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Index : Trees
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- 32. Severe Pruning Injures Trees - Top
- Severe Pruning Injures Trees
Severe pruning of any kind on trees is not recommended. I would term severe pruning to be the removal of more than one quarter of the tree canopy, the leafy part, at any given time.
Severe pruning is usually done under the mistaken belief that it will reduce or control the size of the tree. Nothing could be further from the truth! Severe pruning actually stimulates excessive regrowth. Within a year or two of pruning, trees will actually be larger than they were before pruning.
The new growth that arises from severe pruning is weak, wound-response growth called water sprouts. These shoots can grow six to twelve feet in length. Unlike permanent branches, water sprouts are whips or unbranched shoots. They often grow so thick as to resemble an upside-down broom. Because of their weak attachment to the tree, a strong wind will easily cause these sprouts to break off causing further injury to the tree.
Severe pruning is most often done by cutting back the top of the tree, often by as much as 50 percent. This type of pruning is referred to as topping, hat-racking or rounding over. In this process, no thought is given as to where to cut back on branches to maintain the health of the tree. Rather, all branches are cut at the same point out from the trunk. This leaves large wounds that will not heal properly. Over time, rot will invade these open wounds and move into the largest limbs and even the trunk. When the core wood of the branches and trunk become rotted, the entire tree is weakened and more prone to storm damage.
Even moderate pruning of trees can be damaging during the Summer, due to a condition called sunburn. Trimming allows direct sunlight to penetrate the normally shaded interior of the tree. Thise light-sensitive branches easily become sunburnt, causing the top portion of the branch to die. As a result, the leaves on these branches will fall off, causing even more light exposure and sunburning. Citrus trees are extremely sensitive to sunburn, but even desert trees such as mesquite can be damaged in this fashion.
It is important to remember that reducing the leafy top of a tree also reduces the tree's food supply. It is the leaves which manufacture food for the tree through a process called photosynthesis. Severe pruning puts the tree in a starvation mode and the burning of stored food reserves. If starvation is prolonged the tree is likely to become so weakened it will die.
To keep trees healthy, only prune if necessary and then only prune what is needed. Never prune off more than one quarter to the tree's canopy. If a branch must be shortened, it should be cut back to a lateral or side-branch large enough to continue healthy growth. A good rule of thumb is to cut back to a side branch that is at least one third the diameter of the limb being removed. This technique of pruning will decrease the likelihood of water sprouts developing and will help preserve the natural appearance of the tree.
Finally, I recommend any pruning that requires using a ladder or climbing into the tree be done by a skilled, certified arborist. This work is too dangerous for the novice. Professional arborists can determine what type of pruning, if any, will be necessary for your tree. Choose an arborist that has completed a certification program on tree pruning, such as that conducted by the International Society of Arboriculture. And don't hesitate to ask for, and check, a list of references.
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Written by John Begeman, College of Agriculture, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on July 16, 2000
- Updated: April 19, 2001
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