If correctly pruned when young, large trees generally require less pruning than most other woody plants. However, they may require occasional removal of dead or dying branches.
As a shade tree grows in height and spread, it may be necessary to control its size, reduce excess shade, or prevent branches from rubbing against each other, wires, buildings, or vehicles. Removing weak branches arising from the trunk at a narrow angle also reduces the possibility of branches breaking in storms or of the trunk splitting as the tree matures.
Make thinning cuts to remove limbs just outside the branch collar, the swollen or wrinkled area where the branch meets the trunk (or larger branch). A cut made too long leaves a stub, which dies and leaves a wound that heals slowly. A cut made too close removes or cuts into the branch collar, slowing healing of the pruning wound. This technique of removing a branch just outside the collar is known as natural target pruning.
Maintaining the Leader
Protect and strengthen the main leader unless you need to reduce the tree’s height. Heading the leader eliminates apical dominance and produces vigorous, upright growth just behind the cut. The new shoots are weakly attached and easily broken.
If it is necessary to prune a leader, cut back to a lateral branch with an equal or one-third smaller diameter. The side branch then assumes apical dominance. Initially the new leader bends to assume the position of the original leader, but in a few years this bend is no longer noticeable.
To remove a competing leader, gradually cut it back rather than remove it entirely the first year. Thin out approximately one third the length of a competing leader each year for three years; this will slow it down without causing the tree to lose vigor.
In trees that usually produce multiple leaders as they mature — such as maples, oaks, and lindens — subdue competing leaders by removing one third of the length of the large branches; this can be done in trees up to 15 years old. Subduing competing leaders this way slows their growth and encourages the tree to develop a stronger main leader with scaffold branches attached at more secure angles.
As the tree matures, multiple leaders may form but they will be less dominant than the principal leader and the tree’s structure will be better able to withstand storms.
The best time to prune deciduous trees is late in the dormant season — late winter or early spring — before new growth begins and while the branch structure is easy to observe. Callus tissue, which closes over wounds and pruning cuts, grows most quickly from early spring to midsummer and facilitates closing of dormant season pruning cuts.
However, trees such as maple, dogwood, birch, and elm leak sap from pruning wounds made in late winter or early spring. Although the flow is not harmful and soon abates, it can be uncomfortable to see and work around. A suitable alternate time for pruning these trees is midsummer, when growth has slowed.
Spring-flowering trees, which bear flower buds all winter, can be lightly thinned in late winter with some loss of bloom or pruned immediately after they flower but before their leaves mature.
Thinning, the pruning method to use for most trees, results in a more open shape and encourages the growth of interior branches. Thin crowded branches, branches that are rubbing or likely to rub against each other, and branches aiming toward the interior of the tree rather than to the outside.
It is possible to thin a great number of branches without changing the natural branching pattern or overall appearance of the tree. A properly thinned tree is healthier and more resistant to wind damage. It casts a less dense shadow, more suitable for growing in a lawn or over shade-loving garden plants.
Using a pruning saw, remove a branch at its point of origin on a parent stem, being careful to cut just to the outside of the swollen branch collar. Or cut the branch back to where it makes a Y with a strong lateral branch. Choose a side branch that is growing in a desirable direction and one with a diameter at least two thirds the size of the branch being removed. Thinning cuts should be used in each of the pruning methods described below.
Removing Large Branches
Branches that are rubbing against the house or growing where they shouldn’t be can be thinned or totally removed. Avoid heading them back to a stub; not only is this unsightly, but the resulting growth, which originates near the stub, will quickly become a problem.
Remove a branch too large to hold in one hand with three separate cuts; otherwise the weight of the cut branch will tear a strip of bark from the tree.
Removing Crossed Branches
In addition to inevitably injuring one another, rubbing or crossing branches may lead to limb decay and death. Any of the three pruning operations mentioned in Corrective Pruning for Mature Trees can be used to remove a crossing limb without leaving a large hole in the outline of a tree.