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Last week we talked about philosophies behind pruning and why to avoid topping trees or other injurious pruning practices. This week's topic is the mechanics of pruning shade trees. Shade trees are pruned while they are young to train to a certain size and shape. Older trees are pruned to reduce interference with buildings, wires or other objects, or to reduce wind resistance, improve the air circulation and reduce internal shading. They are also pruned to remove diseased or insect-damaged parts.

Good pruning starts with visualizing how the tree should appear when mature. Trees have a definite size and shape. They range from very narrow, upright trees, to broad spreading or weeping trees. Trees cannot be successfully converted from one growth form to another. Gardeners should realize that all pruning is potentially harmful. Removing live branches means a loss of food producing leaves. This puts the plant on a diet and forces it to reestablish a balance between branches and root systems. Each cut results in a wound that takes energy to close.Does this mean trees and shrubs should never be pruned? Of course it doesn't, but it may be a good excuse when your spouse wants you to prune on cold winter days. Healthy plants tolerate pruning, and hazardous, dead or broken branches should be removed. Generally, less is better when pruning strong, structurally sound wood.

Pruning during the late winter allows a plant to replace lost branches and foliage during the growing season and results in faster closing of the wounds. The existing root system is prepared to support the top growth. Certain trees including birch, maple and walnut are "bleeders." They should be pruned right away or should be pruned while they are in full leaf. Of course, the sap is not blood, and the tree will not bleed to death. It has little effect on tree health as long as the soil moisture is adequate.

Pruning should be done as a thinning and shaping process. Do not leave stubs or flush cuts. Use the three-cut method when removing large branches. Undercut the unwanted branch about 12 inches away from the main trunk. Next, move out 2-3 inches from that cut and cut down so the branch will split cleanly away without tearing the bark or damaging the main trunk. Finally, move back in and make the final cut as illustrated.

The location of the final cut is extremely important for successful wound closure. All cuts should be made at the trunk collar just outside the branch bark ridge. The collar is the area of the greatest growth potential of the tree, and if it is injured, wound closing is slow and incomplete. If the trunk collar is not obvious, you can approximate its position as illustrated in the illustration. The angle of the final cut should approximate the angle formed between the branch bark ridge and an imaginary plumb (vertical) line that runs parallel with the axis of the trunk. In other words, the angle ABC should approximate angle ABD. Make the final cut between B and D.

Branches can be shortened back to a lower, lateral branch. Make the cut just outside the branch bark ridge to a point directly opposite. On forking branches of equal size, make the final cut just outside the branch bark ridge to remove the least desirable upright branch. On dead limbs where the collar has already formed, cut just outside the collar and remove the dead wood. Never injure the collar or cut it back.

Pruning paints have long been a subject of controversy, but most tree workers now consider them to be of no value. In some cases they do more harm than good as the moisture moves from the interior of the tree and is trapped against the pruning paint. The moisture cannot escape and decay as is encouraged. It spreads throughout the tree, causing damage.

Trees do not heal in the traditional definition of the word. Trees are unable to regenerate new tissue in the interior of the tree. That interior of the tree no longer grows new cells. That wood can never be regrown, and injuries can only be walled off or compartmentalized. The tree is able to prevent the staining and the subsequent decay from going further but cannot grow any additional wood in that area. Consequently it is important to protect trees from injuries including those caused by reckless or erratic pruners. Such injuries won't close for years and provide ready access for diseases and insects into the tree.

Pruning is only one part of successful tree care. Do it correctly, with careful consideration to the form and value of the tree. Improper pruning destroys the value of the tree and eventually causes its demise. Correct pruning removes hazards that may affect you or the tree. Timely and careful pruning enhances the tree's value, improves its safety and its overall health.

Additional information and pamphlets on pruning trees is available by writing the National Arbor Day Foundation, 100 Arbor Day Ave., Nebraska City, NE 68410.