This is the first of a two-part series intended to provide essential information on making well-informed pruning cuts, based on the current standards. Part one will focus on the act of making the pruning cut and the types of cuts utilized in managing tree canopy. Part two will concentrate on what to prune, along with the proper timing.

Trees continue to survive in spite of the many challenges they face in the urban environment. We often place trees in less-than-favorable growing locations that don’t allow natural development and maturity, and they often require pruning to develop a durable structure, improve clearance and maintain aesthetics.

Reasons for pruning

Let’s begin with why we want to prune a tree in the first place. The most common reasons typically include aesthetics, structure and reducing risk. Sometimes pruning is necessary to remove dead or dying branches or those affected by insect damage or disease. This helps defend against the spread of the pest and prevent further damage. Also, pruning can increase the vitality of the plant to improve flowering and fruit production.

Another important reason to prune is to reduce the risk of tree failure. This includes removing defective branches on a declining tree or branches damaged by a storm. Risk reduction and the improvement of tree stability are important pruning objectives.

Pruning principles

Any arboricultural practice – including pruning – should not damage or impair the health of the tree. Proper technique and timing are critical to long-term tree health.

The most important principle to remember is that each cut has the potential to change the tree considerably. Pruning trees should not be a common practice used to force them into aesthetic constraints or spaces.

Another key principle is the pruning dose, or the amount of green-tissue mass removed during any single pruning episode. Pruning amounts will depend upon previous pruning cycles and pruning objectives. If extensive pruning is needed, consider extending the pruning over more than one pruning session during multiple growing seasons. Removing dead, damaged or dying tree parts doesn’t figure into the mass when calculating pruning dose.

The tree’s response

So, how does a tree respond to pruning? Trees are complex organisms that respond to pruning in development, root growth and quantity of leaf tissue produced. In simplest terms, pruning creates potentially serious wounds in the tree. However, pruned properly, a healthy tree can completely recover from the wounds caused by pruning cuts. It is important to make proper cuts that allow callus growth to begin to close the wounded area. Each pruning cut requires valuable resources from the tree for healing. The larger the cut, the more time and resources are required to recover. Small cuts always are better than larger pruning cuts. The smaller cuts minimize the amount of tissue exposed to pathogens and expedite the healing time.

Research suggests that in trees that seal poorly (such as maples, birches, poplars and crab apples), pruning cuts should be no larger than 2 inches in diameter. On trees that are better compartmentalizers or better at sealing off wounded areas (most oaks, elms, lindens and hornbeams), 4 inches in diameter should be the maximum-sized branch removed. Limiting the size of the wound better enables the tree to seal the wound. If larger branches need to be removed, consider a progressive pruning cycle. This makes a great case for structural pruning of trees while they are still young and relatively small (more on this later). Wound size and efficiency of the tree’s ability to seal the wound are critical for long-term health.

Pruning objectives

There should be a purpose with every cut, a purpose based on distinct needs or objectives determined by the tree owner or manager and the arborist.

Structural pruning of a young tree is a holistic approach to tree care early in the life cycle. It is the best pruning practice for tree longevity, as well as an economical approach to maintenance. It is far easier and cheaper to prune a younger, smaller tree than a tree that is mature, more sizable and complex. Strong, stable trees should be the goal of any sustainable planting – and this begins with properly trained trees, pruned to improve branch structure and crown development. Structural correction when the tree is young is the best way to reduce risk. However, trees of nearly any developmental age can benefit. This proactive approach to correcting structural faults helps reduce risk and potential failure later as the tree matures, rather than waiting until the tree presents a risk situation.

Maintaining health focuses on reducing tree risk and improving appearance. The process involves pruning to remove dead, dying, diseased, broken or poorly attached branches; crossing or rubbing branches; and, perhaps, adventitious shoots, if too numerous. The primary objective is removing non-beneficial plant parts but minimal live tissue. This is a common pruning practice that not only improves appearance, but also may improve health. Another important objective is to reduce potential risk of failing branches.

Risk mitigation is used extensively by arborists after storms have damaged trees, and it includes selective pruning using proper cuts to repair the damage. This is a common pruning strategy, and is often a regular maintenance procedure in public spaces.

Size management decreases the overall size of the tree and is usually an attempt at making a too-large tree fit into its location. Often, this is the result of a poorly placed tree in a location that cannot allow natural, mature growth. The application of removal and reduction cuts can accomplish objectives such as line-of-sight issues, utility vegetation management or reducing loads to assist with risk mitigation.

Canopy elevation accommodates pedestrian or vehicular access, structural conflicts, line of sight, safety or appearance. Removing the lower tree branches is an important pruning process requiring some knowledge of tree growth. The lowest branches remaining will be the lowest branches on the tree as it matures; proper selection is critical. If the tree is too small to raise to the desired height, a gradual elevation will be required over a period of several years.

Types of cuts

The tree’s response to pruning can be anticipated based on the type of pruning cut. Most plants respond in much the same way to pruning. So, if you understand the responses, you can choose the best cut for the situation. There are three types of pruning cuts: reduction, removal and heading.

Reduction cuts shorten a limb by removing the terminal portion back to a lateral branch of equal or smaller diameter. The cut should be made just beyond the lateral branch, and the remaining branch should be one-third to one-half the size of the branch removed. This remaining branch will then assume the very important terminal role for support and survival.

Removal cuts eliminate a branch back to the trunk or a primary stem just outside the branch collar or branch bark ridge, if the branch collar cannot be identified. For this pruning cut, the part of the plant that remains must have a larger diameter than the part that was removed.

Heading cuts sever shoots or branches from the current year’s growth or branches less than one year old. The cut reduces the length of a stem or branch back to a point without regard to the nearby lateral branches, and takes the branch back to a bud or a node. This also describes cutting an older branch or stem back to any size lateral branch. Often, these cuts are used when restoring trees following storms or when reducing trees where there are no suitable laterals branches.

A detailed description of the application and how to make these pruning cuts can be found in the ANSI pruning standard, ANSI A300 (Part 1) – 2017 Pruning. This is a newly revised standard and bestmanagement practice that should be essential reading for every arborist.

Pruning technique

Proper technique is essential for recovery, health and aesthetics when pruning trees.

The first step is to identify the key components of the branch. This requires careful examination of the branch attachment to identify two very important components: the branch bark ridge and branch collar.

The branch bark ridge is a raised strip of bark at the top of the branch union where the growth and expansion of the trunk or parent stem and adjoining branch push the bark into a ridge structure. This typically is present on every branch union and is an important identifying feature for determining tool placement.

The branch collar is the area where a branch joins another branch or trunk that is created by the intermingling of vascular tissues from both the branch and the stem or trunk. It is typically a slightly swollen area just outside the branch bark ridge and wraps around the stem at the base of the branch. Collars only develop when the branch is much smaller than the parent branch. However, these branch collars are not always present, especially on codominant branches and stems. Many branch bases lack visible collars.

The combination of the branch collar, branch bark ridge and the overlap between the branch and stem are the physiological components that form what is sometimes called the branch protection zone. This zone contains specialized chemical compounds that help resist the spread of disease in the tree and facilitate wound sealing. If the branch collar is damaged or removed, as in the case of a flush cut, the branch loses the ability to defend against invading diseases. As a result, disease organisms are more likely to invade the wounded area and cause decay.

How to make a pruning cut

The practices used for pruning depend on the size of the branch to be cut, whether or not the branch is safely and easily supported by one hand while cutting and whether a simple, single cut can be made with hand pruners, loppers or a handsaw. Branches that are too large to be supported by hand should be removed using the ternary method to avoid tearing or splitting the bark and damaging the branch protection zone. It was formerly called the “double-cut,” which is a misnomer, because it actually takes three cuts to finish the process rather than two, as the name implies. Arborists now refer to this pruning cut as the “tri-cut” or ternary method.

In the ternary method, the first cut, called the undercut, begins on the bottom of the branch anywhere from 6 to 12 inches away from the branch union. The second cut, called the top cut, is made above or just outside of the undercut; proceed with the saw from the top of the branch moving downward. This is the pruning cut that allows the branch to be cut away completely. As the saw moves through the wood, the branch will naturally fall as gravity takes over. This top cut will soon meet the plane of the previous undercut, stopping it and preventing the bark from ripping. After both cuts have been made, the branch should easily fall and be removed. However, the job is not finished! Make the third and final cut just outside the branch bark ridge and the outer portion of the branch collar on the bottom side of the attachment. Now that a proper cut has been made, let the sealing begin!

Poor pruning cuts, leaving rips, stubs or flush cuts, create many issues detrimental to recovery. Pruning without damaging the branch collar and branch bark ridge encourages the formation of a callus that seals the wound and protects the tree. Never “flush-cut” a branch, because that removes the tree’s ability to recover quickly and effectively. Also, never leave the stub behind outside the branch collar area. This leaves a woody material with no support from leaf tissue; it will soon decay and provide a conduit for disease to spread into the remaining branch or stem.

It continues to be accepted that tree-wound dressings are not needed on pruning cuts and provide no benefit to the tree. In fact, many dressings inhibit closure of the wound and slow the sealing process. Many of these are petroleum-based products, which can kill the cells responsible for callus development and wound closure.

Lindsey Purcell is an urban-forestry specialist in the Department of Forestry and Natural Resources at Purdue University in West Lafayette, Indiana. This article is based on his presentation on the same subject at TCI EXPO 2019 in Pittsburgh, Pennsylvania, last fall. To listen to an audio recording of that presentation, go to this page in the digital version of this issue online, under the Publications tab, and click here.

For additional information on tree pruning and response, visit the Purdue Education Store online.