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Oftentimes trees planted within an urban environment have the potential of impacting nearby structures, sidewalks, streets or utilities. Pruning involves the removal of limbs and branches that the tree originally produced and no longer needs. Pruning will also manipulate the canopy of the tree to create a desired result and keep it away from conflicts. The goal is to insure that the tree develops a strong canopy to support the full weight of the branching structure and has sufficient leaf surface for the tree to grow and be healthy.
Correct Pruning Principles
The first principle is to preserve the natural form of the tree, mainly pruning only those portions of the tree that are needed to be removed so that the majority of the canopy can be preserved. The primary goal should be to first remove the weak parts of the tree, such as branches that are dead, declining or severely decayed. If a limb has excessive weight on the end than could result in the limb snapping off, it may decrease the likelihood of limb failure by the removing some of the end weight. This means cutting back the end of the branch to a substantial side branch that is at least one third the diameter of the branch being removed. It is important to prune the limb back to a major side branch that will be able to grow in the direction away from conflicts and to not leave a stub that will either decay or sprout numerous weak shoots. Remember that in most cases, pruning stimulates the tree to produce new growth, since this is the tree’s attempt to replace the total leaf surface lost. Pruning is also usually unsuccessful for addressing insect or disease problems.
It is important to not remove too much foliage from the interior of the tree’s canopy. Ideally about one third of the tree’s leaf surface should be interior. If all of the interior branches are removed, although it might be architecturally pleasing, leaves the tree with the weight of the foliage only on the thinly branched outer perimeter, resulting in the tree having excessive end weight. An extreme example, known as “lion-tailing”, removes all of the interior and secondary branches, only leaving a small tuft of foliage at the end of the branch. These branches are very weak and the tree has lost most of its leaf surface needed for healthy growth.
The DNA within each of its cells dictates to the tree its eventual size and shape; so the tree will attempt to achieve that goal as long as it has the resources to do so. That is why attempts to limit the height of trees by severely and indiscriminately lopping off major limbs, a harmful practice known as “topping”, are usually unsuccessful and are harmful to the structure of the trees. In most cases, topping causes the tree to respond with producing a flush of many closely spaced and fast growing shoots that will try to out-compete one another. In a matter of a few short years the tree has grown nearly as tall as before the topping, but now the canopy is full of weakly wooded and poorly attached limbs that are usually much more liable to fall out of the tree that those that the tree originally produced in a normal growth pattern. Therefore, the City of San José has adopted the ANSI A300 pruning standards for street trees, which among many specifications, allows no more than 25% of the canopy to be removed within a growing season, unless there is an emergency situation.
When removing a branch back to a major limb or trunk, it is important to prune so as not to leave a stub that will decay, but yet not cut into the branch collar, a raised area around the base of the branch were it attaches to the limb or trunk. The goal is to create a small wound as possible, so that the tree will be able to close off the pruning wound quickly, protecting it from decay and insects. If the branch collar is removed in pruning, a condition known as a “flush cut”, then the wound will be larger and will stay open much longer, thus subjecting the area to greater possibility of decay which will likely proceed into the center of the limb or trunk and cause future weakness.
For more information about pruning principles click here (external link)
Pollarding
We discourage in general the pruning technique known as “pollarding”. It is the practice of establishing a basic framework of a trunk and a few major limbs, and then the complete removal on an annual basis all of the previous growing season’s growth back to a knob at the end of the limb. While there are a few species, such as Sycamore and Mulberries, which can tolerate this drastic and disfiguring technique, to be successful it then requires this severe annual pruning to proceed in perpetuity. If the previous year’s growth is not removed, the result will be a very weak structure as previously described with topped trees. If the decision is made to stop pollarding a tree, then it is likely that a skilled professional will be needed to evaluate the branch structure to select those shoots that will be retained and trained to establish a strong branching structure over the course of several growing seasons.
Seeking Professional Advice
Pruning young trees in a correct manner will establish a strong basic branching structure that will serve the tree well for the rest of its life. Pruning of mature trees is done to remove weak or conflicting parts of the trees or branches that are rubbing against each other or structures. Tree care professionals, such as those that are certified by the International Society of Arboriculture, can assist in the proper pruning and care of your trees. Proactive pruning can address problems while they are small rather than waiting until the problems are large and require major surgery. If you received your street tree from our local non-profit urban forestry partner, Our City Forest (998-7337 or ourcityforest.org (external site)), we encourage you to keep in contact with them to receive information on how to properly care for your young trees
Permits required for planting, pruning, and removing Street Trees
Click here for permit request form
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