The smooth hydrangea, botanically known as Hydrangea arborescens, is a popular deciduous shrub prized for its large, rounded flower clusters. Well-known cultivars include ‘Annabelle’ and the sturdier-stemmed ‘Incrediball’. Achieving an impressive display of blooms requires understanding the plant’s unique growth cycle and performing a strategic annual cutback to direct energy toward producing massive flowers.
Understanding Smooth Hydrangea Growth Habit
The smooth hydrangea differs fundamentally from other common types, such as the bigleaf (H. macrophylla) or oakleaf (H. quercifolia), because of where it forms its flower buds. The H. arborescens species blooms exclusively on the current season’s growth, a characteristic known as “new wood.”
This new wood blooming habit means the flower buds are not set until the spring of the same year they flower. Consequently, the plant can be pruned aggressively without the risk of removing the coming season’s blooms. This trait makes the smooth hydrangea a reliable bloomer, even after a severe winter or a heavy cutback. The shrub behaves almost like a herbaceous perennial, dying back to the ground in colder climates but regrowing vigorously each spring.
Optimal Timing for Pruning
The best window for pruning smooth hydrangeas is late winter or very early spring, while the plant is still fully dormant. This timing is ideal because the plant’s structure is fully visible without any obscuring foliage. Pruning before the new growth begins ensures the shrub can immediately channel its energy into developing fresh, strong stems once the growing season starts.
Gardeners should aim to complete this task before the buds begin to visibly swell or turn green, which is typically just before the last expected hard frost. Pruning too late, after significant new growth has already occurred, will not prevent flowering but may delay the bloom time.
Step-by-Step Pruning Techniques
Ensure all cutting tools, such as bypass pruners and loppers, are sharp and sanitized to make clean cuts and prevent disease spread. The primary goal of pruning H. arborescens is to remove the previous year’s growth to encourage robust new stems. This aggressive cutback is the most effective way to maximize bloom size.
The most common technique is the hard cutback, where every stem is cut down to a height of 6 to 12 inches above the ground. This method forces the plant to send up powerful, thick new canes that support the weight of the large flower heads. Cutting closer to the ground, around 6 inches, typically results in fewer but larger blooms, though this can increase the risk of stem flopping in older varieties like ‘Annabelle’.
Beyond the general cutback, focus on completely removing any wood that is damaged, thin, or crossing. Weak or pencil-thin stems should be cut all the way back to the soil line to prevent them from sapping energy. Removing crossing branches improves air circulation through the center of the shrub, which helps reduce fungal issues. The goal is to create an open, sturdy framework for the new season’s growth.
For newer cultivars, like the ‘Incrediball’ series bred for sturdier stems, some gardeners may choose to cut back by only about one-third of the total height. This moderate approach leaves some older woody structure in place, which provides additional support for the subsequent year’s growth. However, for maximum bloom size and to address flopping issues common in older plants, the hard cutback remains the most effective strategy.
Post-Pruning Care and Maintenance
Immediately following the cutback, clean up all discarded stems and debris from the base of the plant to minimize pests or diseases. Applying a fresh layer of organic mulch, such as shredded bark or compost, is beneficial to the newly pruned shrub. The layer should be 2 to 3 inches thick, but keep the mulch a few inches away from the crown to prevent moisture buildup that can lead to rot.
The plant will require energy for the flush of new growth necessary to produce flowers. If a soil test indicates a nutrient deficiency, apply a balanced, slow-release granular fertilizer formulated for shrubs in early spring. Avoid fertilizers with high nitrogen content, as this nutrient encourages leaf and stem growth at the expense of flower production. Regular watering will be necessary as the weather warms and the new stems rapidly expand.