Pruning Mature Trees
This is a wonderful and thorough guide for managing mature trees by Dr. Ed Gilman, Professor Emeritus of Environmental Horticulture at the University of Florida and one of the foremost authorities in tree structure and maintenance.
Pruning mature trees, as well as young trees, is a process, it is not a “one and done”. Just like older humans, older trees do not tolerate stress or injury as well as their younger counterparts. Large pruning jobs on older trees should be separated into multiple visits separated by 6 to 12 months depending on the circumstances. There are many considerations that should be taken into account before pruning mature trees. Such as,
- compartmentalization ability
- overall health
- susceptibility to pests or disease
- time of year
- recent disturbances (storms, construction, etc.)
- Dr. Ed Gilman, Pruning Shade Trees in Landscapes This site by professor emeritus Ed Gilman of the University of Florida Environmental Horticulture Department is a practical compilation of Gilman’s research and that of others who study trees, tree structure, wind effects and tree stability.
Basic Pruning Concepts
Pruning of mature trees should essentially be done (1) to remove dead branches that occur from time to time and (2) to reduce risk of branch failure.
- A few dead branches in the interior and lower parts of a mature tree are a normal occurrence. As lower branches become shaded as the tree grows, these shaded branches do not produce sufficient carbohydrates. So the tree sheds these branches. They die and will eventually fall to the ground unless removed in advance through regular tree maintenance pruning.
- When there are many dead branches or dead branches occur in the upper crown of the tree, that is a sign of other problems such as root problems that could have been caused by construction damage to the roots, soil compaction, flooding or extreme drought. When this situation occurs, have the tree checked by a competent arborist — hopefully an arborist who will not just throw some fertilizer at the problem.
- Mature trees should be pruned from the outside. The idea of reducing stress on long branches is to shorten the branch from the outside. Unfortunately, many tree services prune from the inside out, removing interior branches and watersprouts.
- The extreme form of excessive interior pruning is “Lions tailing” which is a commonly-used but very bad pruning technique where the interior branches are removed supposedly to let the wind blow through. What really happens is the branch center of gravity is raised and the cluster of leaves at the far end of branch is less stable rather than more stable in windy conditions. For more on “Lions tailing” check out the Ed Gilman site Read more about “Lions tailing.”
Less pruning is almost always better for the tree’s health than more pruning.
- Don’t let a tree service make you feel like you are getting your money’s worth by removing more limbs and foliage. Less pruning is better. Here’s why…
- Green leaves are the factories that make food in the form of carbohydrates for the tree. Remove too many leaves and the tree is weakened.
- Carbohydrates manufactured by the leaves are stored in woody tissue in the branches and trunk. When branches are removed, the stored carbohydrates are lost and the tree has to make more.
- Excessive pruning causes a tree to form water sprouts sometimes called suckers. Water sprouts are often seen as clusters of small twigs and branches that sprout from a pruning cut stub or from the interior portion of larger branches. Sprouts are a way the tree has to put out more leaves as quickly as it can to make more carbohydrates. Unknowledgeable tree services will offer to come back for a fee, of course, to “clean up” those ugly sprouts, which the tree service caused in the first place by excessive pruning. Don’t get stuck in the vicious cycle of sprout removal. For more information on water sprouts and suckers, go to the Ed Gilman website on this topic.
- Young trees should be pruned for structural strength, not esthetics, by eliminating double or triple leaders that will form a weak structure as the tree develops over the years.
- Small, young trees can tolerate more pruning (up to 33%) than older, mature trees, which should not be pruned more than 10% to 20%. Old veteran trees should be pruned even less. (Editor’s note: These percentages are generalizations. A healthy mature or veteran tree can tolerate more pruning. And if risk is an issue, then more pruning may be needed beyond the percentages listed by Dr. Gilman above. An experienced arborist with considerable experience with mature and veteran trees should be making this determination of how much to trim.)