Pruning Flowering Shrubs
Pruning is an important horticultural practice for maintaining the health, appearance, and size of flowering shrubs. Knowing when and how to prune promotes vigorous growth and maximizes floral display. This fact sheet details the best timing and techniques for pruning your flowering shrubs based on bloom season and shrub type.
Importance of Pruning
Pruning is an important horticultural technique to maintain the health and appearance of flowering shrubs. Removal of dead, damaged, or diseased wood reduces insect- and disease-related problems and allows pruners to catch issues such as these before they get out of hand. Keeping the center of the shrub open to sunlight and air circulation improves shrub growth while allowing interior leaves to dry quickly after rain or heavy dew, which can reduce the incidence of disease. Removing crossing stems eliminates potential bark damage, reducing the chance of insect- or disease-related problems. Pruning also forces new growth, which, in most cases, produces the most colorful stems and new flowering wood for future years.
Controlling plant size is low on the list of reasons for pruning because pruning is not a substitute for proper plant selection. Most plants have naturally beautiful shapes that can be enhanced and somewhat controlled through proper pruning practices; very few adapt well to shearing. Most plants stay healthy and attractive longer if allowed to grow naturally, so reserve the hedge shears for formal hedges.
Pruning Techniques
The process of removing stems at their point of origin is known as thinning, whereas shortening a stem from the top is known as heading. Technically, shearing is simply making a lot of heading cuts. Thinning cuts are preferable because they open the shrub up to sunlight and air circulation. Heading cuts result in a profusion of growth below the cut, creating a wall of growth on the outside of the shrub that blocks sun from the interior of the shrub and impedes air circulation. Even formally sheared hedges should be opened periodically to encourage new growth from inside.
Shrubs with a suckering growth habit, such as forsythia and lilac, should periodically have the oldest, biggest stems removed at ground level. Rejuvenate badly overgrown specimens by removing the biggest, oldest stems at ground level. This can be done all at once if the shrub is healthy and thriving. If the shrub is unhealthy, this process can be spread out over a 3-year period by removing one-third of the overgrown stems each year. Keep the sturdiest, well-placed younger stems and remove those that are damaged, spindly, or too close to one another.
New suckers will sprout from the roots that will have to be similarly thinned later in summer. Hard pruning should always be done in early spring, before the shrub begins to develop new foliage. It is less stressful for the plant, and you can clearly see the stems when they are leafless.
When to Prune
The correct time to prune your flowering shrubs depends on when they flower. A rough rule of thumb is to prune spring-blooming shrubs soon after they finish flowering because most bloom on "old wood," which means they set next year’s flower buds shortly after they finish blooming this year, whereas those that bloom in summer and fall usually bloom on "new wood," which means they set flower buds on the current season’s growth and can be pruned in late winter or very early spring
Spring-Blooming Shrubs
Like many spring-blooming shrubs, azaleas bloom on old wood. If you wait too long to prune them, you will remove many of next year's blooms when you prune, especially if you shear your azaleas.
Other shrubs that fall into this category include forsythia (Forsythia spp.), Virginia sweetspire (Itea virginica), mock orange (Philadelphus spp.), ninebark (Physocarpus opulifolius), quince (Chaenomeles spp.), rhododendrons including azaleas (Rhododendron spp.), spring-flowering roses (Rosa spp.), spring-blooming spirea (Spiraea prunifolia and S. x vanhouttei), lilacs (Syringa spp.), and viburnums (Viburnum spp.).
Summer-Blooming Shrubs
In contrast to spring-blooming shrubs, shrubs that bloom later in summer and fall tend to bloom on new wood. These shrubs should be pruned in late winter or very early spring before they develop new foliage.
Shrubs that fall into this category include butterfly bush (Buddleia spp.), sweetshrub (Calycanthus floridus), beautyberry (Callicarpa spp.), trumpet vine (Campsis radicans), summersweet (Clethra spp.), buttonbush (Cephalanthus occidentalis), bush honeysuckle (Diervilla spp.), smooth hydrangea (Hydrangea arborescens), panicle hydrangea (Hydrangea paniculata), repeat-blooming roses (Rosa spp. and hybrids), summer-blooming spirea (Spiraea × bumalda and S. japonica), and chaste-tree (Vitex spp.).
Proper timing and techniques are key to successful pruning. The information outlined here can help you enhance the natural beauty of your flowering shrubs and keep them thriving season after season.
Prepared by Sandy Feather, Penn State Extension educator, Green Industry.
