Hydrangeas (Hydrangea spp.) provide shrubby backgrounds of lacy blooms all summer long. Newer hybrids and cultivars have added purple, green and bicolor blooms to the old standby pink, blue and white varieties. Not all hydrangeas need pruning, but many benefit from some annual TLC. Prune your shrubs according to whether they bloom on old or new wood and clean pruning shears with rubbing alcohol or hand sanitizer between shrubs to avoid spreading harmful viruses or bacteria.
New Wood Bloomers
Smooth hydrangeas (Hydrangea arborescens), including "Annabelle" and "Incrediball," bloom midsummer on wood grown in the current year, as do panicle hydrangeas (Hydrangea paniculata), including "Pinky Winky," "Quick Fire" and "Limelight." These shrubs are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 8 or 9.
Prune them down to the ground in early winter where bitterly cold winters would kill back the shrubs. Severe pruning encourages bloom, but may also result in weaker stems.
In milder climates or when larger shrubs are desired, prune smooth and panicle hydrangeas back to maintain shape and remove dead wood. These shrubs can be pruned for shape and health anytime, but spring pruning will destroy the current year's bloom, so ideally finish pruning before growth begins in early spring. Panicle hydrangeas may also be trained as trees by removing suckers whenever they appear.
Old Wood Bloomers
Colorful mophead and lacecap hydrangeas (Hydrangea macrophylla), including "Nikko Blue" and "Hortensia," hardy in USDA zones 6 through 9, and oakleaf hydrangeas (Hydrangea quercifolia) like "Alice" or "Snow Queen," hardy in USDA zone 5 through 9, bloom in early summer on wood grown the previous year. Both macrophylla types are also known as bigleaf hydrangeas. Serrata (Hydrangea serrata, USDA zones 6 to 9) hydrangeas, once listed as a variety of mophead, bloom like mopheads but grow half their size.
Prune these shrubs as soon as blooms begin to age so they have some summer growing time to recover. If shrubs die back during harsh winters, clean out dead branches in spring. Any other pruning from late fall forward will limit bloom. Since shrubs depend on old wood for blooms, shape or reduce shrubs by taking no more than one-third of the length of the oldest branches each year.
Re-blooming mopheads, including the "Endless Summer" series, grow on both old and new wood, so the less pruning you can do, the better. Wait to take dead wood out in spring until growth begins and deadhead flowers back to the first or second set of side buds. If shrubs become overgrown, prune shrubs back one-third or down to a few inches after their second bloom in late summer or in early spring. In the latter case, a season of bloom will be lost, but the shrub should come back strong the following year.
Climbing hydrangea (Hydrangea anomala), a repeat bloomer hardy in USDA zones 5 through 8, needs little pruning except to remove stray or winter-killed branches.
Hydrangeas in Mild Climates
If your hydrangeas grow in USDA zone 8 or warmer, you enjoy a long season of bloom. Where winters are mild with little frost, hydrangeas may actively bloom well into August or September. Prune smooth and panicle hydrangeas back two-thirds of the way to the ground -- they'll need that head start in your mild climate. Deadhead old wood bloomers back to a set of breaking buds when flowers start to fade anytime from September to January.