Hydrangeas are a very easy care shrub, especially for us here on the west coast, but good cultivation will bring out their best.
First of all, like most shrubs, they should be planted in fertile (organically rich), well drained soil, but with the additional caution that you must be able to water them easily! Hydrangeas need hydrating!! This is particularly true in sunny sites and in the first while after planting. Never forget that it takes a while for roots to get out into the surrounding soil, so as far as the new plant is concerned, it is still in a pot, and so should be watered thoroughly AT LEAST once a week.
After the plant is established, it will still need watering in times of drought, and will be one of the first plants in your garden to droop by way of letting you know things are getting dry.
Macrophylla hydrangeas (mobcap/mophead & lacecap) produce their blooms from buds formed the summer before (old wood). Therefore, pruning in autumn or too early in spring can inhibit, even prevent, flowers. When the last frosts are over, the old flower heads should be cut back to the first pair of healthy buds below them. Any dead, or spindly branches should be removed at the base, and where there are untidy crossover branches choose the healthiest of these and remove the other. It should be noted that some of the newer cultivars have been bred to bloom on both old & new wood: but the advice of pruning for strength still applies.
Paniculata, on the other hand, blooms on new wood formed in the growing season and is also hardier than macrophylla. It can therefore be pruned in late winter to early spring as needed. An extremely vigorous plant, the shrub form is going to produce much larger flowers if restricted to 7 to 11 vigorous primary shoots. If grown as a standard, the plant will need rigorous pruning to keep it in good shape, but the same general guide of fewer, stronger shoots applies.
For both types, a good mulch in the spring and fall will help keep weeds down and preserve soil moisture. Feeding once a year with a slow release all purpose fertilizer will promote vigorous growth which will in turn provide for more abundant flowers.
How to Change The Colour of Your Hydrangea
Most blue or pink Hydrangea macrophylla varieties can change their flower colour based on soil acidity and the presence of aluminum in the soil. If the soil is acidic (a pH of less than 7), the plant is able to absorb aluminum from the soil and turn the flowers more blue. Common soil acidifiers include sulfur, peat moss and various fertilizer products like Rhodo and Azalea fertilizers. The colour transformation happens slowly of the course of 1-2 seasons. If you want your flowers to be more pink, sweeten your soil, that is - make it less acidic - more alkaline, with lime, bonemeal and other similar products. A pH higher than 7 reduces the plants ability to absorb aluminum and turns the flowers more pink. Again this change happens over several seasons.