Chapter 12: Perform Structural Pruning to Increase Stability—It’s the Only Pruning That Matters

If we want to help heal Earth, planting and saving trees is important. But we cannot stop there—we also want to help those trees live as long as possible. To achieve maximum longevity for trees in the sub/urban landscape, the two most important things we must do are maximize their structural stability and health. Aside from avoiding harm, like construction in the CRZ, the best ways to do this are pruning to create ideal structure and improving the health of the soil in which they grow.

Ideal structure and soil are the juggernauts of tree longevity in the sub/urban landscape. Let’s start with structure and then we will focus on soil in Chapter 14. Having poor structure increases a tree’s susceptibility to being split or toppled during storms and high wind events. When this happens, it often mortally wounds or kills the tree and often causes property damage. We can reduce this issue through the act of structural pruning, which is the only type of pruning that matters in terms of caring for trees and helping them live long lives in the sub/urban landscape. There are other types of pruning, such as creating clearance from a nearby building, removing dead branches, and generating more space between the ground and the lowest part of the canopy (this is often called raising the canopy). These and other practices may achieve utilitarian and aesthetic objectives, but they do not increase the longevity of the tree. In some cases, they can actually create harm, especially when improper pruning cuts are utilized.

Structural Pruning—What Is it?

Picture that we are standing in a forest. The trees are crowded closely together. Within this environment, trees must grow tall and straight in order to reach sunlight. They must be efficient with branch growth, which means most of the 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. This is the natural environment for most trees.

In contrast, imagine a tree growing in the sub/urban landscape (or maybe you can look out your window to see one right now). Most trees growing in sub/urban spaces have too much room to grow and do not have to compete with other trees for sunlight. This is not a natural environment for most trees. They grow many trunks and long branches that are often crowded together. They do not have ideal structure and are likely to break during storms and heavy winds. If this happens, the falling branch or tree could damage homes, buildings, and cars. 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. All of these are outcomes we want to avoid.

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. In a way, this is using pruning cuts to biologically program a tree. Of course, we cannot control it or tell it what to do, but we can certainly influence how it grows.

By providing our tree with structural pruning, we reduce the possibility of it splitting or failing in various ways. It keeps our property safer and ensures the tree lives a longer, healthier life. It is for these reasons that I rank structural pruning as one of the top two most important ways to care for trees in the sub/urban landscape.

The concept of structural pruning is often confusing at first. This video helps clarify the important points:

Before I teach how to perform structural pruning, let’s quickly divert into a bit of tree biology, which sets the stage for how and why we perform the cuts.

A Bit of Basic Tree Biology

Tree bark serves as an outer layer of defense to keep harmful wood-decay fungi from entering the tree. Once the bark is gone, which happens with any pruning cut, the inner wood is exposed and wood-decay fungi are able to enter the tree. Once they enter, they digest the moist, live wood and cause it to rot. If the tree cannot stop this attack, the fungi will spread, causing further rot, holes, and hollows. These may lead to falling branches, splitting trunks, and death of the tree.

In the forest, trees naturally perform self-pruning—the fancy term for this is cladoptosis. They do this in a way that prevents wood-decay fungi from killing the tree. When a branch is weak or not generating enough food, the tree will begin slowly shedding that branch by cutting off its supply of fluid and nutrients. The branch begins rotting and eventually falls. The tree grows over the area where the branch was attached to the trunk and creates a permanent wall of bark that blocks invading wood-decay fungi (as with everything in this book, I’m simplifying this process for ease of readability).

This defense process is driven by something called the branch collar. The branch collar is located at the base of a branch where it meets the trunk (or a larger branch). The branch collar consists of overlapping layers of tissue that create a strong attachment point with the trunk. There are thin barriers called protection zones between the layers of tissue that help prevent wood decay fungi from entering the trunk. This is a tad technical—the point is that the branch collar plays an essential role in protecting trees from wood-decay fungi during cladoptosis and after.

By carefully studying how trees naturally prune themselves in the forest through cladoptosis, we arrive at a crucial point: Do not damage the branch collar when pruning. If the branch collar is damaged or removed during pruning, the tree will struggle to protect itself from harmful wood-decay fungi and may die. In the process, it will be more likely to break and cause damage. Next let’s learn how to make a proper pruning cut.

Cut #1: A Proper Pruning Cut That Does Not Damage the Branch Collar

Before we begin, I want to acknowledge that the idea of pruning may feel overwhelming, especially when compared to the simplicity of planting a sapling or choosing not to remove a mature tree. I want to put your mind at ease: we will be working with young trees where it’s ok to make mistakes because they are very resilient. We will focus on small branches using basic tools and simple processes. This is a fun and engaging process with visible positive outcomes!

We will need the following tools:

• Bypass pruners: use these for cuts on branches that are less than ⅓-inch in diameter.
• Loppers: use these to make cuts on branches from ⅓-inch to 1 inch in diameter.
• Hand saw: use this to make cuts on branches 1 inch in diameter and larger.
• Optional: Pole saw and pole loppers. These are great for cutting branches out of reach.

Let me add two important warnings before we dive in:

• Wear personal protective equipment required for each tool. I recommend eye and hand protection at a minimum. Wear a helmet when cutting branches overhead.
• Work from the ground on safe, flat surfaces only. Do not work from roofs, ladders, and other sloped and unstable surfaces without professional training and safety equipment. These are dangerous places.

To begin, we must first identify the branch collar. The branch collar is a bulge at the base of the branch where it meets the trunk (or a larger branch). With practice it will become easy to locate the branch collar. Whenever we make a pruning cut, the goal is to make the cut immediately adjacent to the outside of the branch collar. We do not want to remove or damage the branch collar, but we also do not want to cut too far away from the branch collar. Both make it hard or impossible for the tree to create protection against wood-decay fungi.

For branches less than 1 inch in diameter, we use bypass pruners or loppers to make the cut outside of the bulge, on the side furthest from the trunk (or larger branch). For any branch that is larger than 1 inch in diameter, we should use a hand saw to make the pruning cut. But be careful! These saws are sharp. Make sure hands, arms, and legs are not behind the direction of the cut—we do not want to accidentally slice our body after the saw breaks through the final bit of wood. It hurts!

When we use a hand saw we encounter a new issue: as we are cutting the branch, it could fall and peel bark from the underside of the branch collar and the trunk below where the branch attaches. This creates a serious wound that leaves the tree susceptible to attack from wood-decay fungi. We must avoid this or risk killing the tree. To do so, we perform something called the 3-Cut Method. Here is how to perform the 3-Cut Method. Before making the cut, let me first show what this looks like in video format.

We make the first cut under the branch to keep it from peeling bark from the underside. This cut should be made approximately 2–3 inches from the branch collar. With the first cut in place, we move our hand saw beyond this cut, to approximately 4–5 inches from the branch collar. Make this second cut starting from the top of the branch. Please note that the goal of these first two cuts is to reduce the weight of the branch without peeling bark, thereby freeing us to make a very precise final cut. The third and final cut must be made immediately adjacent to the outside of the branch collar. Do not damage the branch collar. Using a sharp saw will help make a great cut.

Congratulations! Now we know how to make a proper pruning cut. Let’s learn one more type of cut before we dive into structural pruning.

Cut #2: A Reduction Cut

A reduction cut is one where we reduce the end of a main branch back to a smaller, side branch (the fancy term is a side lateral). That remaining side branch should be at least one-third of the diameter of the main branch so that it can sustain life for the remainder of the branch.

For small branches less than 1 inch in diameter, use bypass pruners or loppers to make the cut. For any branch larger than 1 inch in diameter, use the 3-Cut Method. Again, the goal with the first two cuts is to reduce the weight of the branch, thereby freeing us to make a very precise final cut without having to worry about peeling bark. For reduction cuts, the third and final cut must be made at a 45° angle to the side branch and should not damage the branch collar of the side branch. This video helps explain this point:

How to Perform Structural Pruning

Now that we know how to perform these pruning cuts, let’s learn how to apply them to perform structural pruning.

The best time to start this process is when trees are young, which I’m arbitrarily defining as anything between 1 and 10 years old, or thereabouts. 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 we need new tools and techniques that require many years of professional training and are far beyond the scope of this book.

In terms of timing, the best time to prune is during the dormant season. 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 and 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. MOST IMPORTANT: Encourage one dominant, upright trunk.
2. Remove bad branches, which are those that:
   • have a diameter greater than 50% of the diameter of the trunk.
   • form angles less than 45° with the trunk.
3. Create vertical and horizontal spacing between branches.

Step 1: Make Cuts to Encourage One Dominant, Upright Trunk

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 these cuts, 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.

Step 2: Remove Bad Branches

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.

Step 3: Create Vertical and Horizontal Spacing Between Branches

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. Especially 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.

For those wanting to learn more about this topic, Dr. Ed Gilman is the preeminent expert on structural pruning. I highly recommend all his books, online articles, and videos.

Step 4: Repeat Structural Pruning Annually

I recommend repeating this process for as long as we are able. At some point, it is likely that the tree will be too tall for us to continue and we will need to hire a trained professional (see the sidebar for help on choosing the right service provider).

Here is a general structural pruning cycle I recommend:

• Structural pruning every year for the first 15 years. If this is not feasible, move to every other year.
• Continue every two to three years during the next 15 years of the tree’s life.
• 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.
• 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 stability and safety, thus providing it with many additional decades of life. In this instance, I recommend hiring a trained professional to perform structural pruning every two to four years for at least six pruning cycles.

Before we discuss soil, let’s take a quick pruning-related diversion.