Pruning Trees

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Pruning Basics: Dominant Stems, Branches, and Codominant Stems
How Trees Shed Branches
How to Prune Branches
When to Prune
Pruning Roots
Pruning for Overhead Wires
The Dangers of Topping Trees
Pruning Crape Myrtles
Repairing Storm Damage

Pruning Basics: Dominant Stems, Branches, and Codominant Stems
The main trunk of a tree is the dominant stem. Some trees have a single trunk, other trees have more than one trunk (multi-stemmed). The dominant stem is usually bigger, longer, and is obviously the dominant growing part of the tree.
true branch
A true branch is a stem that originated to the side of, and subordinate to, the trunk (think of the dominant stem as the part that grew first, and the branch as the part that developed later). True branches have special protection features for the tree, which we can use to the advantage of the trees when we prune them.
branch collar
Where a true branch comes off the trunk, a collar of trunk wood surrounds the base of the branch. This trunk wood at the base of the branch is the branch collar. In some trees the branch collars stand out and are very easy to see, in other trees the branch collars are not visible at all.

A codominant stem occurs when a leader forks into two parts. Each part shares the characteristics of a dominant stem. Usually the junction where the leader splits into codominant stems looks like a big "Y". Why is this so important? It is important because codominant stems are structurally different from branches: they lack the special protection features of the branch.

The branch collar is a very important and recognizable physical features that helps us prune properly. The idea is to imitate the way forest trees shed branches: just outside of the branch collar. 

How Trees Shed Branches (back to top)
In the forest, trees are crowded together. To survive and grow they must capture sunlight. Because the only way to get sunlight in a crowded forest is up, trees grow straight and tall under natural forest conditions. They are competing with each other for sunlight.
forest tree shedding branch
Mature forest trees typically have trunks that are free of branches almost all the way to the canopy. How do they become free of branches? As young trees grow up, their lower branches become shaded out. The lower branches die, and the trunk wood of the tree "pinches" them off. The trunk (think "branch collar") keeps growing, but not the dead branch - it decays. In just a few growing seasons the rotten branch breaks off just outside the branch collar. As the trunk continues to grow, the branch collar fills in the circle, and soon all that can be seen is a rough pattern in the bark. You can often tell where branches were shed (or pruned) just by observing the patterns in the bark.

Scientists, after dissecting many different branches and trunks, observed that the core of the branch that extends down into the trunk has special protection features. This area is known as the branch protection zone because of its ability to defend the tree against infection from decay-causing fungi through the production and mobilization of anti-fungal substances. Although the branch protection zone is inside the tree, it is defined on the outside by the branch collar

How to Prune Branches(back to top)
In the landscape, trees usually are not crowded together like they are in forests. Open grown trees spread out in all directions to take full advantage of the sunlight. Often, this growth habit causes branches to grow where they interfere with various structures or activities of people. Pruning branches has been an important management tool for people living with trees for centuries.
pruning diagram
Ideally, pruning should mimic the way trees shed branches in nature: just outside the branch collar (when there is one). There are three primary types of proper pruning cuts: pruning a true branch, pruning a codominant stem, and shortening a main leader.

To prune a true branch:
You want to make the final cut through the branch wood just outside the branch collar. You want to create the smallest wound possible, so a cut that goes straight across and leaves a "round" wound (rather than an oblong, oval, or elliptical wound) is most desirable. With big heavy branches, is it important to remove the unwanted portion of the branch in sections that small enough for you to control with your free hand. Undercut the branch about half way through, then cut down from the top side. This technique prevents a strip of bark and wood from tearing out of the trunk on the final cut. Avoid pruning when the leaves are forming.

To prune a codominant branch (the photos tell the story):

Before pruning codominant, before

After pruning codominant, after

When to Prune (back to top)
"Prune when the saw is sharp," is the conventional wisdom. Actually, there are two brief times during the year when you should avoid pruning. First, when the leaves are just forming in the spring (because it takes so much stored energy for the tree to make a new set of leaves). Second, when the leaves are changing in the fall (this is when most of that energy is being stored). Other than these two "windows" of just a few weeks, pruning may be done almost any time. Be aware that pruning certain species (maple, dogwood, beech, birch) in late winter or spring will cause sap to flow heavily from the pruning wounds. Heavy sap flow from pruning wounds DOES NO HARM to the tree - but it does make the pruner feel as if something is wrong. Sap flow will stop by itself in a few days to a few weeks. Dead branches may be pruned any time without harming the tree. It is almost always good to remove dead branches, since they are candy sticks for decay-causing fungi. 

Pruning Roots(back to top)
Sometimes it is necessary to prune roots - for underground utilities, for construction or grading, or to keep roots from growing where they may cause damage to paved surfaces. Since most roots are within the top 30 inches of soil, it is often possible to prune roots with a "sharpshooter"- type spade or a garden spade. Keep the edge sharp with a file. The important principle is to always make the final cut with a sharp instrument (shovel, saw, axe, or loppers), and to make it straight across the root. Quickly cover the pruned root back up with soil, and water the area to keep the soil moist for a few weeks. 

Pruning for Overhead Wires(back to top)
The City of Rock Hill supplies electricity to our customers. Throughout much of our electric distribution system, pole space is jointly shared with other utility companies for telephone, Internet and cable TV distribution. The City of Rock Hill strives to maintain our distribution systems, both overhead and underground, in a safe condition. Trees can interfere with these systems, and represent safety and reliability issues for the public and also for line workers.  The City utilizes contracted tree crews to maintain appropriate clearances for the electric system. Our standard for contracted line clearance pruning is the most recent version of The American National Standards Institute (ANSI) A-300, the national standard for pruning trees and shrubs. This standard is regularly reviewed at the national level and revised as necessary.

The City strives to prune large trees without leaving branches stubbed off, pruning either to a lateral branch growing away from the clearance zone or back to the trunk. Topping is an outdated practice that is discouraged for utility right-of-way maintenance. 

The Dangers of Topping Trees(back to top)
Trees have developed their unique framework over 200 million years. Think of a shade tree as an astounding engineering system capable of suspending many tons of weight high in the air. We live, work and play underneath these systems with hardly a thought.

When we top large shade trees, we sabotage the system. We create wounds that cannot heal. We remove the system's source of energy (the canopy of leaves), and force it to restart itself (resprout), relying only on stored energy to fuel the new growth. Defense against disease and decay puts a heavy drain on stored energy. It is like a person who suddenly goes from prosperity to joblessness and deep debt in a matter of hours. If there isn't enough savings (stored energy) to last through the period of unemployment (the time it takes for the tree to regain its lost canopy), the results will be catastrophic.

To add insult to injury, the new sprouts occur at the ends of the stubbed off branches, which are literally open doors to decay. Over time, the sprouts grow quickly, while the place where they are attached decays underneath them. This creates a potential hazard, and is another excellent reason NOT to top large shade trees. Better to remove the tree and start over with the RIGHT tree. Topping small maturing trees (such as Bradford pear) causes the same problems, with the exception of the degree of hazard that is created. A Bradford pear is a lot less likely to kill somebody when it fails than a large oak, elm or maple.

Pruning Crape Myrtles(back to top)
Crape myrtles are popular small maturing trees that flower profusely during the hot summer. They represent a special case because there are two opposing schools of thought on how to prune them. One theory maintains that crape myrtles should be maintained ONLY as small trees and pruned to remove low, interfering, or poorly situated branches. The other theory maintains that crape myrtles should be cut back hard every year, like a shrub, to promote flowering.

Many who subscribe to the "crape myrtle is a small tree" theory are passionately outspoken about how ill-conceived the "prune it like a shrub" theory appears to be. The term "crape murder" has been popularized and promoted by those who believe that crape myrtles should never be severely pruned.  Indeed, there is no data to show that severely pruning crape myrtles causes them to produce more flowers, so why do people go to the trouble and expense?  On the other hand, crape myrtle is such a tough plant here in the South that it can tolerate pruning to the ground and still come back the next year. The reality is that with crape myrtles, beauty is ultimately in the eye of the pruner: they make outstanding small trees, but also easily tolerate being severely pruned as if they were shrubs. 

Repairing Storm Damage(back to top)
Except for small ornamental trees, homeowners are ill suited to repairing serious storm damage to large shade trees. If you have a large tree that is seriously damaged by winds or ice, call a professional arborist to assess the situation. The yellow pages list firms that perform tree work. Look for those firms that have Certified Arborists on their staff, or who are members of the International Society of Arboriculture or the Tree Care Industry Association. Above all, don't attempt to remove large broken limbs or repair the damage yourself. Tree work can be dangerous work, even to those with the training and experience to accomplish it.
storm topple oak, park
Storm damage can be so severe that it makes for very difficult choices. Do you try to save what's left, or should you remove it? If a shade tree loses more than about 30% of its canopy, you enter a situation where certain "proper pruning procedures" (always prune at the branch collar, never leave large stubs) may not apply. In such a case, cuts that could be interpreted as "topping" cuts may actually be appropriate, if the owner doesn't want to further damage or lose the tree, and the arborist judges that the structure of the tree would be better served by performing such work. It is critical to follow up the initial repair with annual professional maintenance to inspect for insects and decay, and manage the sprouts that will eventually become the branches that fill in the "hole" in the canopy.