Pruning Trees for Form and Function

Pruning a tree is a contentious topic because of its inherently destructive nature. Knowing even the basic mechanics of tree biology and proper reasons to warrant cutting a tree’s branches is fundamental in making the decision(s) to cut. Throughout this synopsis, I will be covering basic Biology and Mechanics of all trees, as well as the implications and hazards of natural growth and pruning.

BIO 101 and Definitions

Cambium: A thin layer between the xylem and phloem of most vascular plants that facilitates growth of new cells and responsible for secondary growth.

Xylem: Plant vascular tissue that transports water and minerals from the roots of the plant to the rest of the structure.

Phloem: Facilitates movement of photosynthates (byproducts of photosynthesis) from leaves to roots, flowers, and fruits.

Bark: The tough outer layer that protects and seals the delicate inner layers from exposure.

Sap: A vital fluid that circulates through the vascular system of a tree, facilitating many essential functions; transportation, energy, protection, and food.

Heartwood: This is the inner most (core) wood that makes up the bulk of the tree’s mass. It is surrounded by the sapwood, xylem and phloem. This portion of the tree is no longer living, but kept intact by the many layers that surround it.

Mechanics

Everyone knows that trees produce and contain sap, essentially the tree’s blood and immune system. It is a very passive and effective method the tree uses to seal itself from the outside and for transportation of nutrients from root to the uppermost canopy. When a cut is made to the tree in a way that exposes the sapwood or other “woody” portions of the branch, sap will release around the wound nearly instantly. This action prevents pests and bacteria from entering the wound, and will eventually aid in the cut portion being healed over.

While trees don’t “heal” in the sense that we think, they do conceal. In that, making a proper cut at the correct location would allow the tree to grow over the cut branch, and thus concealing it under a layer of new bark eventually. That wound will be carried within the tree for the rest of its life, and will be evident when the tree eventually dies or is cut open.

This is precisely why we never (almost) “Top” cut trees (heading cut). This method of cutting involves removing a portion of a branch at a location usually not able to facilitate healing. The cut could be made anywhere along the branch, regardless of node placement or terminal buds. Because of this, new growth will vigorously sprout from these cuts, causing a bushy appearance and many new branches to appear.

Topped trees grow many epicormic sprouts like cutting the head off a hydra, this causes a quick increase to weight and leads to poor branch formation.

In the illustration above, you can see the tree was initially “shaped” in a way that was conducive to the home owner or tree trimmer trying to fit a tree in a given space. This often happens beside homes and under power lines, and for good reason. However, it is never advisable to top a tree in this way. Ideally, a more suitable tree would have been planted in the allotted space in the first place, but this is hindsight and much easier said than done.

After the topping cuts have been performed, the tree will immediately send signals to sprout new shoots, these epicormic sprouts, often called “water sprouts” will provide the tree with lush leaves to facilitate photosynthesis. However, it is most likely that none of them will be able to develop into a strong limb or branch, capable to supporting much weight. And due to the sheer number of sprouts that will grow and leaf out in such a short period of time, the existing limbs become stressed with excess weight, causing a further failure and potential injury or damages to property.

Topping a tree could also be interpreted as a “hard pruning” such as what may be done to bushes, roses, and some fruit trees. In these cases this method of pruning could be satisfactory and even advisable if for instance; a fruit tree only produces fruit on new growth. You could see how this method would end up producing more fruit each year following a topping in the dormant season.

Reduction cuts including “Crown Reduction” is the method of reducing the overall size of a tree by selectively cutting a branch or stem(s) in places that will allow a subordinate branch to take over. This method is ideal for a tree that is overgrowing its allotted space, but removal is not warranted or necessary. An example of this approach would be to start at limb’s end that you find to be too long or overgrown, maybe even too heavy, trace the tree back to a point that is about the desirable length. Finding a crotch or junction of two branches, pick one that is ideal to keep, ensuring that it is at least 1/2 to 1/3 the size of the removed wood. This lateral branch will now become the new leader and all further branch development will stem from this chosen spot. Once reduction cuts have been made, it is usually the case that further reductions will be needed to be performed throughout the tree’s life. Typically, this would be two or more years in between each reduction cutting.

This illustration shows a leader being brough back to a branch, and a cut made properly after the bark branch ridge that joins the two.

The branch bark ridge is an important term when it comes to pruning a tree, because it should dictate the location and angle of any cut that would be made. Essentially, it is the raised area of bark in the branch crotch that marks where the branch wood and trunk wood meet. This union is the ideal space for pruning a tree, because it allows one to cut the branch only, leaving the trunk wood untouched and unexposed. Immediately upon cutting, the tree sends a strong chemical signal to begin the healing process by covering the wound. The branch wood will heal over and become concealed in time, but the trunk wood as we learned earlier is technically dead. If this wood becomes exposed to the atmosphere, pests such as termites, boring beetles, birds, fungi, and bacteria may all be welcomed within. These opportunistic organisms will wreak havoc on the tree, and the effects will usually spread throughout, potentially causing death of a healthy tree. This is why it is so important that care and precision be implemented when hiring a professional to trim your trees.

When a cut is made that is parallel to the stem, essentially cutting off the branch collar, this is called a “flush cut”. It is problematic in the sense that it causes the cut’s surface area to be too large, and unnecessarily so. The larger the wound on the tree, the more time to heal and opportunity pests will have to infect it.

When any portion of a tree is removed abruptly, this always has a resounding impact on the overall health of the tree. The wood that was removed was full of sap, and usually contained leaves that allow photosynthesis. This is the method of food production, in which the leaves take in carbon dioxide and sunlight, and release oxygen and water, while producing sugars to feed the growing cells and produce sap.

Because of these implications, it is never advisable to remove more than 20%-30% of any tree’s growth. Imagine if someone took your limbs, or removed a portion of your blood, that is essentially what is happening; the tree requires time to readjust between pruning sessions. New growth needs time to replace what was lost and build up the sap stores that were abruptly removed.

Trees go dormant during the winter months and early spring, before waking again to a new growing season. While they are dormant, they cease growth to conserve energy and protect themselves from adverse conditions and drought. While dormant, trees may need watering much less frequently than during active growth, and especially less during heat intense months. Here in the Sonoran desert, we typically water established trees about once a week in the scorching summers, where temperatures can go above 100 degrees for months on end. But during the winter months when the tree goes dormant, a single deep watering just once a month may be adequate to keep trees thriving and alive. Overwatering is actually a leading cause of death of trees, both young and old.

When trees are dormant, it is the ideal time for pruning, not just because the tree is already fortified for survival, but because of pest pressure as well. Simply put, there are far less things that grow or eat trees and wood during the colder months. Most insects are dormant as well and not seeking to infest new wood, as well as bacteria and fungi will have a more difficult time thriving in the adverse conditions.

Targets for Pruning

Now that we have covered a lot of what a tree does, and how to go about pruning; let’s go over what we should be pruning. You shouldn’t prune a tree because of how it looks, the natural way a tree grows is usually optimal for its health. What we do need to look for are dead branches, crossing branches, co-dominance, and less than ideal structures.

Dead branches are very self explanatory and are usually easily spotted. They are withered, will not contain leaves or new (green) growth and will snap when bent. These branches can always be removed safely, with little to no harm to be done to the tree. Even when dead, a large branch can be heavy; because of this, remove larger branches in smaller sections to avoid all the weight of the branch pulling bark from the stem or leader upon dropping. This tear can sometimes happen by accident and when severe enough, opens the heartwood to exposure.

Crossing branches are two, often competing branches that will rub against one another in the wind, and eventually grow to intertwine. Rubbing of any sort on a tree is detrimental to the bark at the site, and will eventually cause its removal. If the branches are allowed to grow so thick that they can no longer move, they will interlock and continue to grow around the other. This will strangle the xylem and phloem eventually and not allow the uptake the nutrients, the branch(es) will die. When faced with this dilemma, choose the branch that is most ideal for gathering of light and will maintain the overall structure of the tree.

Co-dominant branches are two or more branches that are in direct competition of each others’ space. In most cases, these competing branches are in search of more light. More often than not, competing branches will be located in the middle of the canopy, often growing from a lower spot and reaching high. Many varieties of trees will produce many trunks, called co-dominant leaders. For some, like Mesquites and most other desert native trees, this is normal and should need minimal correction. For other trees, it will eventually cause the trunk to split, usually in a storm. This is most common in Palo-Verde trees of the “Desert Museum” variety. This is a cultivar that was made through breeding more than one species into a fast growing, thornless variety. While convenient and easy growing, the trees often fail in wind storms and need entire removal or large portions removed to maintain structural integrity.

Structure of a branch can be subjective, just like everything else regarding trees. But ultimately, it should come down to the well-being of the tree, not the aesthetic or curb appeal it provides. What I mean by “structure” would be how the branches connect and the overall shape they make in the canopy. A branch sprouting from the stem at ninety degrees will not be as strong as one growing at an upward 45 degree, and should be considered for removal before it develops into a limb. By carefully selecting the healthiest branches and limbs that will provide the best chances for optimal growth within its environment, pruning your trees should be a medium to long term solution for optimal health. When considering if pruning is necessary, please refrain from shaping your trees like you see on T.V. or in towns and parks. A tree should never be shaped, “lions tails, and lollipops” are not natural forms for trees to take and simply stress it further.

Written by Justin Tornatore

Citations:

University of Arizona - Cooperative Extension

Pima County Smartscape - Training Program for Landscape Professionals

International Society of Arboriculture (web resources and study material)