Making the Cut: It Matters When Pruning Trees

Pruning trees seems simple. All you need is a sharp tool and a vision of what you want the tree to look like – right? Well, that might work if you create a topiary with a shrub, but not when it comes to trees. Not only should we understand how the tree grows and its natural form and shape, but we also need to understand the impact of our pruning cuts, which wound living tissue and remove foliage.

There are three main cuts that we need to explore and understand before we get out the loppers or saws.  

Heading Cut

The first cut is a heading cut. The term heading cut can refer to two very different kinds of cuts. One version of a heading cut is a cut made back to a bud (growing point) on a one-year-old twig (a small branch). When making a pruning cut back to a bud, we can choose to redirect the new growth, depending on the bud we have chosen to cut back to. It is often best to prune to a bud that is directed to the outside of the plant, so we avoid future branches that will grow to the interior of the plant and then rub and cross with other branches. A good heading cut is applied to small branches, and training young trees where small branches are removed back to buds to redirect the new growth.

The second version of a heading cut is a cut that is made between growing points (nodes) on a larger branch, and a stub is left. This cut is problematic because the cut wound cannot close over easily when it is not made at a node. Notice we don't say the tree heals. Trees cannot heal their wounds, but rather close or seal with new wood growth over the wound. Because the tree cannot manage heading cuts effectively, the wound remains exposed, and decay will set in. Water sprout growth is also stimulated near the cut as a result of the loss of the growth hormone, auxin, produced in the buds at the tips of the removed branch. Auxin in buds is responsible for the orderly growth within a branch. Making heading cuts causes internal decay, disrupts the tree's natural form, and promotes vigorous, weakly attached water sprouts from below the cut where the rot has occurred. It also does not effectively shorten a branch because it promotes a growth response from the tree.

This second version of a heading cut, leaving a stub on larger branches, is very rarely applied to branches and generally should be avoided. If a tree has been damaged by a storm and there are no laterals to prune back to, a heading cut can be made with attention to managing the water sprouts and decay in years to come. If a tree is pruned and nothing but heading cuts are made, it has been topped or “rounded over.” This is not an effective way to prune trees because the resulting water sprouts are weakly attached, especially where decay has occurred at the cut points. Over time, these water sprouts grow bigger and heavier, and because of the decay found near their attachment points, they are more likely to fail and potentially cause harm to people or property. Also, these water sprouts typically grow fast, negating the likely goal of reducing the size of the tree or certain branches.

Branch Removal Cut

The second cut is called a Branch Removal Cut. It is the removal of a branch back to its point of attachment with a parent branch or the trunk. When the cut is made properly, the entire branch is removed back to a branch collar, leaving the branch collar intact to close the wound and seal out decay. Removing the branch, while leaving the branch collar intact, ensures that the branch protection zone (the area where a branch attaches to a parent branch, within the branch collar) does the work of walling off decay (compartmentalization) and closing the wound with new wood called callus (the first year) and woundwood (in subsequent years). Using branch removal cuts works with the tree's own natural defenses, allowing it to shed lower branches that are shaded in a forested setting, without internal decay damaging the trunk. When a branch is removed, the dead branch wood becomes encapsulated in the wood, becoming a wood knot like you may see in wood furniture or flooring. If you cut off the branch collar, making a flush cut, where the cut is flush with the bark of the trunk or parent branch, you will remove that branch protection zone and cause internal decay. Therefore, it is best to learn how to identify the branch collar and make the cut just outside of it.

Branch removal cuts have a thinning effect on the tree canopy. A branch removal cut is the preferred cut to make when possible because it causes the least impact on the health and structural stability of a tree when applied properly. 

Reduction Cut

The third cut is called a Reduction Cut. It is the shortening of a larger (parent) branch back to a smaller lateral branch, reducing the length of the branch. For a proper reduction cut, the smaller branch should be approximately one-third to one-half the diameter of the parent branch being cut. The smaller branch must be large enough to have enough energy available to effectively close the wound and reduce water sprouting. If the smaller branch is too small, the cut is more like a heading cut. There won't be enough energy available to manage the wound, leading to decay and water sprouting. When a reduction is made back to a lateral branch, there is no branch collar or branch protection zone present at the cut point, so ensuring wound closure depends on the size of the cut and the health and size of the smaller lateral branch that remains adjacent to the cut location.

Reduction cuts have a shortening effect on the tree. These cuts are used to redirect growth away from buildings, electrical conductors, or other potentially conflicting infrastructure. They can also be used to shorten or subordinate a large codominant leader, allowing one of the leaders to grow larger before the entire codominant leader is removed. As discussed with heading cuts, a reduction cut for the purposes of subordination of a branch results in a similar loss of auxin growth hormone in the buds at the end of the cut branch. The loss of the auxin managing growth in the branch will slow the further growth of that branch, allowing it to be subordinated relative to another leader, reducing or eliminating the structural challenges that come with codominant leaders.

Understanding the types of cuts used in pruning is one important part of mastering the art and science of pruning trees and other woody plants. We also need to understand the growth habits of the trees we are pruning, and the impacts of pruning cuts on structural stability and health of the trees. You can learn more about pruning by reading Pruning Landscape Trees or attending a pruning workshop.