Proper Pruning Techniques part 3

Author: Douglas F. Welsh, Extension Horticulturist; Everett Janne, Extension Landscape Horticulturist (retired)

Making Pruning Cuts Correctly

To encourage rapid healing of wounds, make all cuts clean and smooth. This requires good, sharp pruning equipment. Do not leave stubs since they are usually where die back occurs. Avoid tearing the bark when removing large branches. The following provides some specifics on pruning techniques.

Most woody plants fall into two categories based on the arrangement of the buds on the twigs and branches. In general, the bud arrangements determine the plants's typical growth habit. Buds may have an alternate or an opposite arrangement on the twigs. A plant with alternate buds usually is rounded, pyramidal, inverted pyramidal, or columnar in shape. Plants having opposite buds rarely assume any form other than that of a rounded tree or shrub with a rounded crown. The position of the last pair of buds always determines the direction in which the new shoot will grow. Buds on top of the twig probably will grow upward at an angle and to the side on which it is directed. In most instances, it is advisable to cut back each stem to a bud or branch. Selected buds that point to the outside of the plant are more desirable than buds pointing to the inside. By cutting to an outside bud, the new shoots will not grow through the interior of the plants or crisscross.

When cutting back to an intersecting (lateral) branch, choose a branch that forms an angle of no more than 45 degrees with the branch to be removed (Figure 5). Also, the branch that you cut back to should have a diameter of at least half that of the branch to be removed. Make slanting cuts when removing limbs that grow upward; this prevents water from collecting in the cut and expedites healing.

To "open" a woody plant, prune out some of the center growth and cut back terminals to the buds that point outward. In shortening a branch or twig, cut it back to a side branch and make the cut 1/2 inch above the bud. If the cut is too close to the bud, the bud usually dies. If the cut is too far from the bud, the wood above the bud usually dies, causing dead tips on the end of the branches. When the pruning cut is made, the bud or buds nearest to the cut usually produce the new growing point. When a terminal is removed, the nearest side buds grow much more than they normally would, and the bud nearest the pruning cut becomes the new terminal. If more side branching is desired, remove the tips of all limbs. The strength and vigor of the new shoot is often directly proportioned to the amount that the stem is pruned back since the roots are not reduced. For example, if the deciduous shrub is pruned to 1 foot from the ground, the new growth will be vigorous with few flowers the first year. However, if only the tips of the old growth are removed, most of the previous branches are still there and new growth is shorter and less vigorous. Flowers will be more plentiful although smaller. Thus, if a larger number of small flowers and fruits are desired, prune lightly. If fewer but high quality blooms or fruits are wanted in succeeding years, prune extensively.

Thick, Heavy Branches

According to Dr. Alex Shigo, USDA Forest Service plant pathologist, thick and heavy branches should be removed flush to the collar at the base of the branch, not flush with the trunk (Figure 6). The collar is an area of tissue containing a chemically protective zone. In the natural decay of a dead branch, when the decay advancing downward meets the internal protected zone, with an area of very strong wood meets an area of very weak wood. The branch then falls away at this point, leaving a small zone of decayed wood within the collar. The decay is walled off in the collar. This is the natural shedding process when all goes according to nature's plan. When the collar is removed, the protective zone is removed, causing a serious trunk wound. Wood-decay fungi can then easily infect the trunk. If the pruned branch is living, removing the collar at the base still causes injury.

When cutting branches more than 1 1/2 inches in diameter, use a three-part cut. The first step is to saw an undercut from the bottom of the branch about 6 to 12 inches out of the trunk and about one third of the way through the branch. Make a second cut from the top, about 3 inches further from the undercut, until the branch falls away. The resulting stub can then be cut back to the collar of the branch. If there is danger of the branch damaging other limbs or objects on the ground, it should be properly roped and supported, then carefully lowered to the ground.

Topping Versus Thinning

All too often trees are topped ("dehorned") to reduce size or to rejuvenate growth. In either case topping is not a recommended practice; in fact, some refer to it as the "Texas chain saw massacre". Topping is the process whereby a tree is cut back to a few large branches. After 2 to 3 months, regrowth on a topped tree is vigorous, bushy and upright. Topping seriously affects the tree's structure and appearance. The weakly attached regrowth can break off during severe wind or rain storms. Topping may also shorten the life of a tree by making it susceptible to attack by insect and disease.

Thinning is a better means of reducing the size of a tree or rejuvenating growth. In contrast to topping, thinning removes unwanted branches by cutting them back to their point of origin. Thinning conforms to the tree's natural branching habit and results in a more open tree, emphasizing the branches' internal structure. Thinning also strengthens the tree by forcing diameter growth of the remaining branches.

Training Young Trees

Young trees can be trained using pruning techniques which will help promote plant health and long life.

The first pruning after trees and shrubs are purchased consists of removing broken, crossing and pest-infested branches. The traditional recommendation of pruning up to one-third of top growth when transplanting to compensate for root loss is no longer valid, according to recent research. Excessive pruning at transplanting reduces leaf area, which decreases the amount of plant energy generated which are needed to create a healthy root system. When transplanting woody plants, the only necessary pruning is the removal of broken or damaged branches.

The central leader of a tree should not be pruned unless the leader is not wanted, as is the case with some naturally low-branched trees or where multiple-stemmed plants are desired. Trees with a central leader, such as Texas red oak, sweet gum or magnolia, may need little or no pruning except to eliminate branches competing with the central leader. These competing branches should be shortened. Some pruning may be necessary to maintain desired shape and to shorten extra vigorous shoots.

The height of the lowest branch can range from a few inches above the ground for screening or windbreaks, to more than 7 feet above the ground near a street or patio. Removal of lower limbs is usually done over a period of years beginning in the nursery and continuing for several years after transplanting until the desired height is reached.

The concept in training a tree called "the trashy trunk" refers to this gradual raising of the lowest branches of a tree. Lower branches on the main trunk help create a thicker trunk more quickly. A common mistake in pruning young trees is to strip them of small branches leaving only a tuft of leaves at the top of the tree. This training is incorrect and forms a weak "buggy whip" trunk. Remove lower limbs when they reach 1 inch in diameter. This prevents permanent scarring of the trunk caused by removing larger limbs.

Another important concept in training trees is light versus heavy cuts. This refers to the length of the branch being removed and the desired growth response of that branch. On a young, vigorously growing branch, if the terminal end is lightly cut back (less than 6 inches), then lateral branching is induced up and down the branch. On the contrary, if this branch is heavily cut back (from 6 inches to several feet), the one or two buds located just below the cut are forced and grow at a very rapid rate. The importance of this pruning concept lies in the development of bushy, well-shaped trees through light pruning and the often-desired invigorating effect of heavy cuts.

For greater strength, branches selected for permanent scaffolds must have a wide angle of attachment to the trunk. Branch angles less than 30 degrees from the main trunk result in a very high percentage of breakage, while those between 60 and 70 degrees have a very low breakage rate.

Vertical branch spacing and radial branch distribution are important (Figure 7). If this has not been done in the nursery, start it at transplanting.

Major scaffold branches of shade trees should be vertically spaced at least 8 inches apart and preferably 20 to 24 inches apart. Closely spaced scaffolds have fewer lateral branches resulting in long, thin branches with poor structural strength.

Radial branch distribution should allow five to seven scaffolds to fill the circle of space around a trunk. Radial spacing prevents one limb from overshadowing another, which in turn reduces competition for light and nutrients. Remove or prune shoots that are too low, too close or too vigorous in relation to the leader and to selected scaffold branches.

Pruning Mature Trees

The home gardener should limit pruning of mature trees to smaller branches that can be reached from the ground. Leave the trimming of large branches and work off the ground to professional arborists who are skilled climbers and have proper equipment and insurance. Trees generally require less pruning than other ornamentals in the landscape but may occasionally need corrective pruning to maintain health and vigor. Mature trees are generally pruned only for sanitation, safety or to restrict size. Trees are best pruned during the dormant season. This is especially true for oaks to help prevent the spread of oak wilt

An experienced tree professional can easily distinguish between live and dead wood in winter. Winter pruning is often preferred because it is easy to visualize shaping when foliage is gone. Such work can also be done at a lower cost in winter because fewer precautions are necessary to avoid garden and flower bed damage and cleanup is easier.

Repairing Tree Injury

Injuries to trees that expose the wood or kill the bark may allow insects or disease organisms to enter the tree. Proper treatment protects the tree and promotes faster healing. Few trees reach maturity without receiving one or more wounds from a variety of sources. Yet trees have survived for centuries to become the oldest living creatures on earth despite wounding. Some recent work has involved dissecting trees in an effort to understand how they compartmentalize and close an injury. Trees do not heal in the true sense of the word. Injured tree tissue is never repaired and returned to the former state as is a cut on a person's hand. Trees react by closing the wound and compartmentalizing or isolating the injured tissue from the surrounding tissue. During compartmentalization enclosure, contents from the injured cells leak onto the uninjured surface where they oxidize and form a barrier to prevent further infection. Then the most recently laid down wood is altered as is the tissue around the injury. This is accompanied by discoloration, the extent of which depends on the kind of tree, the vigor, kind of wound, location of the wound and the time of wounding. New growth rings are laid down the following spring and new tissue begins to grow over the injured tissue. Over a period of time, the new tissue closes the wound.

Homeowners can help the plant compartmentalize the damage more rapidly than it does in nature. If bark has been crushed or stripped from the trunk, remove the injured bark, shape the wound. Cut away all damaged bark and remove isolated scraps from the wound area. For fastest healing, shape the edge of the wound, as nearly as possible, to an elongated ellipse. If this shape cannot be obtained, shape the top and bottom of the wounded area so they come to a point, even if the wound must be enlarged slightly. Remove all splintered wood and smooth the surface of the exposed area with a chisel.

Some true injuries result in cavities or hollows within the main trunk or large branch of a tree. For many years gardeners have tried to fill these cavities with bricks, concrete and other materials in an effort to seal the cavity from rain, insects and diseases. Armed with the knowledge of the plant's ability to compartmentalize any wound it is not recommended to fill tree cavities. If water does not drain easily out of the cavity, many arborists will recommend trimming the cavity opening so that water can drain out. If this is not possible, a weep hole may be drilled into the bottom of the cavity to allow water to drain freely. Other than these actions, simply keeping the cavity clean of debris and leaves is all that is recommended.

For a year or more after a tree has been struck by lightning, it is often difficult to determine the extent of damage since much of the injury may be internal. Trees that seem badly damaged may live while others apparently only mildly injured may die. If the tree can be saved, remove all shattered parts and damaged limbs; then smooth and paint exposed wood.

In storm-damaged trees, remove all broken branches and reshape the tree as well as possible at the particular time. Try to encourage new branch development in areas with broken branches. Broken trunks, split crotches or cracked limbs often are mended by restoring the damaged part to its original position and holding it there permanently. Consult professional arborists to install screw rods or cables in trees where this work is necessary. ...

(from The Agriculture Program of the Texas A&M University System - Extension Horticulture Information Resource --- used by permission)