The Art and Science of Tree Trimming
By Anthony Tresselt
Tree Services Magazine, July 2011
For the modern arborist, proper tree pruning is both art and science. The science is a blending of research, experience, knowledge and education. This allows an arborist to cut with the tree's health as a priority.
The art of pruning is applying scientific methods to mold a plant into a safer, healthier, more accommodating aspect of the landscape. Overall plant health remains key, but through thinning, reduction and cleaning cuts, the modern arborist can improve a plant's aesthetics and make it less hazardous without long-term detriment to plant health.
It is important to remember that pruning is a tool used to manipulate plants to meet human needs. However, all pruning should have the underlying premise of tree health in mind. Pruning that decreases plant health in the long term should rarely be considered; if the circumstances warrant that approach, removal may be the best option.
The great majority of trees and shrubs do not require pruning. Only where human needs and aesthetics overlap with the world of trees and shrubs is pruning necessary. Pruning should have a specific purpose in mind and goals clearly stated. Proper goals help assure that long-term plant health is a priority.
Pruning without overriding goals may be unnecessary and detrimental to tree health. Every time a cut is placed in a live section of a tree or shrub, the plant is wounded and its health detrimentally affected. Only with a larger goal of hazard mitigation, vista improvement, tree hygiene, obstacle clearance or any number of other valid pruning goals can this wounding be justified. Furthermore, the establishment of pruning goals guides the specifics of the pruning task. This is vital for the person making the cuts.
Seal, not heal—A solid grounding in the science of tree biology and physiology forms the foundation for proper pruning. One way trees protect themselves from the harmful aspects of the environment is through the process of compartmentalization. This is referred to as compartmentalization of decay in trees (CODIT). CODIT allows a tree or shrub to seal off decaying or diseased portions and form healthy new growth around these pockets. Pruning cuts should be made with the CODIT process in mind. Therefore, good science frowns on cuts made flush with the parent stem, cuts that rip into the branch bark collar or leave excessive branch material behind (i.e. inter-nodal cuts).
Proper pruning cuts preserve the sections of the plant that promote CODIT and are made in areas that are most likely to allow the tree to quickly seal off the pruning wound.
Natural response—The easiest portion of a tree or shrub to manage is the crown, which connects the tree or shrub to the atmosphere and sunlight, allowing the plant to make and store energy. However, this is but one-third of the total plant system. The trunk or structural stems form the framework to physically support the crown and provide a conduit for vital water and nutrient flow. The root system supports the trunk and the crown and interfaces the plant with the vital aspects of soil and water.
Alteration in one of the three pieces of the tree/shrub puzzle (roots, trunk or crown) affects the entire system. It would be inaccurate to say that a tree or shrub maintains a balance between the three pieces, but there certainly is a natural rhythm that a tree or shrub must maintain to stay alive. Good science dictates that we keep all three aspects of trees and shrubs in mind as we manage the crown through pruning.
A tree's natural response to pruning is growth. Managed growth is a good thing, however, over-pruning causes an uncontrollable and undesirable flush of new growth. This new growth occurs quickly and can form bad tree structure through misdirected and weakly attached branches and stems.
Over-pruning also reduces a plant's energy stores and its ability to store new energy. Trees rely on leaves to produce energy through photosynthesis. The sugars and starches a leaf produces are stored and used to initiate new growth. Every time a live branch is removed, the plant can produce less energy and energy stores are removed. Remove too many leaves and a tree must resort to stored energy or decline, sometimes both.
Over-pruning also affects the tree's ability to circulate water and nutrients. The transpiration of water from the roots out through the tree's foliage "pumps" nutrients and life-giving water into a tree's circulatory system. Remove too many leaves and you short circuit the "pump."
Finally, the indiscriminate removal of foliage can cause a tree to become structurally unsound due to uneven loading of branches and stems.
In the most basic sense, biological responses, such as CODIT, photosynthesis, transpiration and plant structure form the basic scientific elements of pruning. An understanding of these elements and how they work together is vital to proper pruning.
Cuts—Making cuts is the art of pruning. In general, the form of every cut is similar. Natural target pruning cuts preserve a tree's ability to compartmentalize by leaving some cells and structures behind and removing others. What differs is the goal or outcome of the cut. Remember, without goals and expectations, pruning becomes haphazard, unjustifiable wounding.
Crown cleaning—This is the removal of deadwood in the canopy of a tree. Deadwood is removed to reduce hazards. Removing deadwood also reduces the habitat for detrimental pathogens in the canopy and allows a tree to complete the CODIT process more quickly. When removing deadwood, the arborist must be sure to not damage live tissue. Trees may begin the CODIT process long before the branch dies. Cutting into live tissue to remove deadwood sets the CODIT process back and may prevent it altogether. Crown cleaning also involves the removal of branches that are diseased or in decline.
Crown thinning—This is the process of removing branches to improve light and air penetration, reduce weight and direct growth to desirable areas of a tree. Proper crown thinning is a balance of choices an arborist must make. Branches that conflict with other parts of the tree may be removed provided their removal does not cause excessive foliage loss. Structurally weak limbs may also be thinned to prevent future problems.
Crown reduction—These cuts enable an arborist to reduce a tree from surrounding features in the landscape. They also allow the overall size and height to be maintained within limits. These cuts should be no larger than one-third the size of the parent stem and retain a viable lateral. This allows proper CODIT and structural stability. Crown reduction is often a multiyear-long process for mature trees. Excessive crown reduction may lead to over-thinning and related problems.
Crown raising—Similar to crown reduction cuts, these serve to elevate the crown over landscape features. These may include sidewalks, roadways, buildings or other plants. Crown raising cuts are also often used to improve sight lines for people on the ground. They follow the basic form of crown reduction cuts and may be the most common cuts made, since they are the lowest in the canopy.
The pruning cuts just discussed often overlap in form and function. Anytime one cut can be used to accomplish multiple tasks, such as a cut that reduces and thins and cleans simultaneously, the opportunity should be taken. Accomplishing many goals with one cut limits wounding and speeds production. Both are vital concerns for the modern arborist.
Tony Tresselt, a writer, ISA certified arborist, TCIA certified tree care safety professional and instructor for North American Training Solutions, works for Arborist Enterprises in Lancaster, Pa.