Power To Prune

Pruning trees seems like a simple task, but it’s actually a task that is simple to err at.

Published November 17, 1999

Being a professional in any line of work means keeping your knowledge base current. Techniques, methods and principles accepted 20 years ago may be obsolete today.

And so it goes with pruning. Considered both an art and a science, pruning recommendations have changed dramatically over the last 20 to 30 years. In fact, practices such as flush cutting of branches, topping of trees and treating pruning wounds with asphalt-based dressings are now relics of the past.

Landscape professionals must combine their talents with a thorough knowledge of hundreds of plants, and each plant’s response to pruning, to do the job correctly.

Why Prune? There are many valid reasons for pruning trees and shrubs. The most important reasons are for safety, health and appearance.

Safety pruning often becomes necessary to correct a “plant in the wrong place” situation. Examples of safety pruning include trimming branches from trees and shrubs that interfere with lines of sight for automobile, bicycle and pedestrian traffic, eliminating branches that grow into utility lines and removing dead or structurally unsound branches from trees. Sometimes, completely eliminating a tree from the landscape is the best option when large sections must be removed to ensure safety.

Pruning for health might involve removing crowded, rubbing or crossing branches to improve airflow and decrease the chance of self-wounding. It may also mean eliminating diseased or insect-ridden branches. Promoting a strong framework through judicious pruning is also pruning for health, as trees and shrubs neglected at an early age frequently have structural problems later in life.

There is a certain amount of subjectivity involved when the goal of pruning is to improve or enhance the ornamental quality of trees and shrubs. Some situations call for plants to be molded into formal or rigid shapes, while others require a plant’s natural tendencies be accentuated, not modified.

Each style has its place, however, and trees and shrubs pruned to take advantage of their natural growth habit usually require less maintenance than their carefully sculpted counterparts.

When to Prune. Trees and shrubs respond to any wound, including those inflicted during the pruning operation, by “walling-off” or compartmentalizing injured tissue. Once these areas are walled off, the isolated wood dies, but the rest of the plant is protected from decay-causing pathogens and rot.

Because this wound response takes place most rapidly just prior to the onset of growth in the spring when energy reserves are at their highest, late winter (February through April) is commonly regarded as the best time to prune. Late winter is also a time when woody plants will not have to cope with opportunistic insects and disease-causing pathogens. Finally, the absence of foliage makes it easier to view the general structure of deciduous trees and shrubs and makes it easier to remove the appropriate branches.

Pruning can be performed at other times of the year with few problems. In fact, pruning to remove broken, dead or diseased branches should be done as soon as these defects are noticed. But pruning should not take place in the spring when leaves are forming because energy reserves are low and the bark “slips” or tears easily at this time.

Autumn pruning, particularly during the period when leaves are falling, should also be avoided because this is when new, absorbing roots are forming, and the wound response drains energy during this period of growth.

Making the Cut. Contained within the branch collar (the flared base of most branches) is an important barrier or protection zone preventing the spread of decay into the trunk or parent branch. The branch collar and raised strip of bark, the branch bark ridge, mark this boundary between the branch and trunk.

Pruning cuts that injure or eliminate the collar will destroy the protection zone, leaving vulnerable tissues open to invasion and infection. Hence, the proper pruning cut should be made to the outside of the branch bark ridge and collar. Contractors should also be sure to avoid flush cuts because they violate the protection zone and leave large wounds that make it difficult for trees and shrubs to defend against infection.

When removing larger limbs (usually those with a diameter exceeding 1 inch), a three-cut technique should be used to prevent the branch from tearing away as it is being removed. The one-third to one-half size initial cut through the limb is made on the underside of the branch, 1 to 2 feet from the trunk. The second cut is made on top of the branch, 1 inch farther out.

As the second cut is made, the weight of the branch will cause it to break at the pivot point between the two cuts. Once the branch falls, the third and final cut is made outside the branch collar.

Wound Dressings. These materials have fallen out of favor after experimental work proved that they do not prevent decay in wood, and, in some cases, actually slow the tree’s response and protect wood-rotting fungi.

Recently, non-phytotoxic wound dressings, such as orange shellac, have been recommended for pruning wounds made on several oak species and American elm in the spring and early summer. While the use of wound dressings may be justified to prevent the transmission of fungal pathogens causing oak wilt and Dutch elm disease, a better approach would be to avoid pruning susceptible species during periods when insects for these diseases are active.

Doing It
    Dormantly

    There’s little disagreement among researchers that the dormant season, defined as the period between the dropping of leaves in the fall and the expansion of buds in the spring, is the best time for tree pruning. But there are some keys to remember during dormant pruning, according to John Ball, associate professor of forestry, South Dakota State University, Brookings, S.D.:

    • Pruning will result in sap flow or “bleeding” in the spring in certain species because of root pressure or stem pressure to push sap flow. This won’t harm the trees, but contractors should alert customers to it before hand.
    • Pruning trees that are considered marginally hardy for a region early in the dormant period can result in an increase of winter damage around the wounded cell, particularly if winter conditions are worse than normal. Heavy pruning of marginally heavy species can kill the entire tree.
    • Over pruning during the dormant season is a common mistake without leaves on the trees to help direct contractors. Such over pruning can easily lead to increased ice damage, however, because the branches cannot support each other’s weight. Also, branches that were shaded for bud formation may produce less efficient leaves because the leaves cannot readily adapt to the increased sunlight after being developed in the shade.
    – Bob West

Pruning Deciduous Trees. Ideally, tree pruning should begin at time of planting. Trees that receive proper pruning when they are young usually have fewer structural problems at maturity than those left to fend for themselves. But avoid the temptation to thin a young tree’s crown. Excessive pruning removes leaves needed to manufacture carbohydrates for the developing tree.

Severe pruning also removes stored energy in branches and buds the tree will need for root growth. Studies have shown that post-planting growth is more rapid and trees establish sooner if pruning at planting time is limited to only weak, dead, diseased, rubbing or injured branches.

Also, avoid removing the many small side branches along the trunk. Lateral branches help the trunk increase in base diameter, or caliper, and make for a sturdier tree. Laterals also help shade the trunk, reducing the chance of sunscald injury, and ward off equipment operators, animals and vandals. In general, two-thirds of the tree height should be left as crown.

After the young tree has become established in the landscape (usually one year after planting), pruning really becomes a job of training. There are two general concepts contractors can use as guides: first, training or pruning should take place progressively over the next three to five years, and second, no more pruning should take place in a single year than is needed to enhance shape or structural strength.

The first step is identifying those primary limbs, called scaffold limbs, that will eventually make up the tree’s framework. The height to the lowest limb will be determined, in part, by the anticipated activities that will occur under or near the tree.

Scaffold branches selected should be spaced evenly and distributed radially around the trunk, and they should be roughly one-half the diameter of the trunk or less. Optimally, major limbs on large-growing shade trees should be spaced 18 to 24 inches apart on alternative sides of the trunk. For smaller-growing trees, a spacing of 6 to 12 inches is recommended. And never let one limb grow directly over a lower one.

Unless the tree has a natural multi-stemmed habit, it should be trained to have a single, central, dominant leader. The central leader is the uppermost vertical stem extending from the trunk. Laterals that threaten to grow taller than the leader should be pruned back. Double leaders, if left unattended, can pose problems for trees as they age. Either of the two stems, usually the weaker stem, should be removed.

Branches that ascend from trees at steep angles and double leaders that grow close together are often poorly attached because of the formation of included bark. This is bark that turns inward at the point where branch and trunk, or double leaders, meet. As affected branches increase in size, they often tear away from the tree. These weakly attached branches should be pruned from the tree as early as possible.

As training continues in subsequent years, other practices will be necessary to maintain tree health and vigor. Laterals that have grown higher than the terminal leader or beyond the perimeter of the crown should be pruned back. Any laterals that have grown inward toward the center of the crown should be removed back to their point of origin. And structurally weak water sprouts and basal sprouts, commonly known as suckers, should be removed to preserve beauty and natural growth habit.

Mature trees should be pruned cautiously. Dead branches and those that are structurally unsound should be removed as soon as they are noticed. But because they depend on carbohydrates produced by leaves and stored in branches, mature trees should never be pruned or thinned excessively.

Under no circumstances should trees ever be topped. Large branch stubs that result from topping are open invitations to insects and wood-rotting pathogens. Once decay enters the stub, it may progress into the trunk, weakening the tree and creating a hazard for people and property.

The author is assistant professor in the department of horticulture at Iowa State University, Ames, Iowa.

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