Chapter 10 Developing special forms on young plants.
1) Contrast options for developing multi-trunked trees.
2) Train multi-trunked trees to a single leader.
3) Describe the special forms trees can be pruned to and how to create them.
4) Present nursery production and landscape practices far developing special forms.
Modified central leader
DEVELOPING A SINGLE LEADER ON A SMALL ORNAMENTAL TREE OR SHRUB
Trees that stay small at maturity can be trained to a single trunk or left to row into a multi-trunked habit. Most will grow into a multi-trunked habit if left unpruned. The strongest form resulting in the least likelihood of breakage is the single trunk with stems and branches less than half the trunk diameter (Figure 10-1). To train a plant to a single trunk, allow a recently planted liner to grow for about a year. Reduce the length of the stems that compete with one leader in the center of the canopy (Figure 10-2, center left). Cut back aggressive branches to smaller lateral branches using reduction cuts, or simply use heading cuts on young plants (Figure 8-2). Continue this procedure regularly until the desired form is achieved. If needed, remove lateral branches at appropriate times to push more growing into the main leader. You might need to use a stake or splint if you wish to grow a straight trunk on certain trees (see Chapters 8 and 9).
DEVELOPING MULTIPLE TRUNKS
Large-maturing trees are usually not suited for this form. A number of small-maturing trees and shrubs have showy bark and trunk structure that can be displayed nicely by creating a multi-trunked or low-branched tree (Appendix 2). Many develop a multi-stemmed form without pruning. Multi-stemmed shrubs and trees develop one of several forms. Stems can emerge naturally or be developed from a short, single trunk pruned to encourage multiple leader development. For the strongest tree, develop 4 to 6 inches of vertical spacing between multiple trunks arising from a short stem (Figure 10-1, top left, center, and center left). When the tree is young, keep the branch diameter less than one-half the diameter of the main trunk through regular subordination pruning. This allows trunk tissue to develop a collar around the base of the branch and will increase the strength of the attachment. The least desirable form is present when all stems originate from the same position on the trunk, or when included bark forms (Figure 10-1, right). One trunk could split from the plant due to this weak structure.
There are several methods of developing multi-trunked plants. One is to plant two or three seedlings 4 to 12 inches apart, forming a multi-stemmed effect (Figure 10-3, option B). It is crucially important to leave temporary branches along the trunks to help them develop a thick, tapered strong trunk. As the trees grow older in the landscape, the trunks may eventually touch; this may cause them to push each other apart because they are unlikely to fuse. Root systems can also develop asymmetrically, causing a trunk to fall over. Because this is a relatively new planting technique, time will reveal the merits of this practice. It is probably fine for shrubs and small-growing trees such as lilacs, crape-myrtle, ligustrum, and bottlebrush, because if one of the trunks falls over in the future, little damage to property will occur due to its small size. However, this technique should not be practiced on trees maturing at greater than 30 to 40 feet tall due to the potential for injuring a person or property if one of the large trunks should split from the tree. One trunk may grow slowly, spoiling the multi-trunk effect.
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A second method is to make a heading cut on the leader of a young sapling (Figure 10-3, option A). This will stimulate the development of sprouts and lateral shoots. One shoot will become the leader and the others the major branches to make up the structural framework of the tree. All sprouts from this cut are headed, each one to a different length (see Figure 10-3, E). The shoot cut back the least will become the leader; the others will be branches. This provides several stems of different lengths. Each sprout from these cuts is also pruned to a different length. Continue heading sprouts in this fashion until the tree takes on the desired form. Each time you head the sprouts, cut one of them back only slightly. This will become the leader stem. Training a tree in this fashion can create a strong, multi-trunked tree. Apples and crabapples are pruned in this fashion to create strong trees capable of holding fruit. This form is sometimes referred to as the modified central leader system.
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A third method is to head the leader once and let the tree grow with no further training (Figure 10-3, option C). This creates a nicely formed tree, but it will have a weak structure because included bark will often develop in the unions where stems are clustered together. This could cause the tree to split apart as it grows bigger.
A fourth method simply allows the plant to grow with little pruning the first couple years (Figure 10-2, top). Then remove lower branches from main stems to produce a multi-trunked effect. As long as the tree does not develop included bark nor grow too large, this structure is usually strong enough to hold the tree together for a period of years.
DEVELOPING FRUIT TREES
Deciduous fruit trees are typically trained to one of two forms. Apple trees are often trained to a modified central leader whereas peaches, nectarines, plums, and apricots are commonly trained with an open center. There are some orchards that train peaches to a single central leader with no reduction in fruit yield compared to the open-center system (this book does not examine the details of commercial fruit production). Others train plants as espalier or to other forms. Trees trained to the modified central leader (Figure 10-3, option A) system usually have from five to eight scaffold branches spaced 6 to 10 inches apart along a central trunk that can be straight or may zig-zag up through the canopy. Scaffold branches are those that are the largest diameter on the tree. Limbs with wider crotch angles support more fruit than upright limbs, often because included bark forms in the crotches of vigorous, upright limbs. Branches with included bark can break easily. If major branches are forming at desirable positions on the tree, but they are at a narrow angle with the trunk, consider increasing the crotch angle by inserting a wooden dowel sharpened to a point at both ends in the crotch. A nail can be driven into each end of the dowel and sharpened to hold it in place. (The sharpened ends help keep the dowel from slipping off the branch and trunk.) This could slow the growth rate of the branch by directing it to grow more horizontally and helps secure it to the trunk. Subordination cuts (Figure 8-3) also will help reduce the splitting due to included bark by slowing the growth rate on the cut branch, and they can be used in conjunction with crotch spreading. Branches that are subordinated and spread may generate new sprouts along the tops of the branches.
Trees trained to an open-center or open-vase system (Figure 10-3, option D) have three to four scaffold branches originating from a short (2- to 4-foot-long) central leader. The best structure places scaffold branches at least 4 to 6 inches apart (Figure 10-1, top center, center left) so the collar can develop properly around the base of the branches. Scaffold development should begin within the first year or two after planting. Remove root suckers and vigorous sprouts as they emerge early in the growing season in order to direct growth into the developing scaffold branches. Scaffold branches should be selected and should dominate the tree structure by the second or third year after planting. Once the main scaffold branches have been established, in subsequent years you may have to shorten or remove vigorous sprouts in the interior of the canopy in order to keep the tree open in the center. The open-vase system can be more profitable than the central leader system for certain orchards.
DEVELOPING AND MAINTAINING A STANDARD FORM
A tree or large shrub trained to one short, straight trunk with a dense, round canopy is commonly referred to as a standard. A number of small trees are suited for this type of training (Appendix 3). The canopy of a standard has numerous branches. Frequently pinched or headed, the canopy develops into a dense mass of foliage originating from many small-diameter branches. Numerous branches often originate from the same point on the trunk, which would be undesirable if the tree were to be grown to a large mature specimen; however, standards are meant to be maintained with regular clipping and are not designed to become large plants. Therefore, this branch attachment is fine. Never purchase standards if they will not be maintained as such because their branch structure is weak and not suitable for growth into large trees. In many ways, the 'Bradford' Callery pear is grown as a large standard that becomes weak as it grows older.
When necessary, the trunk of a young standard is held straight with a stake or by some other means. This ensures that the tree is secured in an upright position to create a straight trunk. The support is removed as soon as the tree is able to stand under its own weight. Until the tree can stand erect on its own, low-vigor, temporary lateral branches should be left on the lower trunk to increase trunk diameter and strength. Leave as many as possible. Keep them clipped to about 12 inches long to prevent a large-diameter branch from forming that would leave a large wound when removed. Immediately remove any that grow to more than about pinky width. These temporary low branches will be pruned back to the trunk prior to sale in the nursery. Allowing the wind to move the trunk will also help make it stronger.
Standards can take on different forms depending on how they are trained and pruned in the nursery. The easiest method of training is to make a heading cut on the trunk of a young sapling and let the sprouts grow (Figure 10-4, option B). Each sprout can be headed again and again to create a nicely shaped, compact plant (option C). Unfortunately both methods result in a weak plant that is susceptible to splitting at the point where the four main branches meet the trunk, especially if the tree will grow to be taller than about 15 feet.
A better method is to head the leader and then head each of the sprouts to a different length (option A). The sprout cut back the least becomes the leader, the one cut back slightly more becomes the major branch at that point on the trunk, and the other two become much less dominant because they were cut back severely (Figure 10-3, E). Head each of the sprouts that originates from these cuts in the same manner (i.e., cut them back to different lengths). Continue heading in this fashion about every 6 inches up the trunk until the plant develops the form and size you want. Notice in Figure 10-4, option A, that the two branches indicated with arrows are larger and have more secondary branches than the others. These are spaced several inches apart on the trunk, not clustered together as in options B and C. This should help the tree stay together in a wind storm and make it more sturdy overall.
If a standard is to be planted near a street or in a parking lot or other location where pedestrians or vehicles will walk or operate beneath it, the canopy or head should begin at least 6 to 7 feet from the ground. Provided that the tree can stand up without support, the trunk below this point should be clear of branches.
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CREATING SPECIAL EFFECTS
So-called architectural pruning shapes and maintains trees unnaturally to a specific form and size with regular pruning. For example, pleaching interlaces or twines young branches and trunks on saplings together to form a hedge, archway, or tunnel. Curtain creates a flat wall surface with annual heading that is typically accomplished with power shears. Topiary creates an animal, column, ball, or other shape with regular (at least annual) heading cuts. Heading cuts are usually made with power shears forming a smooth surface of foliage on the outside of the canopy. Please note that rounding over (topping) trees every three to five years beneath power lines or in other situations is not topiary because shoots more than a year or two old are headed. Many trees are suited for topiary, including those in the following genera: Acer, Cupressus, Eugenia, Ilex, Podocarpus, Prunus, Quercus (small-leaf types such as live oak), and Taxus. Bonsai is the art of maintaining small trees that involves root pruning and directing shoot growth with pruning and wires. Espalier and pollarding are explained next in detail.
DEVELOPING AN ESPALIER
Plants can be located close to a structure and trained into an espalier. Espalier is a specialized, high maintenance pruning technique requiring patience and regular pruning. A shrub or tree (see Appendix 4) is trained to grow more-or-less flat against a wooden or metal trellis, garden, garage or house wall, fence, or other support. It is a great way to shade the west or east side of a home or office building from the summer sun. Consider painting or mending the wall before planting.
Start the process by installing a young plant 6 to 36 inches away from the support. In deciding how close to plant, be sure to account for trunk and root flare growth over time. Branches growing toward and away from the structure can be bent and tied parallel to the structure. Those that are too big to bend or are in the wrong position may have to be removed back to the trunk; or you could leave a stub with a few buds and train the new sprouts to the desired position. Leave most or all branches growing parallel to the structure.
There are several ways to train an espalier. One is to attempt to space branches several inches or more apart along a single, straight trunk. No branches are opposite each other. Another system locates branches directly opposite each other. And yet another system develops several trunks equal in diameter with branches along each one. There are many more ways to do this.
Develop the basic framework early, removing appropriate branches and pruning with heading cuts regularly and moderately. Light to moderate pruning prevents excessive sprouting. On the other hand, there may be other cases where you want sprouting to occur and can use this to your advantage. If necessary, attach branches using string ties to a decay resistant support to encourage a flat, two-dimensional structure. Replace or adjust ties regularly to prevent them from girdling secured branches.
Any number of shapes and sizes can be developed. Once the main framework and desired size is in place, some horticulturists use pollarding cuts to maintain the espaliered plant at a certain size; others use heading and reduction cuts. Like any pruning technique, this one is best mastered with practice.
DEVELOPING AND MAINTAINING A POLLARD
For centuries in Europe, trees were maintained at a designated height with regular pruning. This practice, called pollarding maintains a tree at any specified height, sometimes for centuries, and provides a formal look to the landscape. Not everyone cares for the look of a pollarded tree, especially in winter. Traditionally, trees have been maintained at 20 to 30 feet tall. Pollarding can be used to keep a large-maturing tree small if it was improperly located in a restricted soil space such as a planter, narrow soil strip, parking lot island, or sidewalk cutout. It is also useful to control size if improperly planted too close to structures such as a building, street light, or electric wire. Once begun, it is essential that pollarding continue.
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Preferably, the pollarding process begins when the tree is very young. Over a six- to ten-year period, and before pollarding begins, the main tree architecture can be developed. In the typical form, upright or horizontal branches with wide angles of attachment spaced along a dominant trunk are chosen for the main scaffold limbs. Many other forms can be created. After this, young stems are headed with a slanted cut at the exact position that the tree will be cut to at each subsequent pruning. Choose these points carefully because they cannot be repositioned at a later time and be sure each is located so sprouts will receive adequate sunlight. If shaded, that part of the plant could decline. In the dormant season, all sprouts but one or two from the original heading cuts are removed back to the point where the original heading cut was made (Figures 10-5 and 10-6). Always remove the most aggressive sprouts. Some people remove all sprouts back to the original heading cut. For the first or second prunings, leave 1 to 2 inches of stub if necessary to ensure that there will be buds to initiate next year's sprouts.
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A knob of tissue called a pollard head resembling a ball develops several years after the first cut was made. Astarch-rich pollard head is a collection of buds, callus, and collars. The head is never to be removed. Most shoots grow from this tissue, which enlarges slightly each year. Most are oriented upright, do not branch, and grow at a rapid rate. They are cut back to this knob at each pruning. Shoots originating below the pollard head should be removed each time the tree is pruned. Do not discard removed shoots; they make great kindling for fires and can be used to tan leather, for animal fodder, and weaving.
A clear distinction must be made between pollarding and topping (Table 10-1). Topping heads branches and stems regardless of their age and diameter and initiates cracks and decay inside the tree, whereas pollarding cuts branches less than 2 years old and minimizes decay. Pollarding is a high-maintenance practice requiring annual or biannual pruning, but it can create unique trees that live for a very long time. Because of the regular pruning requirement, many European communities are abandoning the practice and planting small-maturing trees instead. Others are topping the trees periodically (e.g., every five years). Subsequent topping occurs a foot or more out from the last topping (Figure 10-7). Although decay results from this treatment, trees remain small and some can live for many decades before declining or failing. Pollarding has a place in landscapes and should be tried more often than it is currently. Trees that respond well to pollarding and have lived a long time under this treatment are included in the following genera: Acer, Aesculus, Alnus, Crataegus, Fraxinus, Liriodendron, Morus, Platanus, Quercus, Tilia, and Ulmus. Small-maturing trees such as crape-myrtle also respond well to pollarding (Figure 10-8).
DEVELOPING WEEPING PLANTS
Many trees are grown in ornamental weeping forms (Table 10-2). Some growers may refer to this as a mop-top form because the canopy resembles a mop. Some of these are budded or grafted on top of a straight stem (Figure 10-9). Weeping trees left un-pruned can develop long, stringy branches and a thin canopy that might not be appealing to customers (Figure 10-9, bottom). The canopy density can be increased and the form improved by reducing the length of branches using heading cuts (Figure 10-9, top center). This is best done in the dormant season. Several shoots typically form just behind the pruning cuts. These new shoots are shorter than the removed portion of the cut branch resulting in a more compact, attractive plant.
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CHECK YOUR KNOWLEDGE
1) The practice of removing all shoots every one or two years back to the same position on the tree is called:
b. rounding over.
c. stag horning.
2) For the strongest multi-trunked tree:
a. develop at least 4 to 6 inches of vertical space between multi-trunks arising from a short stem.
b. trees should not be grown with multi-trunks.
c. all trunks should arise from the same point on top of a short stem.
d. use reduction cuts to subordinate all stems.
3) Most "standards" are not suited for growing into large specimens because:
a. species chosen for this treatment are usually weak wooded.
b. main branches usually originate from one point on the trunk.
c. they were headed in the nursery.
d. they were staked for too long in the nursery.
4) A tree or large shrub trained to one short, straight trunk with a dense head of foliage is called a:
a. is another word for topping.
b. can be used to keep a tree or shrub at a specified size.
c. should begin in the nursery for best results.
d. is often practiced under power lines and other utilities.
6) Which of the following may respond poorly to pollarding?
Answers: d, a, b, d, b, d
1) Small-maturing trees can be trained to grow to a multi-trunked habit. What would be the disadvantages of allowing a large-maturing tree such as an oak to grow with multiple trunks?
2) Describe several urban situations where pollarding might be a good option for managing trees.
1) Find a young, large-maturing tree such as a sycamore, oak, or similar plant and make appropriate pollarding cuts to begin the pollarding process.
2) Take a young nursery tree that normally is grown with several stems arising from one location on the trunk and train it to grow with the main stems spaced 4 to 6 inches along a main trunk.
3) Stand next to a standard and discuss why it is not suited for growing into a large specimen.
TABLE 10-1. Comparison of topping and pollarding. Topping * Commonly practiced, yet inappropriate, technique * Make heading cuts through regardless of diameter or age * Practiced on trees regardless of age * Tree may or may not be pruned again * Practiced on many types of trees * Initiates decay in cut branches and trunk * Leads to weak structure and could place landscape at risk * Can lead to short life Pollarding * Rarely practiced, yet appropriate, technique * Make heading cuts through one to a few years old, no more than about 1 inch in diameter * Training begins when tree is young * All shoots removed every one to three years back to the same position on the tree, never below the original cut * Appropriate species include sycamore, lindens, horsechestnuts, and others * Does not initiate trunk decay * Maintains good structure * Pollarded trees can live for centuries TABLE 10-2. Some weeping trees suitable for pruning into a mop-top. Acacia pendula Larix x marschlinsii 'Varied Direction' Acer palmatum 'Dissectum' Malus sp Callistemon viminalis Morus alba 'Chaparral' Caragana arborescens 'Pendula' Pinus densiflora 'Pendula' Caragana pygmaea 'Pendula' Pinus strobus 'Pendula' Catalpa bignonioides 'Nana' Prunus subhirtella 'Pendula' Cedrus deodara 'Pendula' Robinia pseudoacacia Cercidiphyllum japonicum Salix caprea 'Pendula' 'Pendula' Duranta erecta Sophora japonica 'Pendula' Gleditsia triacanthos Tsuga canadensis 'Pendula' 'Emerald Kascade' Ilex vomitoria 'Pendula' Ulmus alata 'Lace Parasol' Juniperus scopulorum Ulmus glabra 'Camperdownii' 'Tolleson's Blue Weeping'
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|Author:||Gilman, Edward F.|
|Publication:||An Illustrated Guide to Pruning, 2nd ed.|
|Date:||Jan 1, 2002|
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