Hydrangeas

Hydrangeas, it is safe to say, are pruned in spring.  There are two ways to prune hydrangeas, depending on whether they bloom on old wood or new wood.  

Old wood bloomers

The old wood bloomers, such as the mop-head (H. macrophylla), and lace-cap simply get tidied up.  This involves cutting out the oldest stems right at the base, getting rid of weak, spindly growth and trimming the dried flower heads off.  There are a few more species of hydrangea that also only need a light spring pruning.

New wood bloomers

New wood bloomers including the cone-heads (H. paniculata - peegee) get a more extensive spring pruning to invigorate new growth.  This involves removing dead, diseased and crossing branches, so that the plant will have an open framework.  It also involves shortening last year's growth to within two buds of the old wood.

Hydrangea care in general

Besides pruning, hydrangeas mostly require a sunny, or partly sunny exposure.  The hydra in hydrangea means water, so they need plenty of it, but the soil must drain well.  Soil drainage is greatly improved by adding compost and/or sand.  Finally, there is a product available at garden stores that will gradually turn the flowers blue.