The Bigleaf Hydrangea (Hydrangea macrophylla), known for its vibrant pink, blue, or purple mophead and lacecap blooms, is a garden favorite. However, the timing of pruning this shrub is a frequent source of confusion. Understanding when to prune is necessary for successful flowering, as an improperly timed cut eliminates the flower buds for the following year.
The Critical Factor: Understanding Bloom Wood
The crucial element determining when to prune a Bigleaf Hydrangea is “old wood blooming.” This means that the flower buds for next year’s blooms are set on the stems that grew during the previous summer, not in the spring. These dormant buds develop in the late summer and fall, survive the winter, and are ready to open the following summer. Pruning during the dormant season (late fall, winter, and early spring) removes these pre-formed flower buds. Cutting back the stems at these times results in a plant that produces only leaves, not flowers.
Ideal Timing for Traditional Varieties
For traditional Bigleaf Hydrangea varieties that bloom exclusively on old wood, pruning must occur immediately after the flowers fade in the summer. This narrow window, typically mid-July through early August, is the only time to prune without sacrificing next year’s blooms. Pruning right after the current flowers are spent allows the plant time to produce new growth and set next year’s flower buds before dormancy. The strict deadline for pruning is mid-to-late August, depending on the local climate. If significant size reduction is required, perform a series of light prunings over two or three summers rather than a single, aggressive cut.
Timing for Reblooming Cultivars
Modern cultivars, often called reblooming hydrangeas, are an exception because they bloom on both old and new wood. Varieties like ‘Endless Summer’ produce flowers on the previous year’s growth and the current season’s growth, offering a more forgiving pruning schedule. They benefit from light pruning immediately following the first flush of blooms in early summer. This minimal pruning, focused on deadheading spent flowers, encourages the next wave of blooms on the new wood. If major structural pruning is necessary, do it in late winter or early spring to remove dead or winter-damaged wood, ensuring a flower display later in the season.
Pruning Techniques for Health and Shape
When pruning, focus on thinning and maintenance rather than severe shaping. Deadheading, the removal of spent flowers, should be done by cutting the stem just above the first set of healthy leaves or an outward-facing bud. This technique improves the plant’s appearance and redirects energy away from seed production. Stems that are dead, diseased, or damaged can be removed at any time of year without harming future bloom potential. For older, overgrown shrubs, rejuvenation pruning involves cutting approximately one-third of the oldest, thickest stems all the way to the ground, encouraging strong new growth.