Pruning is sometimes done in conjunction with another horticultural procedure called training. Training involves cutting, repositioning, and guiding the course of development of branches and limbs of plants according to a specific objective. During training, limbs may be bent and tied to support structures or even removed altogether, resulting in creative and attractive shapes. In the landscape, aesthetics appears to be more important than productivity, whereas the reverse is true in orchard management. 19.1 GENERAL PRINCIPLES OF PRUNING AND TRAINING The success of pruning and training plants depends on the understanding and observance of certain principles: 1. Evaluate the whole plant. Pruning affects the entire plant, whether physiologically or physically. Removing a limb may change plant form or shape and may also affect the plant’s capacity for performing certain physiological functions such as photosynthesis and transpiration. By assessing the whole plant, one can make a decision as to which part of the plant to cut to give the overall best results. 2. Think before you cut. Cutting is an irreversible operation. Thus, a limb should be cut only when there is a good reason to do so. It is best to cut in stages, especially when one is relatively inexperienced in pruning. More of the branch or plant part can be removed if the first cut is not adequate. 3. Apical dominance is broken when a stem is cut. Apical dominance resides in the terminal bud and gives direction to plant growth. While the apical bud is present and in control, lateral buds are suppressed. Breaking apical dominance stimulates new growth. New growth tends to arise below a wound on the stem because of interference in apical dominance. Apical dominance is strongly associated with vertical growth. Thus, any attempt to alter vertical growth induces a response in plants to correct the change. For example, when a branch that is growing vertically is bent and forced to remain horizontal, the buds in the leaf axis are stimulated to grow vertically. This happens because apical dominance is reduced by changing the vertical growth to horizontal growth. The new side shoots are likely to develop into reproductive shoots, flowers, and fruits. 4. Pruning invigorates regrowth. When pruning is used to reduce the size of a plant, it encourages vigorous new growth. The more severe the pruning, the more vigorous the regrowth. Thus, if a plant is growing in an unbalanced fashion, for example, the weaker side should be pruned severely to encourage vigorous new growth. For this reason, it may be best to select plants that will fit the available space when they are mature, thereby eliminating pruning that would produce vigorous new growth. 5. Pruning can be used to direct growth. By removing apical dominance, the direction of growth is transported to the topmost lateral bud. Thus, by selecting which bud will become the topmost bud, the regrowth is given direction because buds are positioned to face certain directions. 6. Timing is critical. Plants have different flowering habits that must be considered in pruning. Shrubs that flower in late summer and autumn produce flowers on the current season’s growth. These plants are pruned in spring so that they will produce vigorous shoots that will flower later the same year. On the other hand, shrubs that flower in spring or early summer produce flowers on the previous season’s growth and thus are pruned after flowering. This way, the new growth has time to develop and be ready for flowering in the following year. Certain plants lose much sap when cut. Such plants should not be pruned in spring when sap production is at its peak. 19.1 General Principles of Pruning and Training 567
7. Pruning can be used to create special effects. With pruning and training, plants can be manipulated to produce unique shapes and forms in the landscape. Such techniques include pollarding and coppicing. 568 Chapter 19 Pruning 19.2 OBJECTIVES OF PRUNING Although the manner of pruning varies, the general objectives remain similar. All of the objectives may not be required or accomplished in any one particular instance, since pruning may be used for a specific purpose at a particular stage in the growth of a plant. The four general purposes of pruning are plant sanitation, aesthetics, reproduction, and physiology. 19.2.1 PLANT SANITATION Pruning may be undertaken to remove plant parts that create an unsanitary condition. Specific actions geared toward improved plant sanitation include the following: 1. Broken branches and dead tissue on plants provide surfaces on which disease-causing organisms grow and thus jeopardize the health of plants. Broken branches pose grave danger to people. 2. Diseased plant parts may be removed to prevent the spread of infection. 3. The plant may be cleaned by removing unsightly dried or dead parts. 4. The canopy can be opened up so that air can circulate freely and thereby reduce humid conditions that predispose plants to disease. 5. An open canopy enables effective spraying by allowing pesticides to penetrate the canopy. 6. Removal of excessive undergrowth not only keeps the landscape clean but also reduces hazards from brush fires. 19.2.2 AESTHETIC OBJECTIVES In ornamental horticulture, the visual appeal of plants is of paramount importance to gardeners, especially if plants are in the landscape. Pruning is employed for shaping the form of the plant. After determining the desired shape, branches are strategically removed or their growth controlled to maintain the shape. In formal gardens or on certain public grounds such as those found in zoological or botanical gardens, certain plant species are grown, trained, and trimmed into geometric figures or readily recognizable or abstract shapes. This art form is called topiary. Pruned plants can by themselves be attractive elements in the general landscape. However, more pleasing components can be produced if the style of pruning takes into account other elements in the landscape such as the architecture of the house, the terrain, and artificial structures such as statues and fountains. 19.2.3 REPRODUCTIVE OBJECTIVES Pruning may be undertaken to enhance the reproductive capacity of the plant in several ways: 1. The canopy may be opened up by cutting out the branches in the center. This allows light to penetrate the canopy for fruiting to occur on inner branches. 2. Fruiting may be regulated by encouraging the growth and development of fruiting shoots while reducing nonreproductive shoots. 3. Pruning may be done to balance reproductive and vegetative growth for optimal yield.