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Tomatoes 2008
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Pruning Landscape Plants To prune or not to prune? This is a question gardeners often face. Many feel they should prune, but they are not sure why or how. Pruning is an accepted practice for the orchard and in the rose garden, but it may be rather haphazard in the landscape. Most often it is only performed when a shrub or tree begins to encroach on surrounding plants, a path or a building. What then is pruning? Why, when and how should it be done? Pruning is the removal of a part or parts of a woody plant for a specific purpose. This section explains the reasons for pruning, the proper techniques and when various types of plants should be pruned. Reasons for Pruning
Training Trees. The first pruning after trees and shrubs are received consists of removing broken, crossing and pest-infested branches. With trees, the traditional rule of pruning one-third of the top growth at transplanting to compensate for root loss is no longer valid for properly-pruned, nursery-grown plants. According to recent research, excessive pruning at transplanting reduces plant size and does not aid in plant survival. 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 burr oak, locust, cottonwood, 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 greatest 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 have 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. If this has not been done in the nursery, it can be started 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 will 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. Maintaining Plant Health. In pruning to maintain plant health, the first consideration is sanitation to eliminate dead, dying or diseased wood. Any dying branch or stub can be the entry point or buildup chamber for insects or fungi that could spread to other parts of the tree. When removing wood infected by disease, such as a fungal canker or fire blight, it is important that the cut be made in healthy wood beyond the point of infection. The development of a sound framework will help prevent branches from shading other branches lower on the plant. Evergreen shrubs will usually benefit from an occasional thinning of foliage. Thinning allows light and air to penetrate throughout the shrub, resulting in even foliage growth. Improving the Quality of Flowers, Fruit, Foliage or Stems. The more flowers and fruit a plant produces, the smaller they become, as seen on an unpruned rose bush or fruit tree. By reducing the amount of wood, pruning diverts energy into the production of larger, though possibly fewer, flowers and/or fruit. Most flowering shrubs will bloom either on last year’s growth or on new growth. Properly-timed pruning increases the production of wood that bears flowers. Restricting Growth. Over time, trees and shrubs often grow to sizes that exceed the space allowed for them. When space is limited, regular pruning becomes necessary to keep plants in bounds. Regular pruning is necessary on formal hedges to maintain a uniform growth rate. To reduce labor, select plants that will not exceed allotted space. Know the mature size of the plants and place them accordingly in the landscape. Pruning Techniques Over the years great discussions (arguments) have been had concerning pruning techniques from topping to hedge clipping. The following techniques are well documented and provide guidelines in pruning trees and shrubs properly. Pruning Twigs and Small Branches. When pruning twigs and small branches, always cut back to a vigorous bud or an intersecting branch. When cutting back to a bud, choose a bud that is pointing in the direction you wish the new growth to take. Be sure not to leave a stub over the bud or cut too close to the bud. When cutting back to an intersecting (lateral) branch, choose a branch that forms an angle less than 45 degrees with the branch to be removed. Also, the branch that you cut back to should have a diameter 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. Pruning 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. The collar is an area of tissue that contains a chemically-protective zone. In the natural decay of a dead branch, when the decay advancing downward meets the internal protected zone, an area of very strong wood meets an area of very weak wood. The branch falls away at this point, leaving a small zone of decayed wood within the collar. The decay is walled off in the collar. When all goes according to nature’s plan, this is the natural shedding process. When the collar is removed, the protective zone is removed, causing a serious trunk wound. Wood-decay fungi can easily infect the trunk. If the pruned branch is living, removal of the collar at the base still causes injury. When cutting branches more than 1½ 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 from 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 trunk than 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 cut branch damaging other limbs or objects on the ground, it should be properly roped and supported, then carefully lowered to the ground after the second cut. For over half a century, recommendations for pruning have been to flush-cut and paint. The flush-cut increases tree injury, and the paint only hides it. Pruning paint is primarily cosmetic, a psychological treatment for the person doing the pruning to show that he or she has done something to “help” the tree. In fact, paints or wound dressings may trap moisture and increase disease problems. The only exception to the need for pruning paint is a result of the threat of disease in New Mexico. Painting freshly-cut limbs seems to reduce the chance of infection caused by insects which transmit disease through new wounds. Topping versus Thinning. All too often trees are topped to reduce size or 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 can seriously affect 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 trees by making them susceptible to attacks by insects 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. Pollarding. Pollarding is a pruning technique used extensively in Europe. It results in thousands of healthy trees lining city streets in what is considered a very adverse environment. For example, London planetrees have lived for 100 years through the use of pollarding. In a sense, pollarding allows an urban home gardener to grow a “bonsai” of a mature tree. Pollarding is often confused with topping; both techniques severely prune the tree back to large diameter branches. The difference is that with pollarding, follow-up thinning is performed 1 and perhaps 2 years later. By thinning out the dense flush of new growth, the tree’s integrity and strength are assured. Pruning Deciduous Shrubs. Pruning recommendations for most deciduous shrubs include 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, a lateral side branch, the “Y” of a branch junction or at ground level (Figure 7.30). 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 habit of growth. Plants can be maintained at a given height and width for years by thinning out. This method of pruning is best done with hand pruning shears, loppers or a saw, but it should not be done with hedge shears. Thin out the oldest and tallest stems first. In gradual renewal pruning, a few of the oldest and tallest branches are annually removed at or slightly above ground level. Some thinning out pruning may be necessary to shorten long branches or maintain a symmetrical shape. To rejuvenate an old overgrown shrub, remove one-third of the oldest, tallest branches at or slightly above ground level before new growth starts. The general pruning procedure shown for crape myrtle applies to many large shrub and small tree species. If a shrub is grown for its flowers, pruning must be timed to minimize disruption of blooming. Spring flowering shrubs bloom on last season’s growth and should be pruned soon after they bloom. This allows for vigorous summertime growth and results in plenty of flower buds the following year. Examples of shrubs that bloom on last season’s growth include the following: Cercis canadensis Redbud Chaenomeles japonica Japanese quince Chionanthus virginicus Fringe tree Forsythia spp. All forsythia species Lonicera spp. Honeysuckle Rapheolepis indica Indian hawthorn Rosa spp. Rambling rose species Spiraea spp. Early white spirea species Viburnum spp. Viburnum species Some shrubs that bloom after June usually do so from buds which are formed on shoots that grow the same spring. Such shrubs should be pruned in late winter to promote vigorous shoot growth in the spring. Examples of shrubs that bloom on current season’s growth include the following: Abelia X grandiflora Glossy abelia Buddleia davidii or B. globosa Butterfly bush Hibiscus syriacus Shrub althea Hypericum spp. St. Johnswort Lagerstroemia indica Crape myrtle Rosa spp. Bush rose Vitex agnus-castus Chaste tree Pruning Evergreen Shrubs. For most evergreen shrubs, thinning out is the most desirable pruning procedure. Some evergreens can be sheared for a stiff, formal appearance; however, they still need to be thinned occasionally. Prune both evergreen and deciduous shrubs grown for foliage in late winter before new growth starts. Minor corrective pruning can be done at any time. 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 edgings. A well-shaped hedge is no accident. It must be trained from the beginning. The establishment of a deciduous hedge begins with the selection of nursery stock. Choose young trees or shrubs 1 to 2 feet high, preferably multiple-stemmed. Cut the plants back to 6 or 8 inches when planting; this induces low branching. Late in the first season or before bud-break in the next season, prune off half of the new growth. In the following year, again trim off half. In the third year, start shaping. Trim to the desired shape before the hedge grows to its desired size. Never allow the plants to grow untrimmed to the final height before shaping; 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 (Figure 7.32). After plants have been initially pruned to induce low branching, the low branching is maintained by trimming the top narrower than the bottom so that sunlight can reach all of the plant leaves (Figure 7.33). These questions often arise and the answers depend to some extent on how formal an appearance is desired. How often should this hedge be trimmed? When should I trim? 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 the desired height. Then carefully trim for the next few years to give it the desired shape and fullness. Hedge plants may occasionally decline too much to recover from this treatment, thus making it necessary to replace them. Rejuvenating evergreen hedges is more difficult. As a rule, evergreens cannot stand the severe pruning described. Arborvitae and yew are exceptions. Other evergreen hedges may have to be replaced. Tools. What tools should be used to trim hedges? The traditional pair of scissor-action hedge shears is still the best all-around tool. It will cut much better and closer than electric trimmers which often break and tear twigs. Hand shears can be used on any type of hedge, while electric trimmers do poorly on large-leaved and wiry-twigged varieties, and sometimes jam on thick twigs. Hand shears are also quieter, safer and are less likely to gouge the hedge or harm the operator. Hand pruners are useful in removing a few stray branches and are essential if an informal look is desired. Large individual branches can be removed with loppers or a pruning saw. Chain saws are not recommended for use on hedges. Pruning Roses. All roses need some type of pruning. If roses are not pruned for a number of years, the plants deteriorate in appearance, flowers become smaller and smaller, and disease and insect problems are more likely. Hybrid Tea, Grandiflora and Floribunda roses require annual pruning in the spring just prior to bud break. If rosebushes are pruned too early, injury from late frost may make a second pruning necessary. Steps in pruning. Begin in late winter by removing all dead and diseased wood at least 1 inch below the damaged area. Remove all weak shoots. If two branches rub or are close enough that they will do so soon, remove one. On old, heavy bushes, cut out one or two of the oldest canes each year. Cut back the remaining canes. The height to which a rose should be cut back varies depending on the normal growth habit of the particular cultivar. The average pruning height for Floribundas and Hybrid Teas is between 12 and 18 inches, but taller growing Hybrids and most Grandifloras may be left at 2 feet. Make cuts at a 45 degree angle above a strong outer bud. Aim the cut upward from the inner side of the bush to push growth outward and promote healthy shoots and quality flowers. Other types of roses have special pruning considerations, some of which are described below. Standard or tree roses. A tree rose is a Hybrid Tea, Grandiflora or Floribunda budded at the top of a tall trunk. Prune tree roses as you do Hybrid Teas, cutting the branches to within 6 to 10 inches of the base of the crown in order to encourage rounded, compact and vigorous new growth. Miniature roses. Miniature roses are 6 to 12 inches high with tiny blooms and foliage. Miniature roses do not need special pruning; just cut out dead growth and remove the hips. Ramblers. Old-fashioned Rambler roses have clusters of flowers usually less than 2 inches in diameter. They often produce pliable canes 10 to 15 feet long in one season. Ramblers produce best on 1-year-old wood; this year’s choice blooms come from last year’s growth. Prune immediately after flowering. Entirely remove some of the large old canes, and tie new canes to a support for next year. Large-flowering climbers. Climbing roses have large flowers, more than 2 inches across, that are borne on wood 2 years old or older. Climbing rose canes are larger and sturdier than Rambler canes. Some climbing roses flower only in June, but others, called everblooming climbers, flower more or less continuously. This group should be pruned in autumn before cold weather sets in. First, cut out dead and diseased canes. Then remove one or two of the oldest canes each season at ground level to make room for new canes. The laterals, or side shoots, are shortened 3 to 6 inches after flowering. If the plant is strong, keep five to eight main canes; these should be tied to a trellis, fence, wall or other support. If it is not strong, leave fewer canes. |