Pruning
| Standard Cyclopedia of Horticulture |
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Pruning is the methodical removal of parts of a plant with the object to improve it in some respect for the purposes of the cultivator. Much of the current "pruning" is really repairing, and is now commonly called "tree surgery." See page 354, Vol. I. Under this denomination are comprised a multitude of practices and ideals. It is impossible to give any advice for pruning until one has analyzed the subject and knows the objects for which one is to work and the underlying principles on which the practices must rest. The larger part of the writing on pruning gives mere advice or directions, or details some person's experience, without analyzing or clarifying the subject. The practice must differ with every person and every condition, but the principles are general. The ideas that are associated with pruning may be grouped around three centers: (1) pruning proper, or the removal of a part of a plant for the purpose of bettering the product and improving the character of the remaining part; (2) training, or the disposition or placing of the individual branches, a practice that ordinarily is coincident with pruning proper; (3) trimming, or the shaping of a plant into some definite or arbitrary form. The principles that underlie pruning proper may be associated with two purposes,—the lessening of the struggle for existence amongst the parts of a plant, and the cutting away of certain parts for the purpose of producing some definite effect in the formation of fruit-buds or leaf-buds or in modifying the habit of the plant. There are more branches in the top of any plant than can persist; therefore there is struggle for existence. Those which have the advantage of position persist. Nature prunes. Dying and dead branches in any neglected tree-top are illustrations of this fact. Whenever the struggle for existence is greatly lessened, the remaining branches receive a greater proportion of the plant's energy, and they therefore make stronger growth, yield better produce, or are more productive in flowers and fruit. Pruning is essentially a thinning process. There exist the widest variations of opinion as to the merits of pruning, particularly as it applies to fruit-trees. Some persons oppose any pruning whatever. Undoubtedly a certain type of novice places too high estimate on pruning, as if it were the one essential operation; others carry the practice to needless extremes; but the reasons for pruning lie in the nature of the plant, and the useful results are attested by long experience. It is one of the cardinal practices in the growing of many kinds of plants, along with tilling, fertilizing, combating pests and diseases; and it is not to be considered as a thing apart or as a remedy or corrective for all deficiencies. In itself pruning is not a devitalizing process; it is devitalizing only when it is carried to excess or when the wounds do not heal and disease sets in. It is rather an invigorating process, since it allows more nourishment to be distributed to the remaining parts of the plant. The notion that pruning is devitalizing arises from false analogy with animals, which suffer shock or injury when parts are removed. The fact that pruning is not a devitalizing process is proved by every tree. The tree is a record of successive prunings. Note the number of branches on the seedling tree in the nursery-row or in the forest, and then consider that all these branches, with the exception of the leader itself, will probably perish in the course of time. The forest tree develops a bole and the side limbs are pruned away by natural causes. (Fig. 3198.) Knots are records of this natural pruning. In the greater number of cases the limbs die and are removed when still very young, and they leave small record in the grain of the wood; but all visible knots are histories of the removal of large branches. As a rule, it is only when the knots become knot-holes that injury results. A knot-hole means decay, and this decay may extend into the heart of the tree, finally causing it to become hollow. A discolored or decayed heart is an indication of disease. The disease originates on the outside of the plant; it is the result of inoculation. This inoculation takes place through some bruised or broken part; it is usually an infection of filamentous fungi. These fungi gain a foothold in the dead and dying cells of the wound, and as they grow they are able to destroy the living cells. The larger the wound, the greater is the liability to infection. It is very important, therefore, in the pruning of trees, that the wounds shall be as small as possible and shall heal quickly. This means that the best pruning is that which is practised annually, so that the branches to be removed do not attain to large size. This annual pruning is also most desirable for other reasons, as will be seen. Pruning when transplanting. Woody plants should always be pruned when they are transplanted. This is because the roots are pruned in the very process of removal, and the tops should be reduced in proportion. For some time after the plant is transplanted, it has very little vital connection with the soil, and if all the top is allowed to remain there is much evaporation from it and a dissipation of the energies. How much of the top shall be removed depends on how much of the roots was removed in digging, on climate, and also on personal desires of the operator. It is a general practice to cut back the top of a plant at least one-half on transplanting; in some cases still more of the top is removed. In broad-leaved evergreens, some of the leaves may be cut in two at transplanting, to reduce transpiration. (Fig. 3199, after Wester.) Quite another question is the particular form in which the top shall be left. Some growers prefer to remove all side branches, if it is a fruit-tree, and leave a straight whip. (Fig. 3200.) They are then free to start the new branches where they like. This is allowable with very young trees, and it is much employed with peach trees, inasmuch as these trees are planted when the top is only of one season's growth. (Fig. 2792, page 2496.) If trees are two or three years old and well branched, as is the case with apples and pears, most persons prefer to leave three or four of the main branches to form the starting point of the future top. (Fig. 3201.) These branches may be headed back half or more of their length. Some years ago a method of very severe pruning came into notice under the name of the Stringfellow or stub-root system, taking its name from the late H. M. Stringfellow, of Texas (page 1598, Vol. III), who wrote much concerning it. The fullest presentation of Stringfellow's ideas will be found in his book, "The New Horticulture." It advises that practically all the roots be cut away and that the top be shortened to a straight stick 1 or 2 feet long, without side branches. It is the supposition that when trees are reduced to their lowest terms in this way, the new root-branches that arise will take a more natural form and the tree will assume more of the root character of a seedling. This method of transplanting has not gained acceptance. In most cases, it will be better, particularly in trees that are three years or more old, to prune them only moderately, shortening them in all around, allowing a part of the original root-system and a part of the top to remain. Whatever the way of pruning at transplanting, good live buds should be left on the tree; the practice of pruning two-year-old wood to a whip is therefore to be discouraged, for only dormant buds (if any) then remain on it. Pruning fruit-trees. Fruit-trees are pruned for the purpose of enabling them to produce a superior quality of fruit. In America, they usually are not pruned primarily to make them assume any definite or preconceived shape. It is best, as a rule, to allow each variety of tree to take its own natural or normal form, pruning it only sufficiently, so far as shape is concerned, to remove any unusual or unsymmetrical growths. 1. The fundamental conception in the pruning of fruit-trees is to reduce the struggle for existence, so that the remaining parts may yield larger and finer products. 2. The result of pruning fruit-trees should be to keep the tree in bearing condition, not to force it into such condition. If the tree has received proper care from the time it is planted, it should come into bearing when it reaches the age of natural fruitfulness. Pruning aids to keep the tree in proper bearing condition. When trees have been much neglected, pruning may be the means of reinvigorating them and setting them into a thriftier condition. In such cases it is one of the means of renovating the tree, as are tilling, fertilizing, and spraying. 3. Heavy pruning of the top in any year tends to produce very vigorous growth on remaining parts. This is because the same amount of root energy is concentrated into a smaller extent of top, thereby causing a heavier growth. This is particularly true if the pruning is performed when the plant is dormant. 4. Heavy pruning of the root tends to lessen the production of wood, because the same amount of top receives a less supply of soil-water. 5. Trees that grow much to wood are likely to be relatively unproductive. It is an old maxim that checking growth induces fruitfulness, so long as the plant remains healthy. This, of course, does not mean that trees of decreased vigor are more fruitful, nor that the maintenance of full growth from the first is to be avoided. Orchards that are kept in a vigorous thrifty condition are most productive, other things being equal; but when very thrifty trees do not bear, the checking of the growth may induce the desired results. If the tree is thrown into redundant growth every two or three years by very heavy pruning, it tends to continue to produce shoots at the expense of fruit. When a tree is to be brought into bearing condition by general good treatment, the aim should be to keep it in that condition by a relatively light annual pruning. Violent pruning is allowable only when trees have been neglected and it is necessary to bring them back into bearing condition by renewal or to re-shape them. 6. The operator should know where the fruit-buds are borne before undertaking the pruning of any fruit-tree; otherwise he may destroy too many of them. If he knows the position of the fruit-buds, he may prune in such way as to thin the fruit even without the removal of much wood, and thereby to reduce the struggle for existence to a minimum. Every species of tree has its own method of fruit-bearing. The pear bears its fruit largely on old spurs. The peach bears mostly on the long wood of the last season's growth, particularly when trees are young. If one is to thin the fruit of the pear by pruning, therefore, it is necessary to remove part of the spurs. In the peach it is necessary to cut out or to cut back a part of the previous year's growth. Each species of plant is a law unto itself in these regards. 7. Heading-in under certain conditions (which the operator must judge by observation) tends to promote fruitfulness. If the heading-in is very severe it may amount to a heavy pruning, and in that case it may set the plant into shoot-bearing rather than into fruit-bearing. It is not to be supposed that heading-in is necessarily to be advised in order to make trees bear. They may bear just as well if they are never headed-in, provided they are otherwise well pruned and well cared for. Whether one shall head-in the fruit-trees or not, is in part a personal question. If the trees are growing too rapidly, it is well to head them back. This may be necessary when trees are growing on very fertile soil in order to keep them within bounds; but the heading-in under these conditions may not conduce to greater fruitfulness When trees are planted too close together, it may also be necessary in order to prevent the plantation from becoming too thick. Some growers like a low-headed and rounded top; this is a question of personal preference and of the general management of the plantation. If the orchardist desires such form, it is necessary to head-in the tree. It should be remembered that the more a tree is headed-in the thicker it tends to become in the crown and the more inside pruning is necessary. Whenever there is danger of fruit-rot, as in plums and early peaches, it is a question whether the thick form of top is the most advisable. 8. Pinching-in the annual growths in early summer tends to augment the development of fruit-buds, although these buds may not be developed the very year in which the pinching-in is performed. This is a special practice, however, which can be employed only on small areas and with particular trees. It is essentially a garden practice and not an orchard practice. In the orchard, one must depend for fruitfulness on the general good care of the plantation, and in this care pruning is one of the essential factors. 9. Pruning fruit-trees usually resolves itself into a thorough and systematic thinning out of the weak, imperfect and interfering branches. Thereby, the energy of the plant is saved and is deflected to those parts that are capable of bearing a useful product. The sun and air are admitted. The tree becomes manageable for spraying and for picking. All the fruits have an opportunity to develop. How much or how little to thin, is a special question. In humid climates, much thinning may be necessary. In dry hot climates, as on the Plains, but little thinning is allowable, else the branches may sun-scald. Figs. 3202 and 3203 illustrate two pruning ideals. 10. Scraping the rough bark from old trunks may be a desirable practice, since it destroys the breeding places of insects and fungi. Trees that have been continuously thrifty, however—that have received uniformly good tillage, fertilizing, pruning, spraying— rarely need to be scraped, as the bark remains relatively smooth and firm. Only the loose outer bark should be removed. On ornamental trees, the bark is a part of the characteristic beauty, and it should not be scraped. Although not a pruning question, this is closely associated with pruning practices. Pruning ornamental plants. Ornamental trees and shrubs are pruned for three purposes: (1) to enable them to produce greater quantity of bloom; (2) to make them take some desired form; (3) to remove unusual or injured growths. The pruning of woody plants for the production of flowers is controlled largely by the flower-bearing habit of the plant. Most early-blooming plants develop their flower-buds the year before. Heavy pruning, therefore, particularly heading-in, when the plants are dormant, cuts off the flower-buds and the amount of bloom is lessened. If these plants are pruned just after the flowers are passed in spring, the best results will be secured, since the new growths will then develop flower-buds for the year following. Among spring-flowering shrubs that may be pruned after flowering (while in leaf), are deutzias, diervillas or weigelas, forsythias, lilacs, flowering almond, wistaria, exochorda, and many spireas and viburnums. It may be advisable, however, to prune such plants in winter for the purpose of thinning them, thereby allowing the flower-buds that remain to produce larger bloom. In most ornamental plants, however, it is the number of flowers rather than the size of each which is desired. Plants that bloom late in the season, as hydrangea and most species of clematis, make their flower-buds on shoots which arise that very season. With such plants, it is well to prune rather heavily while they are dormant in order to cause them to throw up a profusion of strong shoots in the spring. These shoots will bear that summer. Among the summer-flowering shrubs that may best be pruned when dormant, are hydrangeas, althea or hibiscus, ligustrums, trumpet creeper, ceanothus, potentillas, vitex, symphoricarpos, and many kinds of clematis, lonicera, jasminum, and some spireas. Pruning to make the plant assume some definite form is essentially a method of shearing or heading-in. If it is desired to have a very regular and definite shape, it is well to shear the plant at least two or three times a year in order to keep down the exuberant growths. It is a common practice to shear the plants only in the winter, but if this shearing is somewhat violent, as is usually the case, the plant throws up numerous strong shoots very early in spring and it remains shapeless during a large part of the growing season. Except in very special cases and for formal landscape work, it is much better to let shrubs and trees assume their natural and characteristic forms: these forms, in fact, constitute the beauty of the species. Training. There is relatively little careful training of plants in North America, largely because of the expense of the skilled labor necessary to perform it. Land is also relatively cheap, and room can be given for the natural development of most plants. In many parts of the Old World, fruit-plants must be grown in very small areas, and it may be necessary to train them on walls, sides of buildings, or on trellises of various kinds. Trained fruit-trees may generally be referred to one of three categories: the wall tree, which is trained against a continuous surface; the espalier, which is trained on a trellis, the branches starting at nearly right angles from a central shaft; the cordon, or training to a single or double strand near the ground. Properly, an espalier is a trellis (page 1146), but the word is commonly used for the plant that is trained on the trellis. There are many variations in the methods of training and pruning in each of these three classes, and the methods are such as can scarcely be well elucidated in writing. The Old-World literature is replete with instructions. In recent American literature, the fullest account is to be found in "The Pruning-Manual." In order that trees may be well trained on walls, espaliers, and cordons, it is necessary that the training be begun in the nursery. The Old-World nurseries grow plants that are trained for various uses, but the American nurseries do not. If, therefore, the American is to train trees in any of these formal shapes, he should secure specimens that are not more than one year from the bud or graft, and begin the training himself. The illustrations (Figs. 3204-3206) suggest some of the special methods of training fruit-trees. On such trees, if skilfully trained and carried out in patient detail, the best excellence in individual fruits may be attained. Pruning after frost-injury. When woody plants have been much injured by freezing, it is the best practice to remove all dead parts as soon as the line of demarcation is evident. The kind of corrective pruning to be employed when trees have been much shattered by winter cold is a subject that needs further investigation. It is not a single or a simple problem, as much depends on the previous state of the trees and on other conditions. Speaking of peach trees, Chandler writes (Research Bulletin No. 8, Missouri Experiment Station): "Pruning the trees severely following a winter when the wood has been killed, although apparently in the best condition of maturity, seems to reduce the amount of killing. However, such pruning following winters when the wood has been killed on account of its not having reached the proper condition of maturity in the fall, generally due to the presence of wet weather following a drought the season before, is liable to result in greater loss than if no pruning were done." On the proper practice to pursue in the case of frozen citrous trees, T. F. Hunt issued the following advice to California growers following the freeze of January, 1913: "Relative to badly injured trees, it appears best not to prune until the new growth has started. It is best to delay the pruning until a distinct line of demarcation develops between the injured and uninjured wood. At the Citrus Experiment Station last, year five- year-old lemon trees were frosted. Good results were obtained by waiting until the new growth had reached from 4 to 5 inches in length; in that instance about six weeks were required. Allowing the injured limbs to remain not only makes it possible to determine how much it is best to prune, but the limbs and leaves afford shade to the bark of the tree, which is accustomed to protection. It may be desirable, in some instances, to spray the trunks and limbs of large and severely pruned trees with whitewash in order to reflect the sun's rays. Wrapping the trunks of young trees with loose sun protectors would seem extremely desirable. "It seems reasonably certain that no injury to the tree can result from any of the materials passing from the frozen oranges into the tree. An examination of those oranges which have been too badly frozen to be fit for shipment shows that most of them only partially have been killed; consequently, they are presumably respiring carbon dioxide. This loss of energy would be saved if the oranges were removed from the tree. There is no experimental evidence, however, to show whether this loss is sufficient to warrant the cost of early removal by hand." When to prune. It will be gleaned from the above discussion that the time of pruning depends on many circumstances, and chiefly on the result which it is desired to reach. So far as the healing of the wound is concerned, it is perhaps best to prune when the vegetative activities begin in spring so that the wound is quickly covered or "healed." For the purpose of checking growth and producing other definite results, it may be necessary to prune at other times of the year. As a general rule, however, the best tune to prune is in late autumn to early spring, when labor can be had and before the rush of spring work comes on. In practice, it resolves itself largely into a question of the convenience of the operator. The wound. The wound made by severing a branch heals by means of a callus which forms from the growing tissue between the bark and wood. (Fig. 3207.) This tissue rolls over the wound, finally joining in the center and completely covering the old wood. The old wood itself takes no part in the healing process; in fact, it dies. When the healing is complete, the old wood is merely covered and preserved from external injury and infection, much as fruit in a jar is preserved by being protected with a tight cover. There is no dressing that will hasten the healing process except as it keeps the wood from decay. In other words, the whole object of dressing a wound is to protect it. The dressing hinders bacteria and fungi from securing a foothold and thereby prevents the rot. Wounds that are exposed for some years nearly always become unsound at the center because of the intrusion of these organisms, and even if the wounds should subsequently heal over, the infection may still extend down the heart of the tree and finally cause its death. The best covering for a wound is one that protects it best from weather, microbes, and fungi and which persists the longest. Ordinarily, good white lead paint, applied heavily and renewed occasionally, is a good pro-tection for fruit trees. Grafting-wax may afford a fair protection, if it is applied warm and thin so that it soaks into the tissue. If it is merely spread over the surface, it soon blisters and becomes loose and affords relatively little protection. For shade and forest trees, which are treated by "tree surgeons," special dressings and disinfectants are employed. The rapidity with which wounds heal depends very largely on their position on the tree and the way in which they are made. Wounds along the main branches, which are the leading avenues for distribution of food, heal more speedily than those on the weaker side branches. If the wound is close to the branch it may be expected to heal better. (Figs. 3208, 3209.) If a stub is left several inches long (Fig. 3210), it seldom heals until it rots back to the main branch or trunk; and by that time the decayed part may have extended deep into the tissue of the tree. It is a common notion that a limb should be cut at right angles to the direction of the limb itself and beyond the bulge at its base. It is a better plan, however, to make the wound parallel to the direction of the branch or trunk that remains, and closer to it. This wound may have a somewhat larger superficial area, but it is much nearer the source of the healing food-supply. Tools. A good large-bladed large-handled sharp knife, a narrow and pointed rather fine-toothed saw, and a pair of shears are the essential pruning tools. Many forms of these three primary implements are on the market. In tools that require such incessant use, when the work is in progress, and that meet such resisting obstacles, only the best makes and materials should be secured. The operator must learn by practice how to use them, for even in such simple implements as these there is a right way and a wrong way. Fig. 3211 (from Wester), for example, shows an improper way of using shears, cutting at such a long acute angle rather than crosswise or merely oblique as to render the work difficult, leave a long sharp stub, and injure the shears. A good ladder is also necessary for large trees. When the top or head of the tree is low enough, the pointed ladder shown in Fig. 3212 (from Wester, a bamboo ladder used in the Philippines) is one of the best types; it is easily inserted among the branches and it may be rested securely in a crotch. Many other pruning implements are useful for special work, and suggestions of these will be found in various bulletins and in the catalogues of dealers. See also the discussion in Vol. IV, page 1950- L. H. B.
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Pruning in landscaping and gardening is the practice of removing diseased, non-productive, or otherwise unwanted portions from a plant. The purpose of pruning is to shape the plant by controling or directing plant growth, to maintain the health of the plant, or to increase the yield or quality of flowers and fruits. Proper pruning is as much a skill as it is an art, since badly pruned plants can become diseased or grow in undesirable ways.
Proponents of pruning, both gardeners and orchardists, often argue that it improves the health of the plant and makes sturdier structure, often referred to as the scaffold; opponents believe that pruning harms plants' "natural" forms.
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Types of pruning
From the least to the most severe forms of pruning:
- Pinching: Removing new buds or growth of herbaceous plants in order to control growth. This is the least damaging method for pruning and is the first opportunity to control the plant, encouraging bushiness or height in a plant by removing its terminal growth or lateral growths, respectively. For instance, pinching off the terminal buds of herbaceous plants, such as chrysanthemums encourages denser growth and more profuse or delayed flowering.
- Dead Heading: A form of pruning used on small to medium flowering shrubs to remove spent flowers. This is a common method used on Roses. The practice of "deadheading" is also a type of pinch pruning: it removes spent flowers before they begin to set seed, to concentrate a plant's energy on continued flower production, for the current year or the following year, as when lilacs are dead-headed after flowering.
- Shearing: A form of pruning employed on hedges or topiary, in which most of the growing points are tipped back, to produce artificially dense growth. Shearing in general is a more indiscriminate form of pruning, where the plant is simply cut to shape, however plants that have been trained to be sheared generally produce buds and on the "surfaces" where they are to be sheared. It is commonly performed with hedge trimmers.
- Cutting-back: Removing a portion of a growing stem down to a set of desirable buds or side-branching stems. This is commonly performed in well trained plants for a variety of reasons, for example to stimulate growth of flowers, fruit or branches, as a preventative measure to wind and snow damage on long stems and branches, and finally to encourage growth of the stems in a desirable direction. Also commonly known as heading-back.
- Thinning: A more drastic form of pruning which involves removing entire branches. This is usually employed to revitalize a plant by removing over-mature, weak, problematic, and excessive growths. When performed correctly, thinning encourages the formation of new growths that will more readily bear fruit and flowers. This is a common technique in pruning roses and for simplifying and "opening-up" the branching of neglected trees, or for renewing shrubs with multiple branches like Forsythia or Spirea.
- Heading: Heading is a very severe form of pruning which involves removing all branches and growths down to a few large branches or to the trunk of the tree. When performmed correctly, heading can be used to begin training younger trees for pollarding. When heading a tree down to a stump, the technique is called coppicing. When heading is performed incorrectly or on old trees, the result is at best a non-aesthetically pleasing tree crown or, at worst, the death of the tree.
Technique
The general rule to pruning is to always cut in a location where growth will occur, whether the cut is next to a bud or another branch. Cutting a branch beyond where growth will occur effectively kills all portions of that branch back to the closest branch, bud, or dormant bud clusters, leaving a stub of dead wood. The withered stub will eventually rot away and fall off. Prior to that, however, it will prevent the plant from forming a callus over the cut surface, which will in turn invite insects and infection. All cuts should be relatively smooth since this will aid in healing.
Also, the pruning cut should not be too large when compared to the growing point. For instance, a large cut on a 20 cm trunk down to a 15 cm branch should be fine, but the same cut to the trunk down to a 1 cm twig or bud is considerably less ideal and should be avoided if possible.
Pruning to bud
A correct pruning cut will allow for quick healing and promote vigorous growth from the closest bud to the cut. The cut should be close enough to the bud to reduce the size of the stub of dead wood that will form from the cut, but far enough away to prevent the bud from being adversely affected by the cut though desiccation. Cutting too close to the bud (under-cutting) sometimes results in the death of the bud, which results in a scenario similar to cutting too far away from the bud (over-cutting). In general, a correct cut should be angled at a moderate 35-45 degree slant such that its lowest point is situated on the same level as the tip of the growth bud. This technique is usually applied when pinching or when cutting-back.
Pruning to a main branch
The pruning cut should occur slightly away from and follow the branch collar. When cutting away branches growing directly from the roots, the cut should be flush and level to the ground. This technique is usually applied when thinning or to remove larger dead or damaged branches.
When using pruning shears or loppers to remove a branch back to a main branch, the "hook" portion of the shears should always face away from the main branch. This ensures that the blade will not leave a protruding stub and the hook will not damage the branch collar or parts of the main branch.
Large heavy branches
Depending on the weight of the branch, the first cut should be a notch on the underside of the branch about a third to half of the way through. The bulk of the branch should then be removed with a follow-through cut slightly above the first cut, thus leaving a limb stub. The purpose of this is to stop the weight of the branch from tearing the bark of the tree from the underside, which would normally occur if the removal was done with one cut. The limb stub ensures that any cracking of the wood resulting from the branch separation is limited to the portion of the wood to be removed. The branch collar should then be located, and can be identified by the strip of rough bark running down from the topside of the branch at its junction with the stem. The cut for removing the limb stub should be just outside the branch collar, leaving a small bump. The bump and the branch collar should not be removed since this action can reduce healing time, which could result in an infection.
Time period
Pruning small branches can be done at any time of year. Large branches, with more than 5-10% of the plant's crown, can be pruned either during dormancy in winter, or, for species where winter frost can harm a recently-pruned plant, in mid summer just after flowering. Autumn should be avoided, as the spores of disease and decay fungi are abundant at this time of year.
Some woody plants that tend to bleed profusely from cuts, such as maples, or which callous over slowly, such as magnolias, are better pruned in summer or at the onset of dormancy instea d. Woody plants that flower early in the season, on spurs that form on wood that has matured the year before, such as apples, should be pruned right after flowering, as later pruning will sacrifice flowers the following season. Forsythia, azaleas and lilacs all fall into this category.
