Pruning Trees - What to Avoid and How to Do it Right
Pruning is a really important part of tree care, but it is useful only when it’s done in the correct way. Improper techniques can irreparably damage trees, leading to shorter lifespans of your trees and a higher risk of branch or trunk failure.
Well-pruned trees can provide you with years of shade and enjoyment. Pruning done properly reduces the many risks associated with trees; eliminates possible problems with buildings, streets, and pavements; improves the structure of the tree, and makes them really attractive. A good Tree Surgeon always makes a concerted effort to achieve desired goals while causing as little damage to the tree as possible. In most cases, correct tree pruning will remove only dead branches and a minimum amount of live tissue. When unusual circumstances mean that more live branches need to be removed, a good Tree Surgeon will plan the work for dormant periods of the year and will spread the pruning over a couple of seasons when possible.
To help keep your own trees strong and healthy, please avoid the following practices:
Wrong practice: Topping
A common myth is that trees sometimes get too tall and should be topped to make them safer. In fact, the long-term result of topping is to make trees less safe. New growth from topping cuts (also called heading cuts) tends to be poorly attached. There is also more extensive decay at the site of heading cuts. As the new branches get larger, they frequently break away from the tree and fall.
It is sometimes prudent to reduce the size of trees. Some species are brittle and tend to overextend themselves. Pecan trees are a prime example. In addition to developing long, arching branches, they can get very heavy when the fall crop of nuts is at its fullest, and branches commonly break under these loads.
The correct way to reduce a tree's canopy is to reduce branches from the tips. By making proper pruning cuts that shorten the longest limbs, we can reduce both the weight of the branches and also the amount of area that will be caught by strong winds. If you imagine holding a small dumbbell or other weight, it is much easier to hold it close to your body than with your arm extended. This same principle applies to trees.
Removing a small amount of strain at the end of a branch makes a big difference in how much stress acts on its entire length.
Wrong Practice: Lion-tailing
Also called "poodle-dogging" or "stripping out," lion-tailing is the removal of a large part of the interior growth in a tree. It is sometimes called "removing suckers" by uninformed tree workers. In fact, interior branches are not sucking anything from the tree; they are doing just the opposite. Every leaf on a tree creates 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 as starches and sugars. These stored compounds help the tree survive through stressful times, such as drought or soil compaction.
When interior branches are removed, the tree loses some of its ability to produce this energy. But there are more reasons to avoid lion-tailing. When all the interior growth is removed from a tree, the bark is suddenly exposed to sunlight. This can lead to sun scald, which often causes the bark to die and exposes the interior wood to decay. The result is weaker limbs that may be likely to break years later.
Also, small branches support large branches. The diameter of a parent branch will be larger ahead of the side 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 very ends of a long limb, there is a very little taper, and the branches are more prone to breaking. Because all of the weight and wind load are pushed to the ends, they have a greater effect on weak points in the limb, making them still more likely to break.
But that is not all. When interior branches are retained, they dampen the effects of wind movement on the parent branches. Each side branch dissipates a little bit of energy, so more side branches means more dampening. Again, the parent branches are less likely to break.
Finally, interior growth offers an opportunity when extreme weather, mechanical damage, or some other cause does break a limb. Rather than having to make a heading cut that will lead to problems down the road (see "Topping" above) or take the entire branch back to its point of origin, we can often save part of the broken branch by cutting it back to an interior limb.
So, the correct way to prune a tree is to retain as many interior live branches as possible. We try not to remove green tissue unless it is broken, it hangs too low over the street or sidewalks, or it is causing damage to structures underneath. There are exceptions (see "Topping" above), but a good arborist knows that a successful pruning job will result in mainly dead branches going into the chipper or brush trailer at the end of the day.
Wrong Practice: Overpruning
Continuing on that same note, removing too much live tissue is bad for trees regardless of where it comes from. Industry standards recommend never to remove more than one-third of a tree's living branches in a single season. Prudent arborists try never to remove more than one-fourth. When we take more than that, the tree loses huge potential energy production and large amounts of stored energy (in the form of starches and sugars). At the same time, it must expend energy to seal over the numerous wounds created. All the while, the tree must continue to support remaining branches and roots. The tree is forced to rely on stored starches and sugars, depleting reserves. A tree that is overpruned thus becomes more susceptible to dying as a result of outside stressors, such as drought, insects, or diseases.
The correct method is to avoid removing more than one-fourth of a tree's living canopy in any one year. If larger amounts must be removed, it is better to reduce the targeted limbs gradually 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 nearly so in the summer. After a severe reduction, a tree should be left to recover at least two years before any further pruning is done.
Wrong Practice: Overlifting
"Lifting" a tree's canopy, also called "limbing up," is a necessary part of a tree's life in urban environments. Branches that are too low can damage cars, houses or other structures. When low branches extend over a street, passing vehicles can break them off the tree, which is much more devastating to the tree (not to mention the vehicle) than is a pruning cut. When the tree is low over a sidewalk or lawn, people can be injured and turf grass can suffer.
Though good arborists routinely lift low canopies, it is important to avoid doing too much at one time. A good rule of thumb is that, when viewed from a distance, the bottom one-third of the tree should be stem and the top two-thirds should be canopy (leaves and branches). Having more low branches is not a problem for the tree, but having less means the trunk will have less taper, thus be more prone to breakage (see "Lion-tailing," above). Also, the lost leaf area will lead to diminished energy production, forcing the tree to rely on stored energy reserves (see "Overpruning," above).
When low tree branches on a young tree conflict with the above guidelines, the correct way to deal with the problem is to cut the problem branches off gradually over two or more years. By taking the ends of the low branches back to a lateral, we slow the growth of that branch and the tree sends more energy to higher branches, but the low branch still contributes to the development of strong taper in the stem until it is ultimately removed. As higher branches develop and take a more dominant position in the tree, the low limbs are often shaded out and die naturally, which is much less damaging to the tree.
Wrong Practice: Flush cuts
When a branch is removed from a tree, it is very important that it be done correctly. A common myth is that we should cut the branch as close to the stem as possible so that the tree can more quickly heal the wound. In fact, this practice is devastating to trees. When a new branch sprouts, its parent forms special tissue around its based called a branch collar. This tissue swells up around the new limb the way water in a stream swells around a large rock. Flush cuts remove this tissue, compromising the tree's ability to grow new wood over the outside of the wound. In fact, some flush cuts never completely close, exposing the interior wood to decay and disease 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 cannot see the defect until years later when the branch breaks.
The correct way to prune a branch is to cut it just beyond the branch collar. This will make an almost circular wound in most trees, which is a smaller area for the tree to cover. Fortunately for those of us who prune trees, it is also the shortest, 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 protrusion, depending on the species, the age of the branch, and individual genetics of the tree. A trained eye quickly learns to distinguish the proper location and angle of a good pruning cut. An untrained eye frequently makes bad cuts that can lead to problems later. Though leaving too much of a stub is not a preferred practice, leaving too much is less damaging to the tree than not leaving enough. When in doubt, cut stubs a little long and wait to see how the tree reacts. You can always take more off later if necessary, but you can never put it back.