Many gardeners have questions about pruning. The main concern is what plants to prune and when to prune them. The major factor in deciding when to prune is whether or when the plant flowers or sets blooms.
Light, corrective pruning can be done any time of the year and will reduce the need for severe pruning, which might weaken the plant. When severe pruning becomes necessary it should be done in late winter or early spring. Pruning in late summer should be limited because new growth might not have time to mature before cold weather.
Woody plants heal pruning cuts by producing calluses that gradually grow over the wound. Internally, they compartmentalize or wall-off the damaged tissue from healthy wood. These responses to wounding occur more rapidly just prior to the onset of new growth in the spring and just after maximum leaf expansion in June.
The time of pruning also will affect the amount of new growth produced. Plants respond to late-winter and early-spring pruning by growing vigorously to replace the removed wood. When pruned during the summer, only a limited amount of new growth is produced. Susceptibility to cold weather is increased after pruning. Wood around winter-pruned cuts is more susceptible to damage and drying out.
Shrubs grown mainly for their foliage are easier to prune because flowers won’t be reduced by pruning. Shrubs that are grown for their foliage are pruned in late winter and early summer. Many of these are considered to be “bread and butter” plants and are used as a foundation planting in the landscape. Examples of these shrubs would be boxwoods, cleyera, holly species, pittosporum and podocarpus.
For plants that produce flowers, pruning is more complicated and should be based on when a plant flowers. Spring-flowering (before May) shrubs such as azalea, clematis, doublefile viburnum, forsythia, saucer and star magnolia, and spirea set flowers on buds that are formed the previous summer or fall. If these shrubs are pruned during late summer, fall or winter, many of the flower buds will be removed. To ensure maximum flowering, these shrubs should be pruned as soon as possible after flowers fade in the spring. Little or no pruning should be done after the Fourth of July.
Summer-flowering (May or later) shrubs flower on new growth produced in the current growing season. They should be pruned late winter or early spring before new growth begins. Summer- and fall-flowering shrubs include abelia, beautyberry, butterfly bush, clethra, crape myrtle and Rose of Sharon.
Hydrangeas are an exception to the rule. They are summer bloomers but should be pruned immediately after flowering. This shrub will bloom on new wood and old wood depending on the type of hydrangea planted. How and when to prune hydrangeas is an issue for many gardeners, even those with years of experience.
There are several popular types of hydrangea. The most common is the bigleaf, Hydrangea macrophylla. This hydrangea blooms on last year’s wood. Another example of a hydrangea that blooms on old wood is the Oakleaf hydrangea, Hydrangea quercifolia. The Oakleaf hydrangea and the Bigleaf hydrangea should be pruned as blooms begin to fade.
An example of a hydrangea that blooms on new wood is the Pee Gee hydrangea. Around the first of March, prune these plants severely to leave two buds at the base of each stem. These Hydrangeas will still get new growth and flower because they bloom on new wood. If you prune severely, the plant will be smaller but will have larger blooms.
Crape Myrtles should be pruned selectively, not “murdered,” as Sid Mullis explained in an article that ran Jan. 3, 2012, in The Augusta Chronicle. Please refer to this article for more information on pruning Crape myrtles.