground and most liekly break. Still, some species only flower on 'old wood'. In these occaisions, new wood resulting from pruning will not produce flowers for you the following season.
Did you know that hydrangeas are moderately toxic if eaten as all parts of the plant actually contain cyanogenetic glycosides? | 
