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University of Illinois Extension

Pruning Trees in Spring

March 5, 2008

There is an old saying that you can prune a tree whenever the saw is sharp. However, the effects of pruning a tree can vary with the season and according to how much of the tree is pruned away.

Early spring pruning of trees is often preferred because plants are approaching a period of rapid growth and pruning wounds will heal quickly. In general, if all you want to do is trim out dead, weak or diseased parts of the tree, this type of pruning can be done at any time of the year without drastic effects on the tree. If you prune to thin the trees–to allow light for grass and shrubs–prune in late winter or early spring. Heavy pruning done after growth starts in spring can weaken a tree because it forces the tree to use stored food for re-growth instead of for new growth. Heavy pruning during late summer should also be avoided because it can stimulate lush, succulent growth.

Elm, ash and oak are an exception. Prune elm and oak during late summer, fall or winter to avoid the change of infection with Dutch elm disease and oak wilt disease. Insects can spread these diseases, and fresh wounds made during the growing season are likely attractants. Ash trees under stress should not be pruned during June or July because of the likelihood of an infestation with ash borer.

Birch and maple are known as bleeders and will "bleed" clear sap if pruned in early spring. Bleeding will not harm a tree but causes concern to owners because of sap loss. To avoid sap loss, prune after trees are fully leafed out.

Many people feel large trees should be topped. This is an unfortunate misconception. Rather than eliminating "top heaviness," topping produces a weak tree subject to major problems.

When a tree is topped, large branches are drastically cut back in length. A majority of the small, leaf bearing branches is also removed. Topping a tree leaves many large stubs that can provide an ideal entrance for insects and diseases. The shearing effect results in a mass of growth the following year, near the end of each cut. These branches are weak and easily broken.

A more severe problem caused by topping occurs in the root system. A delicate balance exists between the leaf and root area. When a large amount of branches are removed, portions of the root system will die as there are not enough leaves to manufacture food for the root system. Disease organisms that lead to rotting frequently attack these dead and dying roots.

Improper pruning of limbs can also lead to weak, hollowed trunks. Pruning cuts should be made close to the trunk, a bud or branch. Do not leave stubs. Stubs are an open invitation to rot and decay and eventually can lead to rot entering healthy tissue and the trunk. Proper pruning averts this problem.

Proper pruning techniques include: removing dead or dying branches, eliminating narrow, v-shaped crotch angles, trimming out limbs that grow towards the center of the tree, removing limbs that rub together, making cuts close to the trunk or branch and not leaving stubs.

Source: David J. Robson, Extension Specialist, Pesticide Safety, drobson@illinois.edu

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