Tree pruning: what to avoid and how to do it right

Pruning is an important part of tree care, but it’s only useful when done correctly. Improper techniques can irreparably damage trees, reducing lifespan and increasing the risk of branch or trunk failure.

Well-trimmed trees can provide years of shade and enjoyment for their homeowners and neighbors. Proper pruning reduces the risks associated with trees; eliminates conflicts with buildings, streets and walkways; improves its structure; and makes them more attractive. A good arborist always strives to achieve desired goals while causing minimal damage to the tree. In most cases, a good pruning job will remove only dead branches and a minimum of living tissue. When unusual circumstances require more live branches to be removed, a good arborist will try to schedule the work to be done during down periods and spread the work over several seasons when possible.

To help keep your trees strong and healthy, avoid the following practices:

Incorrect Practice: Topping

Topping is when branches are cut off abruptly, leaving large stumps. This practice, sometimes called “hatracking”, is especially common on Crape Myrtles, earning it the nickname “crape murder”.

A common myth is that trees sometimes get too tall and need to be topped to make them safer. In fact, the long-term result of topping is to make trees less safe. New growth from top cuttings (also called head cuttings) tends to adhere poorly. There is also more extensive decay at the site of the course breaks. As the new branches grow, they often break off the tree and fall.

Sometimes it is prudent to reduce the size of the trees. Some species are fragile and tend to overextend. Pecan trees are a good example. In addition to developing long, arching branches, they can become very heavy when the fall nut harvest is at its peak, and branches commonly snap under these loads.

The correct way to reduce the crown of a tree is to reduce the branches at the tips. By making proper pruning cuts that shorten the longest branches, we can reduce both the weight of the branches and the amount of area that will be caught in high winds. If you imagine yourself holding a small dumbbell or other weight, it’s much easier to hold it close to your body than it is at arm’s length. This same principle applies to trees.

Removing a small amount of tension at the end of a branch makes a big difference in the amount of tension acting along its entire length.

Incorrect Practice: Lion’s Tail

Also called “poodle-dogging” or “stripping out,” lion’s tail is the removal of a large part of a tree’s inner growth. This is sometimes called “removing suckers” by uninformed tree workers. In fact, the inner branches are not sucking anything from the tree; they are doing the opposite. Each leaf on a tree generates energy from sunlight through a process called photosynthesis. This energy is transferred throughout the tree, where it is stored in the roots and woody tissue in the form of starches and sugars. These stored compounds help the tree survive stressful times, such as drought or soil compaction.

When the inner branches are removed, the tree loses some of its ability to produce this energy. But there are more reasons to avoid lion’s tail. When all the inner growth is removed from a tree, the bark is suddenly exposed to sunlight. This can lead to sunburn, often causing the bark to die and exposing the interior wood to decay. The result is weaker limbs that are likely to break years later.

Also, small branches support large branches. The diameter of a parent branch will be greater in front of the lateral branch than beyond it. The greater the taper of a branch, the stronger it will be. When all the leaves and side branches are at the ends of a long branch, there is very little taper and the branches are more prone to breakage. Because all of the weight and wind load is pushed towards the ends, they have a greater effect on weak points in the limb, making them even more prone to breaking.

But that is not all. When the inner branches are retained, they dampen the effects of wind movement on the parent branches. Each side branch dissipates a bit of energy, so more side branches means more damping. Again, the parent branches are less likely to break.

Finally, ingrowth offers an opportunity when extreme weather, mechanical damage, or some other cause breaks a limb. Rather than having to make a course cut that will cause problems in the future (see “Topping” above) or bring the whole branch back to its point of origin, we can often salvage part of the broken branch by cutting it back to a inner member.

So the proper way to prune a tree is to retain as many interior living branches as possible. We try not to remove green fabric unless it is torn, hanging too low over the street or sidewalks, or causing damage to structures below. There are exceptions (see “Topping” above), but a good arborist knows that a successful pruning job will mostly result in dead branches going to the chipper or brush trailer at the end of the day.

Bad practice: excessive pruning

Continuing on the same note, removing too much living tissue is bad for trees, regardless of where they come from. Industry standards recommend never removing more than one-third of a tree’s live branches in a single season. Prudent arborists never attempt to remove more than one quarter. When we take in more than that, the tree loses a lot of potential for energy production and large amounts of stored energy (in the form of starches and sugars). At the same time, he must expend energy to seal the numerous wounds created. Meanwhile, the tree must continue to support the remaining branches and roots. The tree is forced to rely on stored starches and sugars, depleting the reserves. A tree that is over-pruned becomes more susceptible to dying as a result of external stressors, such as drought, insects, or disease.

The correct method is to avoid removing more than a quarter of the live crown of a tree in a year. If larger amounts must be removed, it is best to gradually reduce the target limbs over two or more years. When large amounts of green tissue must be pruned from a tree, it is best to wait until the tree is dormant in the winter, or near summer. After severe thinning, a tree should be allowed to recover for at least two years before any further pruning is done.

Bad Practice: Overload

“Lifting” the top of a tree, also called “limbing,” is a necessary part of a tree’s life in urban settings. Branches that are too low can damage cars, houses, or other structures. When low branches spread out over a street, passing vehicles can rip them off the tree, which is much more devastating to the tree (not to mention the vehicle) than a pruning cut. When the tree is low on a sidewalk or lawn, people can be injured and the lawn can suffer.

Although good arborists routinely raise low crowns, it is important to avoid doing too much at one time. A good rule of thumb is that, viewed from a distance, the bottom third of the tree should be the stem and the top two-thirds should be the canopy (leaves and branches). Having more low branches isn’t a problem for the tree, but having fewer means the trunk will have less taper, making it more prone to breaking (see “Lion’s Tail,” above). Also, lost leaf area will lead to decreased energy production, forcing the tree to rely on stored energy reserves (see “Overpruning,” above).

When the lower branches of a young tree conflict with the above guidelines, the correct way to approach the problem is to cut back the problematic branches gradually over two or more years. By moving the ends of the lower branches to the side, we retard the growth of that branch and the tree sends more energy to the higher branches, but the lower branch still contributes to the development of a strong stem taper until it is finally removed. . . As the higher branches develop and take a more dominant position on the tree, the lower branches are often shaded and die back naturally, which is much less damaging to the tree.

Bad Practice: Flush Cuts

When removing a branch from a tree, it is very important that it is done correctly. A common myth is that we should cut the branch as close to the stem as possible so that the tree can heal the wound faster. In fact, this practice is devastating to trees. When a new branch sprouts, its parent forms a special tissue around its base called a branch collar. This tissue swells around the new limb in the same way that stream water swells around a large rock. Flush cuts remove this tissue, compromising the tree’s ability to grow new wood on the outside of the wound. In fact, some flush cuts never close completely, exposing the interior wood to decay and disease-causing organisms. Often some tissue will eventually cover the wound, but not before cracks have formed in the wood. These cracks are hidden from view, and sometimes even the best arborists can’t see the defect until years later when the branch breaks.

The proper way to prune a branch is to cut it just past the neck of the branch. This will make a nearly circular wound on most trees, which is a smaller area that the tree needs to cover. Fortunately for those of us who trim trees, it’s also the shortest and easiest cut to make in most cases. The result will vary from a small bump at the base of the cut to a short, stubby bump, depending on the species, the age of the branch, and the individual genetics of the tree. A trained eye quickly learns to distinguish the correct location and angle of a good pruning cut. An untrained eye often makes bad cuts that can cause problems later. Although leaving too much stump is not a preferred practice, leaving too much is less harmful to the tree than not leaving enough. If in doubt, cut the stumps a bit long and wait to see how the tree reacts. You can always remove it later if you need to, but you can never put it back on.

Incorrect Practice: Climbing with Nails

It used to be common for tree workers to climb up on pickaxes, also called grappling hooks or climbers. They are metal spikes that are tied to the inside of the climber’s boots. The climber stabs them into the wood and they allow him to climb the tree. We have a lot of research now that tells us that this practice is very harmful to trees. Each of the entry points becomes a potential source of spoilage organisms and/or disease. In the short term, the tree will rarely show any signs of damage, but, years later, the tree may break or die due to problems associated with spikes.

The correct method for climbing a tree is to climb with a rope and harness, or climb into the canopy with a ladder and then install an approved rope and harness system. The only times a climber should climb a spiked tree is when the tree is being removed or when an emergency calls for a quick climb to rescue an injured climber. Even many rescues can be done without nails, but some situations can arise that would make them the best option in an emergency. The rest of the time, the spikes should be left on the ground.

Leave a Reply