Pruning Article for Boulder Home and Garden

For many people, pruning is the maintenance job they most fear and dread. And it is good to be wary, because a tree that is badly pruned can dominate a landscape with its ugliness for years, can be more prone to breakage and disease, and can have a much shorter life.

         Tree and shrub pruning have four basic aspects: the practical or aesthetic interests of the owners, the biology of how trees “heal”, the physics of what makes a branch strong or weak, and the art of how to create beautiful forms.

         We want our trees and shrubs to perform and we have particular ideas about what we want and don’t want. We don’t want a tree rubbing against the house or the roof; we want to be able to walk down the sidewalk without ducking. We want the trees to be safe so limbs don’t fall on our children or break something. We want to preserve certain views and to block out others. And some people have formal tastes while others prefer a naturalistic style. Pruning can accommodate these human interests, however there are limits. An ash tree that will mature at 50’ cannot be kept to 20’ under a power line. We cannot chop away aggressively without hurting the health or the beauty of the tree. However if we begin pruning a well-chosen tree when it is young, and prune correctly, we can be very successful in making that tree or shrub fit our practical and aesthetic interests.

         Trees are living beings with a certain biology, and so if we are going to cut into them, we should do it with respect and with a knowledge of how they repair and protect themselves. The most important pruning knowledge for a tree owner is how to make a correct cut. See the sidebar with its illustrations. The world-famous tree expert Dr. Alex Shigo has researched, dissected and observed trees for decades to learn the importance of not cutting off the branch collar. This collar is mostly visible as a swelling where the branch attaches to a bigger branch or trunk. The swelling indicates the presence of a “protection zone” inside the branch, which has cells capable of walling off or “compartmentalizing” an injury, disease or pest. Since a tree cannot heal injured cells like an animal, its defense is simply to create a barrier around the problem. Consequently if we cut off the branch collar, as occurs with a flush cut, the protection zone is removed and fungi and bacteria can enter freely and deeply. Just imagine not having the ability of locomotion, and not being able to escape danger, and you will appreciate the importance of a proper pruning cut.

         The physics of pruning relates both to the individual strengths of different woods and also the proportion of diameter of a branch to its length. Fast-growing trees, especially if they are heavily watered and fertilized, will have long thin branches. This is often seen in silver maples, willows, cottonwoods, boxelders and Siberian elms. Fast growth usually means weak growth, so it is important to shorten the lanky, leggy growth, or when a strong wind or heavy snow comes along, branches will break. As a general rule, if a branch looks spindly, droopy or leggy, shorten it by 10%-20%. Sometimes more is necessary, but even taking 12” of the end of a 10’ branch will reduce the leverage greatly.

           Another point of structural strength or weakness is the crotch, the connection between the branch and its larger source branch or trunk. The strongest connections are at a 60 degree to 90 degree angle. Very narrow crotches of 10 degrees to 30 degrees are very likely to have fibers running parallel rather than interlocking with the source branch. These narrow crotches have permanent cracks that never close, and so they are very often split under the weight of heavy wet snow. To prevent them from splitting, it is best to remove the branches with weak crotches when they are very small, to dwarf by shortening if they are too important to the shape of the tree, or as a last resort, to cable to a stronger branch.

         When shortening a branch it is important to leave a “leader” or side branch at the very end. This leader should be no smaller than a third the diameter of the branch where it is cut. It is also often good to thin out (remove)some of the smaller branches. This reduces the weight, lets snow fall through more easily and lets light get to the leaves below. If branches are rubbing, remove the one that is weakest or least necessary to the form.

         The ancient Greeks knew that proportion, balance and beauty are related, and this fact is the basis of the art of pruning. A properly pruned tree that is compacted 10%-20% usually should look beautiful; it should not look mutilated. The do-it-yourselfer will be able to shape smaller trees with the help of a pole-pruner. In general, the tree shapes that are most strong, stable and beautiful are the sphere and the cone. It is not necessary to create living lollipops, but when the tallest branch is over the trunk (the pillar), the tree is usually balanced. Leaning or arching bonsai-style trees can be sculpted, but they require more diligent and frequent maintenance.

         It is good to stand back and look at the tree frequently as the pruning is being done. A tree is a sculpture, so it should look good from all directions. Sometimes it is necessary for a tree to grow for a couple more years in order to prune it more beautifully. Think in terms of “tree time”: Where will this branch grow in 5 years? In 10 years? If in doubt about what to do, cut less; we can always cut more later, but we can’t put a branch back.

         When is the best time to prune? The wise old saying quips “When your tools are sharp.” And with sharp tools, the job will be easier, safer, and faster “healing” for the tree. Dr. Alex Shigo says that most times are fine for pruning if the tree is not stressed. For stressed trees, avoid pruning when the tree is putting on or dropping leaves. 

         When done correctly, pruning can be of great benefit to trees and shrubs, and can extend their lives. Pruning can also add beauty to your environment, and can be in itself, a very satisfying art form and nurturing activity. Dr. Shigo says “Touch Trees”; I agree.