When done properly, pruning can improve a tree's healthy appearance, as well as increase the life expetancy of the tree. Proper pruning opens the canopy of the tree to permit more air movement and sunlight penetration.

Done improperly, pruning can decrease the tree’s life expetancy or even kill it. Because trees are living organisms, they can be profoundly affected by pruning practices. Tree care professionals adhere to accepted standard of practices when pruning trees.

The American National Standard for tree pruning, designated as ANSI A300, has been in existence since 1995. Its development was approved by the American National Standards Institute. This pruning standard should be followed in all pruning situations and all geographic areas. Knowing how certain tree species grow within a given environment may alter how the recommendations of A300 are interpreted.

The following information is designed to help you understand exactly what will be accomplished in a pruning operation.

Making Cuts

Branches should be removed with thinning cuts. A thinning cut either removes a branch at its point of origin or shortens it back to a lateral branch that is large enough to assume the terminal role.

Branches should not be removed with heading or topping cuts. A heading cut is when a currently growing or one-year-old shoot is cut back to a bud, or when a larger limb is cut back to a stub or a lateral that is not big enough to assume the terminal role. Heading should rarely be used in shade andornamental tree pruning, since it forces the growth of multiple upright sprouts that are weakly attached to the parent stem. Drastic heading can kill the tree outright.

Branch Size

A minimum or maximum diameter size of branches to be removed should be specified in all pruning operations. This establishes how much pruning is to be done.

Pruning Objectives

Pruning objectives should be established prior to beginning any pruning operation. A300 provides two basic objectives.

Hazard Reduction Pruning

Hazard reduction pruning is recommended when the primary objective is to reduce the danger to a specific target caused by visibly defined hazards in a tree. For example, hazard reduction pruning may be the primary objective if a tree had many dead limbs over a park bench.

Maintenance Pruning

Maintenance pruning is recommended when the primary objective is to maintain or improve tree health and structure, and includes hazard-reduction pruning. An example here might be to perform a maintenance pruning operation on a front yard tree.

Pruning Types

Hazard reduction pruning and maintenance pruning should consist of one or more of the pruning types noted below.

Crown cleaning consists of the selective removal of one or more of the following items: dead, dying, or diseased branches, weak branches and watersprouts.

Crown thinning is the selective removal of branches to increase light penetration, air movement, and reduce weight.

Crown raising consists of the removal of the lower branches of a tree to provide clearance.

Crown reduction, also called crown shaping, decreases the height and/or spread of a tree. Consideration should be given to the ability of a species to sustain this type of pruning.

Vista pruning is selective thinning of framework limbs or specific areas of the crown to allow a view of an object from a predetermined point.

Crown restoration pruning should improve the structure, form and appearance of trees which have been severely headed, vandalized, or storm damaged.

When you contract a company to prune trees, you should obtain a verbal or written commitment that, "All pruning shall be done in accordance with the ANSI A300 standard for tree pruning." This means that the overall pruning operation is going to stay within specific bounds. This means . . .

Proper cuts will be made.

Spikes won’t be used to climb. Although spikes can greatly speed the climber’s ascent into the tree, they are injurious to the living tree and should only be used in emergency situations or when the tree has very thick bark.

Not more than one-fourth, or 25 percent, of the foliage of the canopy or individual limbs should be removed in any one season.

When pruning is completed, at least half the foliage should remain evenly distributed in the lower two-thirds of the canopy.

The health and life stage of the tree are important variables in the application of the Pruning Standard. Tim Johnson, chairman of the A300 Committee and owner of a commercial tree service firm, uses the acronym "PEGSMOR" to help his clients understand how the A300 Standard properly applies to trees in different stages of growth.

"P" is for planting. A new transplant needs its foliage, so minimal pruning is the rule. Obviously, if you are compelled to prune a tree that you are just planting, you may want to pick a different tree to plant, or look for another nursery.

"E" is for establishment. If the tree is becoming established, you do not want to prune unless it is absolutely necessary, because the tree needs every one of its leaves to build starch reserves and produce roots.

"G" and "S" refer to growth and structural strength of the young-but-established tree. It is in this life stage that pruning plays the most important role, because pruning can influence both the growth and structural integrity.

"M" is for mature. Other than to remove deadwood, one would prune less as the tree matures.

"O" is for over-mature. Depending on the tree, you either do not want to prune, or you may have to prune drastically. Crown reduction may be appropriate at this time because structural integrity is lacking.

"R" stands for remove and re-plant.

The National Arborist Association is the Secretariat for the ANSI A300 Standard, and assists arborists and consumers with the interpretation and application of the standard.