The Big Leaf Hydrangea, Hydrangea macrophylla, is celebrated for its spectacular, globe-shaped or flat-topped flower clusters. Successfully cultivating these shrubs to produce abundant blooms is dependent on proper pruning timing. Unlike some other common landscape shrubs, the timing of your cuts determines whether the plant will set flower buds for the following season. Pruning H. macrophylla is distinct from managing hydrangeas that bloom on the current season’s growth, such as the Hydrangea paniculata or Hydrangea arborescens varieties. Knowing when to use your pruners is the most important factor for ensuring a vibrant display of flowers year after year.
Understanding the Difference Between Old and New Wood
The reason pruning timing is complex for H. macrophylla lies in its specific blooming habit. The terms “old wood” and “new wood” refer to the age of the stem on which the flower buds develop.
Old wood is the growth produced in the previous growing season, having already survived one winter. Traditional Big Leaf Hydrangeas form their flower buds on these stems during the late summer and fall, where they remain dormant until the following spring.
New wood refers to the stems that emerge during the current growing season. These fresh shoots typically do not have flower buds ready to bloom immediately. Traditional varieties rely exclusively on the old wood for their blooms, making any pruning mistake potentially devastating to the year’s flower display.
Pruning Timing for Traditional Big Leaf Hydrangea Varieties
Traditional Big Leaf Hydrangeas, which bloom only on old wood, require the narrowest and most precisely timed pruning window. The ideal time to prune these varieties is immediately after the flowers have faded, typically from late July through mid-August in most regions. Pruning right after the bloom allows the plant time to recover and develop new stems that will form the next season’s flower buds before winter dormancy sets in.
It is imperative to complete all significant pruning before mid-to-late August. Cutting the shrub after this time risks removing the newly formed flower buds that are already setting for the next year’s bloom. If pruning is necessary for size control, it should be a selective process rather than a hard cutback.
To rejuvenate an older plant, employ renewal pruning, which involves removing the oldest, thickest canes right down to the ground. This encourages new, vigorous growth from the base of the plant. Remove no more than one-third of the total stems annually, ensuring enough old wood remains to produce flowers the following season.
Pruning Timing for Reblooming Big Leaf Hydrangea Varieties
Modern cultivars, often called rebloomers, such as ‘Endless Summer’ or ‘BloomStruck,’ offer more flexibility because they bloom on both old wood and new wood. These varieties produce an initial flush of flowers on the previous year’s growth and continue to bloom on the current season’s new stems. This characteristic makes them more forgiving of late frosts or incorrect timing.
Late Winter/Early Spring Pruning
One opportunity for pruning occurs in late winter or early spring, before the new growth cycle begins. At this time, you can safely perform light shaping and remove any winter-damaged tips or dead wood without sacrificing the season’s blooms. This is safe because the plant will compensate by producing flowers on the subsequent new growth.
Summer Deadheading
A second, less intensive pruning opportunity is available during the summer through deadheading. Removing the spent flowers just above the next set of large, healthy leaves encourages the plant to put energy into developing a second flush of blooms on new wood. If a more significant cut is needed, it should be done in late winter before new buds swell.
Essential Cleanup Pruning
While most significant pruning is tied to the plant’s flowering cycle, there are exceptions that should be addressed at any time of year. This essential cleanup focuses on the “three Ds”: dead, diseased, and damaged wood. Removing these compromised stems is beneficial for the plant’s overall health and helps prevent the spread of pathogens.
This type of pruning can be done regardless of the season because these compromised parts will never produce viable flowers. The best time to identify and remove wood that died over the winter is in the late winter or early spring, just as the plant begins to show new growth. Stems that do not leaf out can be cut back to the ground or to a point of healthy growth.
Deadheading, the removal of spent flower heads, is also a safe, year-round practice. Snip the faded flower head just above the first set of large, healthy leaves. Removing spent blooms improves the shrub’s appearance and can encourage reblooming varieties. Cutting further down into the main stem during the fall or winter is not recommended as it could remove dormant flower buds.