Mophead Hydrangeas (Hydrangea macrophylla) are popular deciduous shrubs, beloved for their large, globe-shaped flower clusters. These plants offer a striking display of blue, pink, or purple blooms throughout the summer months. The long-term success of these shrubs, particularly their ability to flower reliably each year, rests on understanding the correct pruning schedule. Getting the timing right is the most important factor in ensuring a spectacular floral show.
Understanding Mophead Flowering Habits
The confusion surrounding when to prune Mophead Hydrangeas stems from differences in how they produce flowers. Traditional varieties are “old wood” bloomers, meaning they develop flower buds on the woody stems that grew the previous season. These buds are set in late summer or early fall, remain dormant through winter, and bloom the following summer. Pruning these traditional types at the wrong time, such as in fall or spring, inadvertently removes all the waiting flower buds.
A newer category of Hydrangea macrophylla are the “rebloomers” or “endless summer” types. These cultivars bloom on both old wood and “new wood,” which is growth produced during the current season. While they rely on old wood for the first and often heaviest flush of blooms, they can produce a second wave of flowers later in the season on stems that grew that spring. Identifying your plant type is the necessary first step before making any major cuts.
The Optimal Timing for Major Pruning
For Mophead Hydrangeas that bloom only on old wood, the window for significant pruning occurs immediately after their flowering season concludes. This typically falls between mid-July and mid-August in most temperate climates. Pruning during this time allows the plant to heal and dedicate energy to growing the new stems where next year’s flower buds will be set. Cutting back these plants after mid-August carries a high risk of removing the newly formed buds, resulting in a severe reduction or complete loss of flowers.
When performing this major cut, reduce the length of the flowered stems back to a set of healthy buds or a strong pair of leaves. Only remove up to one-third of the total stems in any given year to avoid stressing the plant. For reblooming varieties, the timing is more flexible due to their ability to produce flowers on new growth. The best time for a major size reduction or shaping is in late winter or early spring, before new growth emerges from the base.
This late winter timing allows the gardener to remove any winter-damaged wood and set the overall shape of the shrub before the new season’s growth begins. Even with rebloomers, heavy pruning should be kept to a minimum. These types still produce their earliest and most substantial blooms on the old wood, so excessive spring cutting can reduce the overall flower count.
Year-Round Maintenance: Deadheading and Cleanup
Pruning tasks that do not involve significant size reduction can be performed outside of the main summer window. Deadheading is the removal of spent flower heads, which can be done throughout the summer. To deadhead properly, snip the stem just below the finished flower head and directly above the first healthy set of leaves. This action improves the plant’s appearance and prevents the shrub from expending energy on seed production.
Many gardeners choose to leave spent flower heads on the shrub throughout fall and winter, especially in colder regions. These dried blooms offer visual interest during the dormant season and provide insulation, protecting latent flower buds from frost damage. Any stems that are dead, diseased, or broken should be removed entirely down to the base at any point in the year. Removing this material improves air circulation and helps maintain the plant’s overall health.
Thinning involves removing a few of the oldest, thickest, and weakest canes down to the base. This process rejuvenates an aging shrub by encouraging new, vigorous growth. While this can be done anytime for dead stems, it is best performed during the major pruning window—late summer for old wood types or late winter for rebloomers—to maximize the plant’s health.