Mophead hydrangeas flower on old wood, and should only be pruned in midsummer.

Overpruning Hydrangeas

by Michelle Z. Donahue

Most people come for the flowers: white, pink, blue and a spectrum of purple in between. But whether you’ve fallen in love with a mophead, panicle, smooth, climbing, lacecap or oakleaf hydrangea (Hydrangea spp.), chances are good you’ve wondered how, when and how much to prune your flowering shrub to maximize the show each year. Strictly speaking, most types of hydrangeas don’t need an annual pruning, and while prudent cuts can help the bush look better, aggressive trimming can eliminate potential flowers or damage the plant.

1. About Hydrangea Pruning

Much of the confusion regarding when and how much to prune a hydrangea comes from when flower buds form. Cutting a plant back too early or too late eliminates buds, and knowing if your hydrangea flowers on old or new wood will determine when and how much to prune, if at all. Unless a hydrangea has become leggy or is dense with old stems that inhibit airflow through the plant, pruning is usually unnecessary. Prune only to renew your plant or to shape it.

2. Pruning Old Wood Hydrangeas

Oakleaf hydrangea (Hydrangea quercifolia), a native species that grows in U.S. Department of Agriculture plant hardiness zones 5 through 9, and bigleaf hydrangea (Hydrangea macrophylla), which grows in USDA zones 6 through 9, mostly flower on old wood. This means they produce the current year's flowers on last year's stems. In general, these two types of hydrangeas should be pruned right after flowering but no later than early August, when the following year’s buds begin to form. Bigleaf hydrangea can be pruned every five years or so, while oakleaf is slower growing and should be pruned less often.

3. Pruning New Wood Hydrangeas

New wood bloomers are easier to prune without much danger of reducing the flower display, as they produce flowers on the current year's growth. Smooth hydrangea (Hydrangea arborescens), which is hardy from USDA zones 4 through 9, and panicle hydrangea (Hydrangea paniculata), which grows in USDA zones 3 through 8, both flower on new wood. Prune them at any time in winter, in spring after buds begin to break dormancy, or immediately after flowering.

4. Flower Impacts

Chopping off last year’s healthy stems by overpruning in spring will reduce or wipe out this year’s flower display on oakleaf and bigleaf hydrangeas, though some cultivars may rebloom from trims made to old wood. When deciding which pruning cuts to make, aim to cut back about one-third of the stems, focusing primarily on the oldest wood in the center of the plant, spindly new growth, and dead or damaged wood. Experiment with a light touch to see how your plant responds. New wood bloomers should flower well even if trimmed in the spring, though overpruning may reduce bloom density.

5. Other Effects

Overpruning hydrangeas can have other consequences. Cutting back stems and foliage at any point during the year encourages the plant to produce new growth, which makes the bush look lusher and healthier during the season. Hydrangeas pruned too late in the year may produce a flush of tender, new growth that risks being killed by a freeze. This new growth also draws away resources the shrub needs to prepare for winter dormancy, and may compromise a hydrangea’s winter hardiness. Aggressive pruning may also make a hydrangea more susceptible to disease, as it will have more exposed tissue.

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