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Chapter 14 Considerations for maintaining special sites and trees.

OBJECTIVES

1) Present precautions for pruning mature trees.

2) Contrast pruning strategies for street trees with park and landscape trees.

3) Present special considerations for pruning certain genera and tree forms.

4) Develop an understanding of the challenges of pruning near utilities.

KEYWORDS

Edge trees

Feature trees

Lateral pruning

Mop tops

Ornamental trees

Primary branches

Quaternary branches

Secondary branches

Tertiary branches

MATURE TREES

Pruning large and mature trees focuses on ensuring human safety and passage, minimizing limb failure or total-tree failure near targets such as buildings and cars, and maintaining tree health and vigor. This means (1) minimizing hazardous conditions by cleaning and reducing weight where needed, (2) canopy raising where needed, and (3) maintaining small-diameter interior branches. It may be too late to make meaningful structural changes in trunk and scaffold limb architecture on mature and over-mature trees.

Remove live foliage from a mature or over-mature tree only for good reason! Refrain from removing any live foliage from a stressed tree because they need as much sugar-generating capacity as possible. Removing live tissue on a mature tree removes stored energy and forces the tree to react and expend energy unnecessarily causing many potential problems (Table 14-1). ANSI A300 pruning standards allow up to 25 percent of live foliage removal on mature trees, but this is too much in many circumstances. If you decide to thin an old tree, make cuts primarily on tertiary branches, quaternary branches, and even smaller branches toward the canopy edge only (Figure 14-1). Removing primary branches such as scaffold limbs and more than just a few secondary branches growing from scaffold limbs may leave large pruning wounds and remove too much live tissue. Branches that are more than about a third the diameter of the trunk, and those that are more than about 15 years old (depending on species and other factors) may have poor ability to restrict spread of decay following removal. Consider shortening or thinning the limb by removing tertiary and smaller branches instead of removing the branch entirely.

A common malpractice on large trees is removing many or all interior low branches less than about 4 inches diameter (Figure 14-2). Industry professionals and many educators consider this overthinning or lions-tailing to be no more than an income-generating scheme practiced by uninformed, untrained people practicing arboriculture. Unfortunately, this practice has become commonplace in many communities throughout the United States (see Chapter 13 for more details). When people prune trees in this abusive manner, excessive live tissue is removed from the tree and no structural pruning is performed. This creates poor form and numerous wounds, and the tree becomes more prone to failure especially if there are few trees nearby. Interior branches also provide a local source of carbohydrates needed by the tree to carry on normal defense and other functions. Old trees can decline as a result of removing too much live tissue. Only a small amount of live tissue may be too much to remove on a mature or over-mature tree. There should be a very good reason, such as to reduce likelihood of branch failure, when removing more than 10 percent of the live foliage.

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* Tip: Remove live foliage from mature trees only for good reason.

Older trees that have not been pruned for some time, or those that were never pruned correctly, may have bark inclusions in unions of large branches. It may be difficult to see this from the ground in some trees. Although small branches with included bark are also poorly connected to the tree they may be of lesser priority than the heavier limbs that carry more weight. In addition to a poor structural connection with the trunk, large limbs with included bark can cause unseen injury to the trunk below the union. This occurs because the bark and cambium of both rub against one another as the branch moves in the wind. Look for subtle outward signs of this problem in the union such as bleeding and excessive callus growth at the base of the union. Consider reducing these limbs to lessen the weight toward their ends. This could be combined with cabling and/or bracing systems (see ANSI A300, part 3 Support systems). Many professionals consider this combination the appropriate treatment to lessen likelihood of breakage on large trees.

It is most difficult, lacking any meaningful research in this area, to determine how much to remove (i.e., the dose) when reducing a limb that is potentially at risk of failing. Some refer to this as a limb at risk. Several principles should be considered when making this decision. We know that shortening a limb with a reduction cut reduces end weight, but this can introduce decay at the cut because reduction does not cut back to a natural boundary. Only weak wall 1 retards the spread of decay. We really do not know how much to remove to appropriately reduce an at risk tree by a known amount.

Species play a role in this as well because certain trees are more resistant to decay following reduction cuts than others. It may be reasonable to assume that larger reduction cuts can be made in the trees that compartmentalize decay well (see Table 4-1). Reducing the overall size of the canopy using reduction cuts may be recommended occasionally to clear the tree of structures such as buildings, wires, and lights. Since this can cause severe injury and decline on certain mature trees, consider all other options first!

STREET AND PARKING LOT TREES

Some urban foresters, arborists, and other people familiar with street tree management know that shade trees perform best with one dominant trunk for the first 30 feet or more (Figure 14-3, lower left). Trees trained this way are easier to manage than those with several low leaders or large low branches. Those with large low stems and branches require massive pruning cuts to remove them once they droop in the way of traffic and people (Figure 14-3, top and right). This can lead to the decline of the tree and probably contributes to the short life reported for street trees in many communities.

The best management practices for street tree care include a program that develops a dominant leader high up into the canopy and keeps low branches small (Figure 14-4). This is accomplished by regular pruning in the first fifteen to twenty-five years after planting (see Chapters 11 and 12 for details). Branches above the 30-foot-tall single trunk can be allowed to spread and grow to a larger diameter because they are high enough in many situations to remain clear of vehicles and other obstacles. Even some branches that originate 30 feet from the ground may eventually droop to get in the way on certain species. If this will be the case, shorten them as they grow to prepare them for coming off the tree eventually or to prevent them from getting in the way. Limbs this far up the tree can be guided to grow upward by removing lower branches from the end of the limb. This may allow them to remain on the tree indefinitely. Branches and stems on upright trees can originate low on the trunk, but keep them less than half the trunk diameter with regular pruning.

GOLF COURSE AND PARK TREES

Trees in golf courses, municipal parks, theme parks, cemeteries, and other areas where tall clearance is not required under the canopy can be managed differently than street trees. In some instances, large branches can be left on a lower position in the tree because only lawn mowing equipment needs to pass under them. Management of trees in golf courses and parks depends on where the trees are located.

Trees at the edge of woods

Edge trees have access to sunlight from only one side so they often grow more on that side. This can create an unbalanced situation by encouraging several branches on the sun side to overextend. As these branches grow longer, much of the weight is located far from the trunk so they begin to droop. Look for subtle longitudinal cracks in the branch close to where it meets the trunk. These indicate weakness that could result in breakage. Branches on edge trees might also be blocking play.

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Consider reduction to balance the canopies and to shorten overextended limbs with defects. This often raises the branch and can lessen the likelihood of breakage. Also watch for trees that begin to lean out away from adjacent trees. Root systems or trunks might have been damaged during construction creating an at risk tree--the tree may be at risk of falling over. Consider removing, reducing, cabling, or bracing these trees.

Feature trees

Trees located by themselves surrounded by turf, ground cover, and shrubs are often referred to as feature trees. It is often desirable to allow plenty of sunlight to reach the ground under the tree. Do not be tempted to remove only low branches as this often reduces live crown ratio below 60 percent resulting in a top-heavy condition, excess weight at the ends of limbs, tree decline, and can create a potentially at risk tree condition (Figure 14-1). The tree could blow over.

There are two ways to provide sunlight under a tree canopy. One is to space scaffold limbs far apart creating an open canopy. Instead of scaffold limbs several feet apart, your goal might be to space them 10 feet apart. Trees in a forest often grow this way. Spacing scaffold limbs is easier to do if you begin when trees are young. If the tree is already large and has not received this type of pruning before, proceed toward your goal slowly, over a period of years. You cannot make changes quickly in large trees and maintain the tree in a healthy condition. Start by reducing live branches between scaffold limbs you want to eventually retain (Figure 12-10). Reduce the same branches each year until the desired form and canopy density is reached. Trees tend to fill in holes you create by pruning so this process will have to be an ongoing one for a long time. Trees will remain healthiest if light pruning is conducted each year rather than heavier pruning less often; remove no more than about 5 to 10 percent of the foliage when pruning mature trees annually. The second way to provide more sunlight under the canopy is to perform a traditional thinning operation (Figures 13-2 and 13-3).

Drooping branches on feature trees often require lifting to allow for passage of lawn maintenance and other equipment. Low branches could also be blocking a view or restricting play. Again, be cautious not to overlift or overthin the canopy. Raising can be done by thinning low branches instead of removing them. This makes it difficult to tell the tree was pruned (Figures 13-11 and 13-12) and is considered a more professional approach. Basically, remove the lowest branch, reduce the next one, leave the next one, alternating up the lower third of the tree. This thins the lower canopy and creates a gradually lifting. Removing too many low branches can cause problems.

Trees separating fairways or in clusters

Trees in the middle of a cluster may have a fine form but those on the outside edge often grow as described for edge trees. Plan to manage these as you would edge trees watching for drooping, overextended branches, and unbalanced canopies. The canopies might also have to be raised eventually. Shorten low branches early in their life if they will eventually be removed from the tree (see Chapters 11 and 12).

Root management

Roots sometimes grow into high-value turf areas such as greens, tees, and other athletic turf areas. Tree roots can steal some of the water and nutrients meant for the turf and cause surface irregularities. Roots also can grow under and raise pavement and sidewalks. Pruning roots within about 10 feet of a large tree will certainly stress it and could kill it. If the tree survives it could fall over from lack of ample support roots, especially if soil drainage is poor and all roots are shallow. Roots could rot after root pruning, eventually causing instability. Root barriers near newly installed trees could provide a temporary solution, but some roots eventually grow under many barriers. If roots were pruned close to the tree, consider removing or stabilizing the tree. Some people might recommend canopy reduction and/or thinning to reduce weight in an attempt to keep it from falling.

ORNAMENTAL CHERRIES, CRABAPPLES, AND OTHER ORNAMENTAL TREES

Some of the best-looking ornamental cherries, crabapples, and plums have never been pruned, or only dead branches have been removed. Most people remove too much live tissue from these trees by overthinning or topping them. This causes many water sprouts to generate which quickly grow back to fill in the canopy. Removing excessive live tissue can also lead to decline and eventual death of the tree.

In short, the best strategy is to remove all dead branches including dead stubs, then step back from the tree to determine if you really want more tissue removed. If so, remove no more than 10 to 15 percent of live tissue. This usually means that all crossed or rubbing branches cannot be removed. Remove the rubbing ones that will give you the best aesthetic improvement. Leave as many small twigs as possible along the main branches and trunk since these bear many flowers.

Avoid heading and reduction cuts where possible. These cuts cause great stress on plums and cherries because they decay rapidly. Make only removal cuts on these trees if at all possible, and perform canopy reduction pruning with caution. Do not "clean" the inner portion of the canopy by removing small-diameter branches. A properly thinned tree will have fewer branches than before thinning but every space in the canopy will still be filled with branches. Crabapples and some other small trees often develop one to several branches that shoot out beyond the edge of the canopy. Resist removing or cutting on these since sprouts are sure to follow and fill in to ruin the looks of the tree. At most, shorten them back to the edge of the rest of the canopy. Remove suckers from the roots, graft union, and lower trunk.

If your tree was overpruned, the best course of action is probably to do nothing for two to four years. If you remove sprouts that form the first year or two after overpruning, they are sure to grow back. You will have a yearly pruning job on your hands that will only end when either the tree or you die. Begin removing some of the sprouts when the tree reaches its original size and sprouts begin acting like branches.

FRUIT TREES

In addition to the themes presented in the previous section on ornamental cherries, consider the following. Once major scaffold limbs have been established and dominate the tree structure, live branch pruning can be conducted once or twice each year to clean the canopy. Pruning less often could result in excessive sprouting because too much would be removed at one time. In addition, remove shoots that contribute to crowding (i.e., thin the canopy).

Keeping the center of the tree open by removing upright branches back to more horizontal laterals encourages flowering, helps keep branches low, and makes it easier to pick fruit. Peaches and apples color best in this system because they receive direct sun exposure. If more than 10 percent of live foliage needs to be removed, do it over a period of two years. If you absolutely have to remove more and can live with excessive sprouting, do it late in the dormant season to minimize formation of sprouts and to speed wound closure. Be prepared to fight sprouts for a very long time.

Light summer thinning by removing some live twigs from the edge of the canopy can enhance color on apples and perhaps other fruits but could reduce fruit size. This happens because photosynthetic capacity was removed. Do not remove too much or sunscald on branches could result. On the other hand, thinning the edge of the canopy on citrus enhances fruit color and fruit size, but reduces number of fruit. Remove some young developing fruits on peaches, apples, and other fruit trees to increase the size of the remaining fruits.

Without care, older fruit trees often develop a dense, bushy habit and produce poor-quality fruit. Thin and clean the canopy and make certain that vigorous upright sprouts are removed or cut back with reduction cuts. Remove root sprouts back to their point of origin. Space major limbs apart by removing or shortening other large-diameter branches that are closer than 4 inches. If fruit production is poor remove or shorten upright limbs and shorten major limbs slightly with reduction cuts to stimulate new shoot production. As with most other mature or over-mature trees, do not remove more than 10 percent of the foliage, as this stimulates excessive sprouting. If more needs to be removed, do it over a period of two or three years to prevent the formation of unwanted sprouts, which can spoil your efforts.

CRAPE-MYRTLE AND OTHER SUMMER-FLOWERING TREES

Crape-myrtle and other trees that flower on current season's growth are pruned in a variety of ways. There is little research on the effect of pruning type on growth and flower development. Most has been performed on crape-myrtle. Crape-myrtle pruned hard sprout the most; those that are not pruned sprout very little (Table 14-2). Sprouts on this and other small ornamental trees are a perennial nuisance. Synthetic auxins applied to fresh cuts can slow development of sprouts on some trees. Flower number was greatest when trees were not pruned and professional horticulturists preferred the look of the non-pruned trees. Progressive topping (Figure 10-7) is probably the most common method of maintaining crape-myrtle trees, but it turns out that the resulting flower display and form of the tree is the least preferred.

PALMS AND CYCADS

Palms are pruned to remove dead, diseased, infested, or chlorotic lower leaves; eliminate flowers and fruit; remove leaf bases (boots); or remove sprouts or a stem from a cluster palm for aesthetic reasons. Palms with large dead leaves or fruit left unpruned can pose a potential risk. Removing all leaves from cabbage and Pygmy date palms may reduce transplant shock on those receiving infrequent irrigation after planting. There is no need to remove leaves in an irrigated landscape. The trunk often develops a slight permanent bottleneck in response to this treatment meaning that it will have a smaller diameter than immediately above and below this point. Removing the bud at the top of the trunk will kill a palm. Do not injure the trunk or petioles of remaining leaves with the pruning saw or knife. Spurs used to climb palms are not recommended because they create wounds that do not callus over. Wounds could become entry points for decay organisms and trunk disease (e.g., Thalaviopsis), which compromises strength and health.

Palms are often overpruned (Figure 14-5, right). There is no need to remove green leaves from a palm. Removing green leaves slows growth rate and can cause a narrowing of the trunk and root problems. On some dates and other palms, fusarium wilt can follow and eventually kill those that are overpruned. Insect pests could be attracted to palms following fruit, flower, or leaf removal. Some arborists report that overpruning of green foliage increases the amount of foliage the next season.

With the introduction of the lethal fusarium wilt disease on Canary Island date palm to California and Florida, some professionals recommend not pruning these palms as a measure to prevent spreading the disease. This may not be practical because old leaves that naturally die and turn brown at the base of the canopy remain on the palm for several months or longer. Since this looks unslightly, these are traditionally pruned off to improve appearance. Presently, there is no good solution for this devastating disease. Diseased palms are removed from the site to help prevent the disease from spreading. It has already eliminated this palm from the planting palette of some regions of southern California.

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PRUNING NEAR UTILITY LINES

Electric utilities prune trees to ensure safe, reliable service to their customers and to gain access to utility structures. This practice is referred to as line clearance. Homeowners, horticulturists, and other persons without Electrical Hazard Awareness training must leave this to a line clearance tree trimmer or trainee. Call the utility company or a utility arborist to do this hazardous work. Never prune within 10 feet of a utility conductor unless you have the appropriate training.

Removal of a medium- or large-maturing tree and replanting with a small tree or large shrub is the best method of minimizing pruning requirements near wires. However, this is not viewed as practical in many instances due to the high cost of removing trees. Pruning them is viewed as more economical than removing them. A general rule used by the industry is, unless imminently hazardous, trees are not removed unless the cost of removal is less than 2 1/2 to 3 times the cost of pruning. Most large trees do not meet this criterion because they are expensive to remove. In some instances, wires can be placed underground or moved away from the trees to allow trees to develop properly.

Ice, snow, and wind can break branches growing above or among wires and cause the wires to malfunction or break. The number one cause of outage is branches over wires falling down on the wires. Therefore, medium- and large-maturing trees inappropriately located under or near wires need regular pruning to keep them away. Utility companies often hire specialized line clearance contractors to perform this type of tree pruning. The utility company typically writes the pruning specifications and the contractor prunes trees according to these specifications. Controversy sometimes arises when property owners feel that their trees have been pruned inappropriately or too severely by the line clearance contractor. Citizens should contact the forester at the utility company or the person in charge of line clearance pruning to voice their concerns. Though sometimes mistakes are made, the contractor is almost always following contract specifications as directed by the utility.

Trees pruned with heading cuts generate sprouts that quickly grow back into the wires (Figure 14-6, right). Branches on trees pruned with reduction and remove cuts can be encouraged to grow away from wires (Figure 14-6, left). This is called directional or lateral pruning. In addition to increasing service reliability, directional pruning could reduce pruning costs by increasing the interval between prunings. Pollarding and other forms of architectural pruning are high-maintenance options (see Chapter 10) requiring regular pruning. Pollarding has not been tried on a regular basis in the United States, but deserves consideration in certain instances.

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Trees growing into wires from the side are pruned differently from those under wires (Figure 14-7). Some utility companies prune as shown in option A; others prune less drastically as in option B. Only two pruning cuts were made in each option. The tree in option B may need pruning again sooner than that in option A, but option B results in less debris removal and less trunk decay, and it looks better. Many people would hardly recognize the tree was pruned. Option A can cause more trunk decay because large branches or codominant stems were removed back to the trunk. Large branches and codominant stems lack a branch protection zone at their base. Maintaining a tree as in option B may require more frequent pruning so it could cost more. However, the long-run cost of option A could surpass option B's costs if a severely decayed option A tree fails and damages the line--or, worse, injures someone. Since this is likely to result in litigation against the utility, customers could end up paying more for option A.

In lieu of option A in Figure 14-7, if all lower branches must be removed back to the trunk on the wire side of the tree, reduce the branches as in option B and remove them over two or three pruning cycles, not all at once. If more than half of the foliage must be removed from a branch, or if the sprouts will grow back into the wires quickly, plan on removing the entire branch from the tree. Although there are many exceptions, some utility arborists attempt to make no more than three cuts to remove most of the foliage required to clear the lines of branches. This is not always possible, but provides a guideline to work from.

Lateral branches on trees near wires are often headed to provide clearance (Figure 14-8, option A). This is inappropriate and usually is not in conformance with the ANSI A300 pruning standard. Not only will sprouts quickly grow back into the wires, they are poorly attached to the tree because the headed branches often crack and begin to decay. A much better alternative is a combination of reducing some offending branches and removing others back to the trunk (Figure 14-8, option B).

Depending on the importance of the line, the tree species and condition, utility companies sometimes choose to remove all overhanging branches from one side of the tree. Although this provides the most clearance, it raises aesthetic concerns, and may be extremely damaging to the tree (Figure 14-8, option C). Many trees cannot be expected to recover gracefully from this type of pruning. Removing all branches from one side of the tree at one time is likely to initiate severe trunk decay and cracks and may predispose the tree to sunscald, wind throw, or trunk failure. Bark often dies on the side of the tree facing the wires and roots can decline. If this type of pruning is required to provide clearance consider removing the tree. As an alternative, reduce the offending branches (see Figure 14-8, option B) and remove only one or two back to the trunk at the first pruning. Remove another one back to the trunk at the next pruning cycle and a third one the following time. This will stress the tree less than removing all at once.

It should be apparent that line clearance is controversial. All techniques have serious draw-backs--some worse than others. Moving the wires to a location away from the trees is a possible solution in some cases. Removing large trees near wires and replacing them with small-maturing ones is another good low-maintenance alternative. Communities and utilities will continue to evolve programs to provide for reliable delivery of electricity with minimum impact on trees.

CONIFERS AND OTHER EVERGREENS

Pruning is conducted primarily to change or control the shape, density, or form of the plant or to remove dead, diseased, dying, damaged, or hazardous branches. It is best to locate conifers in the landscape so they will not have to be pruned, because many look odd and are severely injured when extensively pruned. One of the most common types of pruning on large conifers is the removal of the lower branches to create room beneath the canopy for passage of people or vehicles. Branches are removed back to the trunk or, less often, back to a living side branch. Reduction and thinning are usually not appropriate on conifers.

Most conifers grow with one central leader and small-diameter branches, and have a structurally sound form. Little structural pruning is needed. If a double leader develops on a young tree, or in the top portion of an older tree, shorten or remove one of them to prevent the tree from splitting at this point. You may need to install a cable in an older tree if the double leader formed many years ago. If the leader is lost or badly damaged, a branch can be trained to assume the role of the leader (Figure 14-9), but some trees restore themselves without intervention.

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Young arborvitae, falsecypress, Leyland cypress, yew, and some junipers can be cut back into wood 1 or 2 years old that does not have living foliage on it. The best time to do this type of pruning is in the spring, just before new growth emerges. Dormant buds will usually generate foliage and fill in the plant. They can also be sheared on the outside of the canopy to create and maintain a certain shape or size. But remember, shearing could shorten the life of plants.

Because spruces, firs, and most pines (except Canary Island pine and a few others) do not produce shoots from older wood, when pruned they should not be cut back past the current-year twig unless the entire branch is removed back to a lateral branch or the trunk. Abranch with all foliage removed will probably die. Emerging candles on pines can be pinched to create more branches or to slow the growth rate (Figure 14-10). Candles are pinched in the spring just as the needles can be seen emerging from the candle. This encourages several new buds to form at the pinch and creates a more compact plant. Christmas tree growers practice this technique to form symmetrical, dense canopies. The entire candle can be removed directing growth into lateral shoots that were not pinched. Growth can be guided in this manner around a structure or to form an unusual shape. Pines pinched at other times of the year will not form new shoots behind the pinch, and the terminal portion of the branch eventually dies. Spruces and firs can be cut back to a living lateral branch. They can be headed back to any position on the current season's growth because buds exist all along the twig. New growth emerges next spring from buds along last year's twig.

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Branches (other than emerging shoots) on pines may be best pruned during the dormant season so that beetle pests will not be unduly attracted to the tree. Some beetles are attracted to pruned trees in summer droughts and can kill the tree. Most other conifers can be pruned at any time of year, but for maximum tree growth prune just before new growth begins in the spring.

Most small maturing evergreens, such as podocarpus, holly, privet, juniper, viburnums, and wax myrtle, can be pruned anytime. However, in USDA hardiness zones 8 and 9, moderate to severe pruning in the fall through mid-winter could stimulate growth in the dormant season if a warm spell occurs. The tree or shrub could be damaged by subsequent cold weather.

WEEPING TREES

Many weeping trees will grow as a ground cover or sprawling or mounded shrub if not pruned initially to an upright trunk (Table 14-3, right). Some people refer to these as mop tops because their form reminds them of a mop. Even many that are initially trained to one trunk in the nursery develop branches that eventually reach the ground. Branches will grow along the ground unless pruned.

You can enhance growth in the upper portion of the tree and discourage the canopy from sweeping the ground. Encourage growth in branches that are more upright and those at the top of the canopy by shortening branches that will soon reach the ground. Use reduction cuts, where possible, to shorten. This technique is similar to pruning shade trees in that lower branches are shortened to force growth in the upper part of the canopy creating a billowing canopy effect. This technique will help prevent formation of large low branches that might get in the way later. It would be a shame if these attractive, large low branches had to removed later. Some young stems can be headed at the top of the canopy (see Figure 10-9). Do not cut all stems at the top like this; only cut a few scattered in the top. Several sprouts may develop from the heading cuts and grow upright, at least for a short period. Notching a branch or stem above a bud (on the side of the bud away from the trunk) could also encourage a sprout to emerge there.

Other weeping trees generate upright shoots in addition to weeping branches (Table 14-3, left). Several weeping trees such as cultivars of Acer, Picea, some Juniperus, and others normally develop weeping branches from a more or less dominant trunk. To help prevent breakage later, this form should be encouraged by subordinating codominant stems as they develop. Weeping trees that grow to be a large size might also benefit from a dominant leader form.

COLUMNAR AND UPRIGHT TREES

Columnar and upright shaped trees often form a symmetrical, dense canopy. The canopy consists of many small-diameter branches growing from several to many upright stems from the lower, middle, and upper canopy (Table 14-4). Those originating from the lower and middle canopy can grow to be quite large as the trees age. Some become codominant stems with included bark. Despite the bark inclusions, these are normally adequately secured to the tree provided they remain small and do not begin extending outside the edge of the rest of the canopy. When this happens, they become wind loaded and can accumulate ice and snow in storms. Even tight, symmetrical canopies can fail if the larger stems and branches become laden with ice and snow. Small-diameter branches usually have no problem securing themselves to the tree.

If upright stems and branches are regularly subordinated as the tree grows they can be kept in check and remain well secured to the trunk. This is most effective and aesthetically acceptable if the program begins when trees are young. If a large pruning dose is applied for the first time when the tree is in middle age, the canopy might be misshapen beyond what most people will accept. Apply a light dose frequently on trees to prevent large voids. On young trees, it might be best to remove entire stems back to the trunk where possible, especially if there are many smaller-diameter branches near the cut (Figure 14-11). Shortening aggressive stems would also be acceptable. On cultivars with all stems and branches in the upright position, it will be difficult to impossible to apply this treatment.

CHECK YOUR KNOWLEDGE

1) No more than about 10 percent of living foliage should be removed from fruit trees and small flowering trees such as cherries, peaches, and crabapple because:

a. many sprouts will appear along the branches.

b. pruning wounds will close slowly.

c. growth rate will be slowed substantially.

d. the tree canopy will be too open.

2) Only small amounts of live foliage should be removed from mature trees because:

a. it is difficult to reach the ends of the branches where most of the foliage is located.

b. if overpruned, mature trees can become sunscalded, sprout excessively, decay, or roots could decline.

c. mature trees have more dead branches which should be removed instead of live foliage.

d. overpruning can cause formation of wall 4 and a branch protection zone.

3) What is the maximum amount of live foliage that can be removed from a mature tree in one pruning operation according to the ANSI A300 pruning standard?

a. 15 percent

b. 25 percent

c. 35 percent

d. 40 percent

4) Which pruning cut is LEAST appropriate for mature trees?

a. heading

b. reduction

c. removal

d. pollarding

5) Order the pruning cuts from most appropriate to least appropriate for mature trees.

a. heading, removal, reduction

b. removal, reduction, heading

c. reduction, removal, heading

d. removal, heading, reduction

6) When pruning fruit trees it is best to remove what first?

a. sprouts

b. drooping branches

c. upright branches

d. dead branches

7) Assuming the same amount of foliage is removed from the tree in each of the following pruning types, which is likely to cause the greatest harm to a mature tree?

a. canopy raising

b. canopy cleaning

c. canopy thinning

d. canopy reduction

8) Which is the BEST strategy for pruning a medium-aged or mature tree?

a. remove no more than 20 percent of the live foliage

b. remove dead branches first, then live foliage if there is good reason

c. remove no more than 30 percent of the live foliage

d. remove only small-diameter branches

9) What is the BEST method of minimizing the pruning requirement near utility lines?

a. reduction cuts

b. removal and reduction cuts

c. proper directional pruning

d. removing medium- and large-maturing trees

10) Which of the following should not be cut back into 2-year-old wood?

a. arborvitae (Thuja)

b. Leyland cypress (Cupressocyparis)

c. yew (Taxus)

d. fir (Abies)

Answers: a, b, b, a, b, d, d, b, d, d

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CHALLENGE QUESTIONS

1) What do the strategies for pruning palms and pruning shade trees have in common?

2) Describe a strategy for allowing more sunlight to reach under the canopy of a mature oak tree to stimulate better turf growth.

SUGGESTED EXERCISES

1) Find several mature trees on a site. Divide a group into subgroups of about five to seven people. Have each subgroup devise a plan for pruning each tree. Give each subgroup about 15 to 20 minutes for this exercise. Each subgroup will present their plan to the entire group. Be sure the subgroups are instructed to look for all faults and to provide appropriate treatments.

2) Overprune a cherry, plum, or other fruit tree by thinning to remove about 20 to 25 percent of the live foliage. Thin another one by removing about 10 percent. Come back in the growing season a few months later to see the amount of sprouting that occurred. Compare this to a tree that was not pruned.
TABLE 14-1. Risks associated with removing live tissue from old trees.

Forces energy expenditure by initiating compartmentalization
Can cause cracks
Removes energy reserves
Can cause sprouting that further reduces energy reserves
Can cause branch death
Can cause tree death
Reduces available energy storage space

TABLE 14-2. Response of crape-myrtle to pruning types. (1)

                            Pruning Type

                            One-year-old
                            Stems Headed                  Progressive
Response       None         Annually       Pollarding     Topping

Sprouting      minimal      some           some to
                                             moderate     heavy
Flower no.     most         moderate       moderate       least
Panicle size   smallest     intermediate   intermediate   largest
Aesthetics     most         acceptable     acceptable     least
                preferred                                   preferred

(1) Knox and Gilman, unpublished.

TABLE 14-3. Some (1) weeping trees form a billowing
or upright canopy, others form a low, mounded canopy.

Billowing/Upright Canopy       Low, Mounded Canopy

Acacia pendula                 Acer palmatum 'Dissectum'
Acer saccharinum 'Beebe'       Betula pendula 'Youngii'
Betula pendula                 Caragana arborescens 'Pendula'
Callistemon viminalis          Cedrus libani 'Glauca Pendula'
Chamaecyparis nootkatensis     Gleditsia triacanthos
  'Pendula'                      'Emerald Kascade'
Fagus sylvatica 'Pendula'      Larix decidua 'Pendula'
Ilex vomitoria 'Pendula'       Morus alba 'Chaparral'
Juniperus rigida 'Pendula'     Pinus strobus 'Pendula'
J. scopulorum 'Pendula'        Prunus subhirtella 'Pendula'
Picea omorika                  Salix caprea 'Pendula'
Pinus densiflora               Sophora japonica 'Pendula'
Salix babylonica               Tsuga canadensis 'Sargentii'
Syringa pekinensis 'Pendula'   Ulmus alata 'Lace Parasol'
Ulmus parvifolia 'Drake'       Ulmus x vegeta 'Camperdownii'

(1) This is a partial list of plants. See Horticopia
Professional (www.horticopia.com) for more plants.

TABLE 14-4. Some columnar and upright non-coniferous trees that may
benefit from reducing the length* of codominant stems.

Acer x freemanii 'Armstrong' and 'Armstrong II' and Scarlet
Sentinel[TM]

Acer platinoides 'Columnar' and 'Olmsted'
Acer rubrum 'Bowhall' and 'Columnar'
Acer saccharum 'Endowment', 'Goldspire', and 'Newton Sentry'
Agathis robusta
Carpinus betulus 'Fastigiata'
Erythrina variegata var. orientalis 'Tropic Coral'
Fagus sylvatica 'Dawyck'
Gordonia lasianthus
Ginkgo biloba 'Fastigiata' and Princeton Sentry(tm)
Ilex x attenuata (most cultivars)
Magnolia grandiflora (most cultivars)
Magnolia virginiana (cultivars)
Populus alba 'Pyramidalis'
Populus nigra 'Italica'
Prunus sargentii 'Columnaris'
Quercus petrea 'Columna'
Quercus robur 'Skymaster'

* Subordination should begin when the tree is small. Subordinating
these trees later can deform the canopy.
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Article Details
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Author:Gilman, Edward F.
Publication:An Illustrated Guide to Pruning, 2nd ed.
Geographic Code:1USA
Date:Jan 1, 2002
Words:6623
Previous Article:Chapter 13 Pruning types on established trees.
Next Article:Chapter 15 Standards and specifications.
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