Trees and shrubs have differing needs for pruning, depending on the species, but nearly all of them benefit from occasional pruning to shape the plant, to restrict its growth, or to remove dead or diseased limbs. Known when to prune can be tricky, however, especially for trees and shrubs that flower in the spring. Experts tend to divide trees and shrubs into two types—those that flower on "old wood" (which are usually the early-flowering trees) and those that flower on "new wood "(which usually bloom in later spring or in summer).
Old Wood vs. New Wood
Many spring-flowering trees and shrubs set their flower buds the previous fall, on stem growth from that year. When these plants then flower the following spring, they are said to bloom on "old wood"—the stem growth that occurred during the previous year. This is the pattern for many, though not all, of the early spring-flowering trees and shrubs. The earlier your tree or shrub blooms, the more likely it is to fall into this group.
There are other trees and shrubs, however, that bloom later in the season on new growth that appears during the current growing season. These are said to flower on "new wood." Most of these plants bloom in summer or late spring.
Plants to Prune Immediately After Blooming
Most spring-flowering trees and shrubs produce their blooms on old wood—on flower buds that appeared on the previous year's growth. With these, the rule of thumb is to prune them immediately after their spring flowers have faded. This rhythm allows the plants to begin producing the new wood that will set the flower buds to create the next spring's spectacular flowers. You'll have a couple of weeks grace period, after flowering, to get this pruning done before next season's buds begin being set.
Some of the plants that should be pruned immediately after flowering include:
- Azalea (Rhododendron species)
- Beautybush (Kolkwitzia amabilis)
- Bridal wreath spirea (Spirea x vanhouttei)
- Flowering crabapple (Malus species and cultivars)
- Flowering plum (Prunus blireana)
- Flowering quince (Chaenomeles speciosa)
- Forsythia (Forsythia x intermedia)
- Hydrangea, Bigleaf (Hydrangea macrophylla)
- Lilac (Syringa vulgaris)
- Magnolia (Magnolia species and cultivars)
- Mockorange (Philadelphus coronarius)
- Mountain Laurel (Kalmia latifolia)
- Rhododendron (Rhododendron species)
- Redbud (Cercis canadensis)
- Serviceberry (Amelanchier x grandiflora)
- Slender deutzia (Deutzia gracilis)
- Weigela (Weigela florida)
Plants to Prune During Dormancy
Most trees and shrubs that bloom in summer are usually best pruned in the dormant period in late winter or very early spring before new growth begins. If a plant is described as "flowering on new wood," it should be pruned during its dormant period, since this gives it plenty of time to create the new wood that produces good summer flowering.
Here are some of the trees and shrubs for which dormant-period pruning is usually recommended:
- Bradford pear (Pyrus calleryana)
- Butterfly bush (Buddleia Davidii)
- Crape myrtle (Lagerstroemia indica)
- Flowering dogwood (Cornus florida)
- Glossy abelia (Abelia x grandiflora)
- Golden rain tree (Koelreuteria paniculata)
- Hawthorn (Crataegus species and cultivars)
- Honeysuckle (Lonicera fragrantissiam)
- Hydrangea, Peegee (Hydrangea paniculata ‘Grandiflora’)
- Potentilla (Potentilla fruticosa)
- Spirea (except Bridal Wreath) (Spirea japonica)
- Wisteria (Wisteria spp.)
When to Break the Rules
Sometimes it makes sense to go against the standard practice and prune a spring-flowering shrub during the dormant season, even though this means sacrificing the flower buds and losing the blooms for one season.
- When you need to rejuvenate an old tree or shrub and make extensive cuts, it’s much easier to prune when you can see the shape of the plant. This type of pruning is easier to do while the tree or shrub is dormant, before the branches are masked by leaves.
- Any kind of pruning stresses a plant. If you prune while the tree is trying to actively grow in the spring, it is even more of a burden. Dormant pruning allows the tree or shrub to deal with healing the cut without having to worry about producing leaves and flowers or sending out new growth.
- Winter-pruned plants are less susceptible to insect and disease problems. Pruning creates open wounds through which insects, bacteria, and fungal spores can enter. Because these pathogens are not active in winter, pruning at this time lessens the chance of infection.
While dormant-season pruning may cause your tree or shrub to lose its blooms for one season, it generally returns with even more vigorous blooms the following year.