When to prune hydrangeas depends entirely on the specific variety planted. These popular, woody shrubs produce flower buds in one of two distinct ways, a biological difference that dictates the appropriate timing for any major cut. Misunderstanding this growth mechanism can lead to the accidental removal of the next season’s blooms. Therefore, the first step for any gardener is to accurately identify their plant before pruning.
Identifying Your Hydrangea Type
Hydrangeas are categorized based on where they develop flower buds. The “Old Wood” group sets buds on the stems that grew the previous season. These buds are formed and present on the plant throughout the winter months. Common examples include the Bigleaf Hydrangea (Hydrangea macrophylla), known for its mophead or lacecap blooms, and the Oakleaf Hydrangea (Hydrangea quercifolia), recognizable by its distinctive lobed leaves and conical flower clusters.
The “New Wood” group develops flower buds on the growth produced during the current growing season, typically forming them in late spring before flowering in summer. This group is more forgiving, as winter damage or late pruning will not affect the summer bloom. The two most common New Wood types are the Smooth Hydrangea (Hydrangea arborescens), exemplified by the ‘Annabelle’ variety, and the Panicle Hydrangea (Hydrangea paniculata), which features large, cone-shaped flower heads.
Visual identification can be done by observing the foliage. H. macrophylla leaves are generally thick, shiny, and oval-shaped, while H. arborescens leaves are often heart-shaped with a matte finish. H. quercifolia is the easiest to identify due to its lobed leaves and peeling, cinnamon-colored bark.
Pruning Hydrangas That Bloom on Old Wood
Pruning Old Wood varieties at the wrong time removes the flower buds, resulting in a bloomless season. The appropriate time for major pruning is immediately after the flowers fade in the summer, typically from late June through late August. This timing allows the plant to recover and initiate the development of new stems that will host the next year’s flower buds before dormancy begins.
Structural pruning aims to thin the shrub and remove unproductive, older growth to encourage vigorous new stems. Remove no more than one-third of the total stems, focusing on the oldest, woodiest canes by cutting them cleanly down to the ground. This rejuvenation promotes better air circulation and sunlight penetration, strengthening the remaining stems and enhancing overall plant health.
Size-reducing cuts should be made carefully, slicing just above a set of healthy, outward-facing leaves or a dormant bud. Complete any structural pruning before the end of August, as the plant begins to set new flower buds shortly thereafter. Pruning after this deadline, especially in fall or winter, guarantees the removal of developing buds. Reblooming varieties of H. macrophylla produce flowers on both old and new wood, but they still benefit most from summer-only structural pruning.
Pruning Hydrangeas That Bloom on New Wood
New Wood hydrangeas (H. paniculata and H. arborescens) can be pruned during the late winter or very early spring. The plant should be completely dormant and new foliage should not have emerged yet. Since flower buds do not form until the current year’s growth begins, there is no risk of removing future blooms by pruning at this stage.
These varieties are cut back by approximately one-third to two-thirds of their total height to maintain a strong framework. For Panicle Hydrangeas, this hard pruning encourages robust stems capable of supporting the heavy flower heads. For Smooth Hydrangeas, pruning down to about 12 inches above the ground each year encourages the vigorous new growth that produces large blooms.
To shape a Panicle Hydrangea into a tree form, select a few strong, permanent stems and remove all other competing growth at the base. Allow these selected stems to grow vertically. Annual pruning is then used to shorten the lateral branches and maintain the desired canopy shape. This late-winter maintenance focuses the plant’s energy into the strongest stems, leading to better form and a more prolific display of flowers.
Seasonal Maintenance vs. Structural Pruning
Routine maintenance pruning can be performed almost any time of year without risking the next season’s flowering. This includes removing dead, diseased, or damaged wood (the “three D’s”), which is beneficial to the plant’s overall health. These cuts should be made back to healthy wood or to the ground, using a clean, angled cut just above an outward-facing bud.
Deadheading, the removal of spent or faded flower heads, is another form of maintenance. For New Wood varieties, deadheading can be done throughout the growing season to tidy the appearance, or the dried blooms can be left for winter interest. If faded blooms are left on Old Wood hydrangeas, remove them in early spring just before new growth starts. The old flower heads offer a slight degree of protection to the dormant flower buds during severe winter weather.
When deadheading Old Wood types in summer, the cut should be made just above the first set of large, healthy leaves beneath the spent bloom. This precise cut removes the flower while preserving the underlying stem, which contains the developing buds for the following year’s flowers. This cleanup differs from the more aggressive structural pruning, which must adhere to strict summer timing for Old Wood varieties.