How to Perform Structural Pruning

In part one of this series, we learned how to make proper pruning cuts. Now we’ll learn how to use those cuts to perform structural pruning. But first, why is structural pruning so important?

Picture that we are standing in a forest where the trees are crowded closely together. Within this environment, trees must grow tall and straight in order to reach sunlight, which means their branches are spaced apart from one another and the diameter of each branch is significantly less than that of the trunk. These trees have an ideal structure that makes them strong and resilient in the face of heavy winds.

In contrast, imagine a tree growing in the suburban landscape where it has too much room to grow and does not have to compete with other trees for sunlight. Here the tree grows many trunks and long branches that are often crowded together. It does not have ideal structure and is likely to break during storms and heavy winds, which could damage homes, buildings, and cars. Then the tree will either die or begin declining from these new wounds, which results in the loss of a tree and money spent on removal and replacement.

The good news is that we can avoid these negative outcomes through structural pruning, which mimics the competition a tree would experience in the forest. We use pruning cuts to train the tree to develop a tall, central trunk with small, well-spaced branches. As a result, we reduce the possibility of it splitting or failing in various ways and increase the possibility that the tree lives a longer, healthier life.

How to Perform Structural Pruning

The best time to start this process is when trees are young because it’s easy to mold young trees as they grow and they are generally very resilient in terms of withstanding mistakes. The concepts we learn in this chapter can also be applied to large and mature trees, often to great effect, though that type of work necessitates new tools and techniques that require many years of professional training,far beyond the scope of this article.

For a young tree, we should not remove more than 40% of its live growth. For older trees, the limits would be less, generally in the range of 25% for medium-aged trees and 10% for mature trees. This can be adjusted according to the health of the tree; healthier trees can withstand more pruning, while unhealthy trees can withstand less. Whenever we are in doubt, cut less versus more. We cannot reattach a branch once it’s cut, but we can always prune more next year.

These are the three basic steps for structural pruning:

1. Encourage one dominant, upright trunk.
2. Remove the bad branches. Those are branches with a diameter greater than 50% of the diameter of the trunk and/or branches forming angles less than 45° with the trunk.
3. Create vertical and horizontal spacing between branches.

Let’s focus on the first step: Encourage one dominant, upright trunk. Before we begin, here is a video demonstrating this work:

Begin by carefully assessing the tree. Identify the healthiest, most upright trunk—we can do this by tracing an imaginary line from the base of the tree up the biggest trunk, all the way to the point at which it ends at the top of the canopy. This will be our future dominant, upright trunk.

Are there other trunks present? Are there branches that are taller than our desired trunk? If yes, remove these first using one of the two types of pruning cuts. A competing trunk may not have a clearly defined branch collar. Even if we fail to make a perfect pruning cut, young trees are highly resilient and will likely recover. This is not true for older and more mature trees, especially those in poor health.

Using the pruning cuts we learned in part one of this series, we can essentially tell the tree to stop putting growth into other trunks and branches that are competing for height dominance and instead focus growth within what we have deemed to be the main trunk.

If these actions have resulted in 40% of live growth being removed, do not continue. Give the tree one year to recover and begin the process again. If we have not reached 40%, proceed to the next step.

The next step is to use pruning cuts to remove two types of branches: those that have a diameter greater than 50% of the diameter of the trunk and those that form angles less than 45° with the trunk (the ideal branch forms a 90° angle with the trunk). These branches often have weak attachments to the tree and are the ones that tend to break as the tree matures. Find these branches and remove them or reduce them using the pruning cuts we learned earlier.

If these actions have resulted in 40% of live growth being removed, do not continue. Give the tree one year to recover and begin the process again starting at Step 1. If we have not reached 40%, proceed to the next step.

When two or more branches originate adjacent to each other within a vertical or horizontal plane (meaning up/down the trunk or around the circumference of the trunk), they will grow into each other and compete for the same space. As the tree ages and the branches become larger, these have a higher-than-average likelihood of breaking.

In this third step we want to prune to avoid this future scenario. The goal is to prune such that we create both vertical and horizontal spacing between branches.

Before making any cuts, take a moment to evaluate the entirety of the tree and form a plan. Make note or physically mark branches that we think should be removed. When choosing between branches on the same vertical or horizontal plane, always choose to remove the largest of the options. It is counterintuitive, but removing the larger branch helps reduce the possibility that this branch grows to a diameter of greater than 50% of the trunk (i.e., the bad branch we eliminated in the last step).

Begin making cuts and remember not to remove more than 40% of live growth in total (factoring in all previous steps).

Congratulations! We have structurally pruned a tree and helped it become more structurally stable and better able to live a longer life.

Here is a general structural pruning cycle I recommend:

1. Structural pruning every year for the first 15 years. If this is not feasible, move to every other year.
2. Continue every two to three years during the next 15 years of the tree’s life.
3. If this is not feasible, every three to five years is acceptable, though not ideal. In this instance, perform at least four pruning cycles across 20 years.
4. Beyond this, perform structural pruning on an as-needed basis (e.g., to reduce branches growing into pockets of sunlight above the roof).

For trees that are already mature and have never received structural pruning, it’s not too late. Although we cannot make as much of an impact as when we start early, we can still improve a mature tree’s strength and safety, thus providing it with many additional decades of life. In this instance, I recommend hiring a trained professional like Leaf & Limb to perform structural pruning every two to four years for at least six pruning cycles.

Congratulations! You've now mastered the art of structural pruning. Remember, if you ever need professional pruning help, please don't hesitate to reach out to us for a free quote!