Tree Care: Pruning

PRUNING

Proper pruning enhances the beauty of almost any landscape tree and shrub, while improper pruning can ruin or greatly reduce its landscape potential. In most cases, it is better not to prune than to do it incorrectly. In nature, plants go years with little or no pruning, but man can ruin what nature has created. By using improper pruning methods healthy plants are often weakened or deformed. In nature, every plant eventually is pruned in some manner. It may be a simple matter of low branches being shaded by higher ones resulting in the formation of a collar around the base of the branch restricting the flow of moisture and nutrients. Eventually the leaves wither and die and the branch then drops off in a high wind or storm. Often, tender new branches of small plants are broken off by wild animals in their quest for food. In the long run, a plant growing naturally assumes the shape that allows it to make the best use of light in a given location and climate. All one needs to do to appreciate a plant's ability to adapt itself to a location is to walk into a wilderness and see the beauty of natural growing plants.

Pruning, like any other skill, requires knowing what you are doing to achieve success. The old idea that anyone with a chain saw or a pruning saw can be a landscape pruner is far from the truth. More trees are killed or ruined each year from improper pruning than by pests. Remember that pruning is the removal or reduction of certain plant parts that are not required, that are no longer effective, or that are of no use to the plant. It is done to supply additional energy for the development of flowers, fruits, and limbs that remain on the plant. Pruning, which has several definitions, essentially involves removing plant parts to improve the health, landscape effect, or value of the plant. Once the objectives are determined and a few basic principles understood, pruning primarily is a matter of common sense.

The necessity for pruning can be reduced or eliminated by selecting the proper plant for the location. Plants that might grow too large for the site, are not entirely hardy, or become unsightly with age should be used wisely and kept to a minimum in the landscape plan. Advances in plant breeding and selection in the nursery industry provide a wide assortment of plants requiring little or no pruning. However, even the most suitable landscape plants often require some pruning. The guidelines presented in this publication should be helpful when pruning any plant.

Reasons for Pruning

* To train the plant
* To maintain plant health
* To improve the quality of flowers, fruit, foliage or stems
* To restrict growth
* To remove potential hazards

When to Prune

Pruning can actually be done at any time of the year; however, recommended times vary with different plants. Contrary to popular belief, pruning at the wrong time of the year does not kill plants, but continual improper pruning results in damaged or weakened plants. Do not prune at the convenience of the pruner, but rather when it results in the least damage to the plant. There is little chance of damaging the plant if this rule is followed. In general, the best time to prune most plants is during late winter or early spring before growth begins. There are exceptions to this rule, and they will be noted under the discussion of the specific plant groups. The least desirable time is immediately after new growth develops in the spring. A great amount of food stored in roots and stems is used in developing new growth. This food should be replaced by new foliage before it is removed; if not, considerable dwarfing of the plant may occur. This is a common problem encountered in pruning.

Plant Solutions Arborists Making Pruning Cuts Correctly
To encourage rapid healing of wounds, Plant Solutions Arborists make all cuts clean and smooth. This requires good, sharp pruning equipment.

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 we cut back to should have a diameter of at least half that of the branch to be removed. We make slanting cuts when removing limbs that grow upward; this prevents water from collecting in the cut and expedites healing.

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, we 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.

Developmental Pruning to young trees and shrubs

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.

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.

Pruning Mature Trees

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.

A Plant Solutions Arborists 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 in winter because fewer precautions are necessary to avoid garden and flower bed damage and cleanup is easier.

Pruning Shrubs

Deciduous and Flower Shrubs

Pruning recommendations for most deciduous shrubs consist of thinning out, gradual renewal and rejuvenation pruning. In thinning out, a branch or twig is cut off at its point of origin from either the parent stem or ground level (Figure 8). This pruning method results in a more open plant; it does not stimulate excessive new growth, but does allow room for growth of side branches. Considerable growth can be cut off without changing the plant's natural appearance or growth habit . Plants can be maintained at a given height and width for years by thinning out.

Pruning Hedges

Hedges are a row of plants that merge into a solid linear mass. They have served gardeners for centuries as screens, fences, walls and edging.

A well-shaped hedge is no accident. It must be trained from the beginning. Establishing a deciduous hedge begins with selection of nursery stock. Choose young trees or shrubs 1 to 2 feet high, preferably multiple-stemmed.

In the third year, start shaping. Trim to the desired shape before the hedge grows to its desired size. Never allow plants to grow untrimmed to the final height before shearing; by that time, it is too late to get maximum branching at the base. Do not allow lower branches to be shaded out. After the hedge has reached the desired dimensions, trim closely in order to keep the hedge within chosen bounds.

Evergreen nursery stock for hedging need not be as small as deciduous material and should not be cut back when planted. Trim lightly after a year or two. Start shaping as the individual plants merge into a continuous hedge. Do not trim too closely because many needle-bearing evergreens do not easily generate new growth from old wood.

Hedges are often shaped with flat tops and vertical sides; however, this unnatural shape is seldom successful. As far as the plant is concerned, the best shape is a natural form, with a rounded or slightly pointed top and with sides slanting to a wide base (Figure10).

After plants have been initially pruned to include low branching, maintain by trimming the top narrower than the bottom so that sunlight can reach all of the plant leaves (Figure 11).

These questions often arise: How often should a hedge be trimmed? When should I trim? Answers depend to some extent on how formal an appearance is desired. In general, trim before the growth exceeds 1 foot. Hedges of slow-growing plants such as boxwood need to be trimmed sooner. Excessive untrimmed growth will kill lower leaves and will also pull the hedge out of shape. Trimming frequency depends on the kind of shrub, the season and desired neatness.

What can be done with a large, overgrown, bare-bottomed and misshapen hedge? If it is deciduous, the answer is fairly simple. In spring before leaves appear, prune to 1 foot below desired height. Then carefully trim for the next few years to give it the desired shape and fullness. Occasionally, hedge plants may have declined too much to recover from this treatment, making it necessary to replace them.

Rejuvenating evergreen hedges is more difficult. As a rule, evergreens cannot stand the severe pruning described above. Arborvitae and yew are exceptions. Other evergreen hedges may have to be replaced.

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