AB-Dont' Put Off Pruning: Tree Pruning - FOCUS: Tree Care

You can drive a modern car for a long time – neglecting oil changes and forgetting to replace filters and belts – and get away with it. But eventually the time will come when repairs to a damaged engine or transmission will greatly exceed the cost of the routine maintenance you neglected.

Tree care is the same. You can plant a tree and do nothing. If it lives (and surprisingly, many do despite the neglect), years later you might find that the tree may not flower as well as expected, its form is poor, or it drops limbs on buildings or people. As with cars, a little routine maintenance can eliminate future big bills.

TRAINING YOUNG TREES. Pruning is the most neglected area of tree care. Most people ignore the pruning needs of young trees and instead wait until the tree reaches maturity before beginning to prune.

If the tree is trained properly, there is less need to prune at maturity and less possibility of structural defects that can result in tree failure. Training should be performed within the first 10 years after planting. This is the best time to establish the proper spacing and arrangement of the scaffold limbs, which are the lower, permanent, major limbs directly attached to the trunk. Thinning out these limbs while they are small is much less harmful to the tree and minimizes decay formation.

The branches located along the lower 5 or 6 feet of a deciduous tree’s trunk are generally referred to as temporary branches. These branches are eventually removed to provide clearance and improve visibility as the tree becomes taller, but they should be retained for the first three to five years after planting because the food they contribute is critical to lower trunk diameter growth. The lower branches also provide young bark protection against sunscald and splitting.

When removing the lower limbs (crown raising), do not be too quick to "limb up" the tree. At least one-half of the crown’s foliage should originate from branches on the lower two-thirds of the tree.

Of course, there is no rule that says these lower branches have to be removed. Whenever possible, leave these branches on the tree. Trees, such as lindens, develop a very pleasing oval-shaped form if allowed to retain their lower branches.

Most of young trees’ pruning needs are referred to as thinning. Thinning is the selective removal of branches throughout the canopy to improve tree structure. The first branches to evaluate for thinning are the scaffold limbs – those limbs from 5 to 15 feet that will become the lower permanent limbs of the tree.

Scaffold limbs must be properly selected to maintain good form. One of the most persistent tree problems is the development of two main leaders rather than a single leader. This problem is most common with green ash, but it also occurs with maples and lindens. These double leaders, also referred to as codominant stems, are weakly attached and prone to splitting or breaking. Avoid the formation of codominant stems by eliminating one of the two upright stems as soon as they are noticed, preferably when they are less than 1 inch in diameter.

Once codominant stems reach several inches in diameter, subordinate pruning should be considered. This involves shortening one of the two stems so that it is subordinate to the other rather than eliminating one of the stems. However, avoid removing more than one-fourth of the foliage from the subordinated stem. If the codominant stems are not detected until they are mature, attach tree support systems rather than removing one of the stems.

Branches should be properly spaced along the trunk, and this selection should also take place within five years of planting. As the tree matures, branches increase in diameter. If they are too tightly spaced, they may interfere with one another’s development. Close branching may also result in bark splits or cracking. The spacing for scaffold branches for trees with a mature height of less than 30 feet is approximately 6 to 12 inches. Trees with an anticipated mature height greater than 30 feet should have a branch separation of approximately 12 to 18 inches.

There are several other types of branches that should be watched carefully. Many young trees will have sprouts coming up around the base of the tree. These sprouts come from the roots or root flares and are referred to as suckers. Many ornamental trees are cultivars that are grafted or budded onto a seedling-grown rootstock. Thus, the shoots that come from the roots don’t have the same ornamental qualities as the cultivar. For example, a crabapple cultivar may have showy red flowers and small, hard, yellow fruit while the suckers may have small, white flowers and large, soft, red fruit.

The other branches to thin are watersprouts. These are rapidly growing upright shoots that form along the trunk or scaffold branches and are most common on crabapples and other small flowering trees. Watersprouts should not be confused with spurs. Spurs are slow-growing shoots that form along the trunk and branches of some trees, most notably crabapples. These can be separated from watersprouts by their numerous leaf scars that form on the short shoots. Spurs form flowers and fruit for many ornamental trees.

PRUNING MATURE TREES. As a tree matures, the amount of live branches removed should decrease. Mature trees establish a balance between their root system and the canopy branches. Drastically altering this balance by either removing roots or branches may stress the tree. If the tree has been properly trained during the first 10 years, there should be little need for major pruning beyond removing branches as they naturally decline and die.

Oftentimes, people unnecessarily thin out mature tree canopies (called crown cleaning) in the belief that doing so will benefit the tree by permitting more sunlight to reach the interior leaves. However, for most mature trees the interior leaves have adapted to the shaded environment and are often stressed by the sudden exposure to full sunlight. The canopies of mature trees have also evolved so that the individual branches provide mutual protection for wind and ice loads. Excessively thinning the canopies can leave the remaining branches more susceptible to breakage. Unless the branches are structurally unsound or additional clearance from buildings or other structures is needed, mature tree pruning should be limited to removing dead branches.

Pruning cuts used on mature trees should always be thinning cuts, removing a branch at its point of origin (or when reducing the height of a tree, a lateral one-half the diameter) rather than heading cuts. Heading cuts involve cutting to a stub, a practice that results in extensive decay.

Probably the most common form of heading practiced on mature trees is topping. This occurs when the entire top of the tree is pruned back to a specific height. Topping results in rapid formation of weakly attached branches, extensive branch decay and a decline in the root system. While trees rarely immediately die from this harsh treatment, it can result in decline and death over time. Topping is not an acceptable practice.

TIMING MATTERS. Pruning dead and dying branches can be done at any time. However, there are specific time periods for the pruning live branches. The ideal time to do this is during the late dormant season – about a month or two prior to bud-break. During the late dormant period there are minimal pests present and the pruning wounds quickly begin to close with the start of the growing season.

However, light pruning or removing a few small branches – particularly those less than 2 inches in diameter – can be performed at any time of the year.

Heavy pruning should be avoided when the tree’s leaves are first expanding in the spring and when the leaves are falling in autumn. In addition, the removal of large branches (more than 8 inches in diameter) should be avoided during autumn. Many of the decay fungi are releasing spores during this time period and there is a greater risk of increasing decay.

And be aware that pruning birch, maples and walnuts during the late dormant season will usually result in "bleeding." This refers to the flow of sap from the pruning wounds during warm spring days. While the sap may be unsightly and attract wasps and other insects, it won’t harm the tree.

Suckers and watersprouts should not be pruned during the dormant season. Removing them during this time usually results in excessive production the following spring. Instead, they should be thinned in early summer just after the tree has completed its spring growth.