Mature trees usually need little pruning

By Ed Perry

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I've recently discussed pruning young landscape trees, emphasizing the importance of early training for developing structurally strong trees. While young tree pruning is very important, mature trees also need periodic pruning. Even though forest trees grow quite well with only nature's pruning, landscape trees require a higher level of care to maintain their safety and aesthetics. Because improper pruning can seriously damage mature trees, it should be done with an understanding of how the tree responds to each cut. Improper pruning can cause damage that will last for the life of the tree, or worse, shorten the tree's life.

Because each cut has the potential to change the growth of the tree, no branch should be removed without a reason. Common reasons for pruning are to remove dead branches, to remove crowded or rubbing limbs, and to eliminate hazards. Trees may also be pruned to increase light and air penetration to the inside of the tree's crown or to the landscape below. In most cases, mature trees are pruned as a corrective or preventive measure, perhaps the most common being to reduce a tree's size. Routine thinning of branches does not necessarily improve the health of a tree. Trees produce a dense crown of leaves to manufacture the sugar used as energy for growth and development. Removal of foliage through pruning can reduce growth and stored energy reserves. Heavy pruning can cause a serious reduction in vigor, especially in large old trees, where the amount of foliage is relatively small compared to the large amount of wood tissue in the trunk and branches. Proper pruning, with an understanding of tree biology, can maintain good tree health and structure while enhancing the aesthetic and economic values of our landscapes.

So how much should a mature tree be pruned? According to the International Society of Arboriculture (ISA), a professional tree care organization, the amount of live tissue that should be removed depends on the tree size, species, and age, as well as the pruning objectives. Younger trees tolerate the removal of a higher percentage of living tissue better than mature trees do. Another important principle to remember is that a tree can recover from several small pruning wounds faster than from one large wound. A common mistake is to remove too much inner foliage and small branches. It's unfortunately common to see large old trees with all of the small branches removed along the entire length of the larger branches -- called "lion tailing" -- leaving only a turf of leaves at the end. It's important to maintain an even distribution of foliage along large limbs and in the lower portion of the crown. Over-thinning reduces the tree's sugar production capacity and can create tip-heavy limbs that are prone to failure.

The ISA and tree experts stress that mature trees should require little routine pruning. A widely accepted rule of thumb is never to remove more than one-quarter of a tree's leaf-bearing crown. In a mature tree, pruning even that much could have negative effects. Removing even a single, large-diameter limb can create a wound that the tree may not be able to close. The older and larger a tree becomes, the less energy it has in reserve to close wounds and defend against decay or insect attack. The pruning of large mature trees is usually limited to removal of dead or potentially hazardous limbs. Remember, no branch should be removed without a good reason.

Ed Perry, a farm adviser with the UC Cooperative Extension, can be contacted at 525-6800 or ejperry@ucdavis.edu.