Tree trimming is a much more selective process than tree removal, because you need to decide what to cut, and what to leave to make the tree structurally sound, healthy, and good to look at. Furthermore, tree trimming or pruning should only be done at certain times of the year.
There are very few species of trees that can be pruned in summer. The vast majority of tree species should be pruned in late fall, autumn, or winter; no later than early spring. In late spring, the tree is blossoming and budding with new growth that you don’t want to cut off. It is producing leaves that photosynthesize to make food for the tree. Therefore, you don’t want to remove the growth with these leaves on them. Later in the spring, the tree is no longer dormant like it would be in late fall or winter. Its juices are starting to flow and cuts made at this time can invite insect infestations into the tree. For this reason, pruning trees is not carried out in summer. This is especially true of conifer trees such as pine and fir. If you cut live branches off these trees in the summer, a beetle infestation can be invited into these trees and surrounding trees.
Winter is a good time to prune; trees are dormant and losing their leaves, or have already lost them. However, in places like Montana, people just shut down in winter because of sub-zero temperatures and heavy snow, thus not having tree work done. Freezing weather can also adversely affect cuts made on a tree and their proper response to it. Tree work is more difficult for us to do in the winter, with freezing temperatures and heavy snows, but we take whatever measures necessary to get it done.
During summer, dead wood can always be removed. This is true of the dead branches growing on conifers. Limbs should not be removed off the trunk or main scaffolding branches of a tree any higher than the lower one third of the tree. This is true for conifers unless the limbs are dead. Conifers are generally not trimmed or topped, unless it is to remove dead, broken, or dangerous branches, and mistletoe.
Deciduous trees like elm, ash, maple, linden, locust and many others should not be topped either. Topping these trees not only makes them unsightly, but it is also not healthy for them, because it stimulates structurally unsound growth lower down in the tree. If you want the height of a tree lowered, this can be accomplished by means of a method called crown cleaning. This involves thinning out heavy vertical growing limbs that grow much higher than the surrounding growth. If a remaining vertical limb is still too high, it can be cut back to a lateral limb that grows off of it down lower. Doing this is known as a reduction cut which completes the process of crown cleaning to reduce the height of your tree.
Trees can continue to be shaped by removing branches that hang downward or grow too horizontally. Crossing branches, branches that rub on each other, or compete or interfere with each other, can also be removed. After all of this, you’ll have a healthy tree that is structurally sound and pleasing to look at.