The success of achieving a lush display of flowers on a Hydrangea shrub depends on understanding whether the plant forms its flower buds on “old wood” or “new wood.” This distinction dictates the proper timing and technique for pruning, which is the most common reason for a lack of blooms in many gardens. Old wood refers to the stems that grew during the previous season, while new wood is the fresh growth emerging in the current spring. Understanding this categorization ensures a vibrant flowering season.
Decoding the Bloom Habit
The term “old wood” describes stems that have overwintered, meaning the plant developed its flower buds during the late summer or fall of the previous year. These buds remain dormant throughout winter and open early in the summer season. Examples include the Bigleaf Hydrangea (Hydrangea macrophylla) and the Oakleaf Hydrangea (Hydrangea quercifolia).
“New wood” refers to the soft, fresh stems that emerge during the current spring. Hydrangeas that bloom on new wood delay bud development until spring, with flowers opening later in the summer on that season’s growth. This group includes the Panicle Hydrangea (Hydrangea paniculata) and the Smooth Hydrangea (Hydrangea arborescens).
A third category, often called remontant, flowers on both old and new wood. These varieties offer greater flexibility and reliability. They produce an initial flush of flowers from older stems and continue to bloom later in the season on newer growth. This dual capacity provides a longer flowering period and protects against flower loss from late-spring frosts or accidental early pruning.
Identifying Your Hydrangea Variety
Correctly identifying the Hydrangea species is necessary before pruning, especially if the original plant tag is lost. Visual cues from the leaves, bark, and flower shape help with accurate diagnosis. Bigleaf (H. macrophylla) and Smooth (H. arborescens) varieties typically have large, oval, or slightly heart-shaped leaves, though Bigleaf often displays a thicker, shinier texture.
The Oakleaf Hydrangea (H. quercifolia) is easily distinguished by its deeply lobed foliage, resembling oak leaves. Its older stems often develop attractive, peeling bark. Panicle Hydrangeas (H. paniculata) have narrower, more pointed leaves and distinctive cone-shaped flower clusters.
Flower shape is another telling characteristic. Bigleaf varieties produce globe-shaped Mophead flowers or flatter Lacecap clusters. Smooth Hydrangeas are known for their massive, round, snowball-like clusters, while Panicle varieties bear their flowers in elongated, pyramidal shapes. Observing these physical markers allows the gardener to determine the proper bloom habit and apply the correct pruning strategy.
Pruning Strategies Based on Wood Type
Pruning hydrangeas that bloom on old wood, such as H. macrophylla and H. quercifolia, requires careful timing to avoid removing set flower buds. These shrubs should only be pruned immediately after the flowers fade in early to mid-summer. This timing allows the plant sufficient time to develop new buds for the following year before the onset of winter dormancy. Pruning involves selectively removing dead, weak, or crossing stems down to the ground, or occasionally cutting back up to one-third of the oldest canes for rejuvenation.
Hydrangeas that bloom on new wood, specifically H. paniculata and H. arborescens, are tolerant of heavy pruning because flower buds have not yet formed during the dormant season. The ideal time for pruning this group is in late winter or early spring before new growth begins to emerge. Gardeners can cut these shrubs back aggressively, often removing up to two-thirds of the plant’s height. This maintains shape and encourages strong, new stems capable of supporting large flower heads.
For reblooming cultivars, which bloom on both old and new wood, the need for pruning is significantly reduced. The best practice is to perform minimal shaping or remove only dead or damaged wood in the early spring. If a substantial reduction is required, it should be done right after the first flush of old wood blooms fades in summer to preserve the buds that will form on the new wood later in the season.