Tree care doesn’t necessarily end when the snow starts flying. Winter is a good time to prune a number of your trees. Pruning a tree, especially when its young, is one of the most important and generally overlooked maintenance practices. When a tree is young, the cuts are small and good growth and form can easily be achieved. Once a tree becomes large, some serious wounds can be left behind after pruning is done to correct poor growth patterns.
Depending on whether the pruning will be preventive or corrective, examples of reasons to prune include: correcting or limiting the growth of a tree or shrub; eliminating safety hazards such as dead, decaying limbs; removing branches that are rubbing together or are connected at a narrow angle to the main trunk; removing branches so the remaining ones are about one foot apart and come out from all sides of the tree; removing multiple leaders on plants that should have only a single leader as in the cases of evergreens and shade trees.
Winter is the best time to prune trees that are susceptible to diseases at other times of the year. The risk of introducing a disease to the tree is greatly reduced during the winter months. Oaks should ideally be pruned in December, January, or February to minimize the chance of oak wilt infection. The same guidelines apply to elms. Apples, crabapples, pears, mountain ash, hawthorns, and cotoneasters should be pruned between the time they go dormant in the fall and the time growth starts in the spring. This will minimize the spread and chance of infection of a bacterial disease called fireblight.
Some trees "bleed" if pruned in late winter or early spring. The "bleeding" is free flowing sap and causes little or no harm to the tree, but alarms many homeowners. To prevent "bleeding", prune the following group of trees anytime while they are actively growing; ideally in August. These trees include maples (including boxelder), honeylocusts, butternuts and walnuts, birch, ironwood, and blue beech.
Pines put out a single flush of growth each year and stop. They can only be pruned at the candle stage of growth before the candles become woody. The candle stage occurs in the spring and is characterized by long, light-green ‘candle-looking’ growth at the end of a branch. Pruning at other times of the year will cause dead stubs. If you want dense, compact specimen pines, they should be pruned when the candles are elongated to about three-fourths their length. Up to two-thirds of this new growth can be removed. Pruning should not go into last year's growth.
Evergreens that grow continuously throughout the growing season can be pruned at any time, but a good rule of thumb is the earlier in the growing season the better. This would include junipers, white cedar or arborvitae, yews, and hemlocks. The time of the year for pruning of spruces and firs is not critical. Pruning in late winter or before growth starts is probably the best.
Since pruning is vital to the health of a tree, doing a proper job is critical. The pruning cut should leave the branch collar but not leave a stub. The branch collar is the area where the branch enters the main trunk of a tree, it is characterized by a raised ridge most visible in the crotch between branch and trunk.
Never prune oaks and elms in April, May and June.