In general the best time to prune any woody plant is just before new growth starts. Pruning in late winter or early spring while a plant is dormant won’t adversely affect its vigor; but pruning at other times can rob it of stored food energy. Severe pruning during or just after active growth in spring only wastes stored energy. Such pruning can dwarf a plant and is not recommended unless a dwarfing effect is the goal, such as for a bonsai.
Photosynthesis is most active during summer, when plants produce abundant food and new growth. As the days shorten in late summer, growth slows and sugars accumulate in the leaves. Before the foliage drops the food moves from the leaves into the woody branches. Pruning in fall depletes the stored food reserves needed to initiate spring growth. Since many decay fungi produce spores in fall, that is also the time when open wounds are most likely to become infected with decay rot. Later in the dormant season, sugars move farther down the plant and are less likely to be disturbed. Pruning at that time doesn’t waste stored energy and cuts heal more quickly.
New growth can be directed by pruning in late winter or early spring before leaves appear. It is easy at this time of year to examine the structural arrangement of the branches of deciduous trees and shrubs and plan pruning strategies. Dormant season pruning is good for the plant and the gardener. It is a time when few other garden chores make demands and the outdoor activity is excellent exercise.
However, there are cases in which pruning should be done during the growing season. If spring-flowering shrubs are pruned in winter, the flower buds will be removed and the plants won’t blossom that spring. The spring-flowering trees and shrubs that bloom on the previous season’s growth should be pruned immediately after flowering, but before leaves fully expand. Summer-blooming plants, which usually bloom on the current-season’s growth, can be pruned in winter without danger of removing flower buds; in fact, dormant-season pruning will stimulate more flowers.
Midsummer pruning has a dwarfing effect on plants. Removing summer foliage reduces photosynthesis, resulting in less food reserves for the following spring’s growth. Summer pruning is appropriate for slowing the vigorous growth of an immature fruit tree so that it will begin bearing. Dwarf fruit trees especially are subjected to frequent light pruning during the summer. This controls their shape and avoids overly vigorous spring growth.
Summer pruning will prevent an extremely vigorous tree from responding to heavy pruning with a burst of water sprouts and suckers. Summer pruning is also recommended for restricting growth of a tree or shrub that has reached a desired height and spread. However, because wounds will not callus over as rapidly as during the late dormant season, it’s best to keep summer pruning cuts small and save heavy shaping for winter.
The worst time to prune is right after leaves emerge in spring. Stored energy has powered the initiation and expansion of the new foliage, but the leaves have not yet begun to accumulate food to replenish the supply. Bud break in spring is also the time of greatest root growth, another heavy drain on stored reserves. Until food manufacture equals or surpasses food utilization, the plant can ill afford to lose foliage. In addition, the tissue beneath the bark is soft during the spring growth; it is easy to tear the bark when pruning.
With judgment and moderation, some pruning can be done at any time. Dead and dying branches, suckers, and water sprouts should be removed anytime they become apparent. Pinching and small cuts guide growth without removing very much plant material, and can be done any time the plant is growing. And, of course, removing a few stragglers or branches that are out of line during the growing season won’t do any harm.