Hydrangeas can be counted on to provide garden blooms over a long summer and fall season. The most commonly-owned variety is Hydrangea macrophylla, or mopheads, and their frilly cousins, lace-cap hydrangeas. Their large colorful blooms and easy care make them ideal for almost every gardener. Pruning is simple and only needed to keep these enthusiastic bloomers within bounds.
Prune mopheads only when they overreach their space or become so old that they produce very few blooms. Deadheading old blooms and removing dead wood should be done annually, as part of spring cleanup, but pruning may be needed only every few years.
Remember that mopheads produce flower buds on old wood, that is, previous years' growth. In temperate climates (essentially zones 3-6), buds are formed in August. Schedule pruning for June or early July, giving plants a chance to establish their size for the year but avoiding damage to summer blooms.
Cut back plants to the desired size, trimming from the outside. If plants are severely overgrown, remove a third of the largest stems down to the ground, then trim the remaining stems. Removing stems will diminish blooms somewhat for the season but may be the only way to effect a substantial reduction in plant size.