It's hard to imagine that cute little mophead as a big shrub. Gardeners who turn a blind eye to their young shrubs' projected mature height may plant them inappropriately. Bigleaf hydrangeas (Hydrangea macrophylla) do not stay pot-size; they quickly grow to the size of a small car, 6 feet in height and 10 in breadth in U.S. Department of Agriculture plant hardiness zones 6 though 9. Smaller cultivars exist, but the key is to know the mature size of the plant and situate it accordingly. If you don't, you'll need to prune the hydrangea every year for the rest of its life.
Prune just after the flowers fade in summer if your hydrangea blooms on old wood. These hydrangeas develop buds for the following year in August or September. Bigleaf and oakleaf (Hydrangea quercifolia, USDA zones 5 though 9) flower on old wood, and this group includes the popular mopheads and lacecaps.
Prune between summer and early spring if your shrub flowers on new wood. Panicle hydrangea (Hydrangea paniculata, USDA zones 4 through 7) and smooth hydrangea (Hydrangea arborescens, USDA zones 4 though 9) bud on current growth.
Wipe the blades of your clipper with a rag soaked in denatured alcohol before you begin pruning and between clips. Prune out dead stems and branches at their point of origin. Clip off crossing branches to eliminate rubbing wounds. Remove damaged stems and branches at least 6 inches into healthy wood.
Prune out several of the oldest, tallest hydrangea stems at ground level. Remove up to one-third of the total number of stems to reduce the size of the shrub. This type of pruning also rejuvenates the plant by making room for new, green shoots to take the place of the older stems removed.
Clip every stem to the ground in the autumn if your hydrangea blooms on new wood. This produces a shorter bush and bigger flowers the following spring. The severe pruning weakens the plant's support over time and can cause it to flop over under the weight of its flowers. So alternatively leave 18 to 24 inches of stem when you clip to give the plant a more solid foundation.