It happens all the time. On a job or during a training course involving pruning, the question always comes up. It’s so predictable that I avoid the topic until the question is asked. It seems to stick better in the memory if somebody asks and then I answer, as opposed to just blurting the information out immediately. However, the real reason I wait is that both the question and answer are fundamental to proper pruning.

The question: How much should I prune?

Transposition

Before we can explore the question of how much to remove during a pruning session, allow me to flip your thinking a bit. Pruning requires the removal of both living and dead material from a tree or shrub. However, once cut the material stops contributing to the plant’s health. By looking at this transposition, it becomes apparent that as arborists we must focus on what to leave behind, not what to remove.

Just as Michelangelo had a heap of rubble when the statue of David was complete, you will have a pile of brush on the ground after pruning. But unlike the marble David was chiseled from, our refuse is material that was recently living. As such, it must be factored into our pruning equation. Just remember not to focus so much on the debris and forget to form a beautiful tree or shrub.

Biomass

All trees and shrubs have biomass or a measurable quantity of living material. How much depends on numerous factors. Age, species, past structural failures, past pruning and overall health, among others, combine to determine the amount of living tissue present at pruning time. Therefore, determining biomass becomes the first job of the arborist after the pruning specifications have been outlined.

This is done by quantifying in general terms the amount of living material present compared to the size of the plant, the amount of dead material present, and what the plant should ideally look like. This demands a thorough knowledge of the species and how it should look when in ideal health. Determining biomass also gives us a reference point for the next step. Think of determining biomass as Michelangelo did, making sure his initial rock was big enough for his intended outcome.

Health

Tree or shrub health is the largest single determining factor in answering the question: How much should I prune? As we are all aware, the trees and shrubs we are called on to work with often are not in an ideal location or ideal condition. Large trees in small spaces determine certain pruning actions on our part. Small trees in ill health require another set of actions.

In general, a healthy tree will respond to more intensive pruning than a less vigorous tree or shrub. There is also a hidden aspect to the relationship between tree health and pruning. Trees have reactive systems in place to respond to the environment; ill health may cause plant systems to shift to one aspect of survival.

Even same species trees next to each other can be very different in the amount of pruning each could withstand and maintain optimum health. Photo: Tony Tresselt

Even same species trees next to each other can be very different in the amount of pruning each could withstand and maintain optimum health. Photo: Tony Tresselt

For instance, a recently transplanted tree may have root loss. Tree systems may shift energy to root growth as a response. Enter the arborist. A heavy dose of pruning on this tree may shift tree resources into leaf and twig growth. A tree’s natural response to pruning is growth. With only so many energy stores available, this can cause problems; new shoot growth, a possible response to pruning, may take away energy stores better used on root development.

I realize this scenario is oversimplified and lacks many factors. However, it helps us visualize the concept of how tree health and condition should always be considered when pruning. Do not forget that a tree deals with time on a different level than we do. What may serve human needs today could adversely affect tree health tomorrow.

Outcomes

As arborists we prune for specific reasons. Sometimes it’s for aesthetics, other times safety, sometimes purely to enhance the health and future of the tree or shrub. No matter the reason, there should always be clearly defined pruning objectives. Think of the objectives as the destination, the biomass of the plant as the map, and the pruning we do as the route we take.

By determining what a tree should look like, comparing that to how it actually looks, and then establishing a baseline for overall plant health, we can begin to formulate our pruning plan to meet the desired outcome. This is Michelangelo finding a big enough rock and making sure it is sound and ready to support the carving he wanted to craft.

Here are two trees, one with good growth the other without. They are the same type of tree in a similar location, but the pruning should be different for each because there are different levels of biomass. Photo: Tony Tresselt

Here are two trees, one with good growth the other without. They are the same type of tree in a similar location, but the pruning should be different for each because there are different levels of biomass. Photo: Tony Tresselt

Rules of thumb

With all this in mind, we can agree that a healthy tree can sustain more pruning than a tree in ill health for many reasons. Newly established trees or those suffering greatly from the environment or pathogens may only support the removal of dead wood. Vigorous specimens in the full flush of rapid growth may be able to sustain significant biomass removal without detriment.

It is here that you, as arborist on the scene, must leave the realm of the general and move to the specific. Knowledge of growth habits, species, environment, likely pathogens, pruning interval, timing and many other factors will help you determine the amount of pruning suitable to the job at hand. Rules of thumb can only take us so far.

Pruning portions

We can look at one more generality. We can break down pruning amount, or portion, into three basic levels. Level one is high. This is the removal of the maximum amount of biomass the tree or shrub will sustain without being detrimental to its overall health, as well as the removal of deadwood. This may be suitable on very healthy, vigorous and/or immature plants.

The next level is normal. This is the removal of only what is necessary to maintain structure and form and/or diseased material. Crown thinning and/or crown raising cuts could be made, but only as necessary.

The last level is low. This includes only dead, diseased or damaged biomass. All other cuts would be avoided to preserve tree energy and energy supply systems. For very young trees, cuts to help form future structure can be included if they will not compromise the tree’s present health.

The answer to the question “How much to prune?” is not easy. Many variables, factors and knowledge must be combined along with the proper and timely application of them all simultaneously. Like all good questions on pruning, it points to the idea that tree and shrub pruning for the arborist is part science and part art. The science is in the proper placement and execution of cuts, proper timing and proper structure to name a few. The art consists of having a specific, appropriate outcome in mind and applying the science to achieve it.

By establishing pruning portions, the arborist can go a long way toward accomplishing the pruning objectives, maintaining tree and shrub health, and establishing well-formed trees and shrubs that coexist and complement their environment.