There can be many objectives when pruning a tree. These include weight reduction to reduce the potential for branch failure; providing clearance to buildings or infrastructure; reducing shade and wind resistance; maintaining health; promoting flower or fruit production; improving a view; or simply improving the aesthetic look of the tree. Regardless of the aim, cutting a tree and removing foliage will expose the tree to bacterial or fungal infection as well as inducing different stress responses depending on the type of tree. Therefore, pruning should be done properly to allow the tree the best chance to heal and maintain it’s health.
Before making any cuts it is important to have a basic understanding of how branches are attached to each other and to the trunk of the tree. Where a smaller branch sprouts from the parent branch or trunk, there is a branch collar (usually a ridge that runs perpendicular across the base of the smaller branch at about a 40 degree angle (depending on species) the parent branch. It is formed by overlapping wood from the branch and trunk which forms a strong attachment between the two. The physical properties of this area, combined with chemicals produced by the tree make it difficult for infection to spread from the branch into the trunk. For this reason it is important when pruning, to make a clean cut just outside the branch collar. If the cut is made here without tears or breaking, the tree will have the best chance of healing over the cut.
Poor cuts are those made too far from the branch collar, or too flat to the trunk (flush cut). To avoid tearing, make a small cut (1/4 to 1/3) through the underside of the branch a little way out from the collar, then make a second cut on the upper side slightly further out still which overlaps the first. This will enable the branch to snap and separate neatly with splitting or tearing down past the collar. There is a general rule that arborists use when selecting which branches to cut, called the Rule of Thirds. This rule states that where possible the branch being removed should be no more than 1/3 the diameter of the parent branch or stem from which it originates. This rule is here to ensure that the cut is not so big that the tree can’t possibly heal over it, and leaves a strong enough branch or stem to assume what’s called ‘apical dominance’ or in other words become the leading part of the remaining stem.
When cuts are made too far from the collar a stub is left. This stub will die back to the branch collar leaving dead material that the tree takes a long time to grow over and is perfect for bacterial and fungal infection. If this becomes infected there is potential for the infection to spread to the trunk of the tree. Flush cuts, although they may not look like it, actually expose wood tissue from the trunk or stem. This opens up the parent branch or trunk to infection and decay in major parts of the tree can be catastrophic. So remember - find the branch collar and follow just outside it for every cut!
There are three main types of pruning cut; thinning, reduction and heading. Thinning cuts remove selected branches, generally within the crown, to improve light penetration through the crown and increase air flow. Reduction cuts however, remove branches at the exterior of the crown to keep the tree contained. The afore mentioned pruning techniques apply completely to these types of pruning. Heading cuts, on the other hand, require a different technique and for the most part will require more maintenance. Heading cuts are when a branch is cut to a bud or to a lateral branch too small to assume apical dominance. In trees heading cuts stimulate a stress response which causes multiple ‘suckers’ or small straight branches to appear from the nearest buds. These are called epicormic growth and it is basically the tree trying panicking and trying to quickly replace lost foliage. In certain species and situations heading can be a good way to contain a tree while maintaining a full looking crown (known as ‘pollarding’), but care must be taken to select species which respond well to this type of pruning and to keep up the maintenance so the fast growing suckers do not get out of hand.
A common technique many people have heard of is ‘topping’, where the top (apical meristem) of the tree is removed to a predetermined height. Topping used to be commonplace, but as more has been learned about the response of trees to these cuts it has been outlawed in most situations among professional arborists. Topping often leads to branch die back, decay and sucker growth which if left unchecked can produce multiple unstable tops. It can appear to be a cheap way to fix the problem of a tree that is too tall, but the real costs come later when the tree has develops health problems as a result.
If the rule of thirds is followed, the cuts are made carefully and only minimal amounts of foliage are removed, the tree will be left with the greatest chance of healing from it’s surgery and continuing to thrive.