by Richard P. Marini, Extension Specialist, Horticulture, Virginia Tech

   Proper training and pruning of trees is a major component of a profitable apple orchard operation. Successful pruning is an art based upon scientific principles of tree growth and physiology and an experienced understanding of tree response to various pruning cuts and practices. Each tree is an individual and should be treated accordingly. Varieties differ in growth characteristics and response to pruning cuts, rootstocks, soil and growing conditions. It is important that orchard designs, objectives and goals be clearly defined and that pruning principles are developed accordingly. Medium- to high-density plantings require greater commitment to detailed training and pruning than low-density orchards and should not be attempted unless such a commitment is made.

   There are several training systems for apple. The training system discussed here is the central leader, which is the most common system in commercial orchards and is easily adapted to non-commercial situations. The central leader system is used for freestanding trees on the relatively vigorous standard and semi-dwarf rootstocks.

   Objectives of Training and Pruning

   The objectives of tree training and pruning are to develop and maintain small, conical shaped (Christmas tree shape) trees that are capable of early production of large crops of high quality fruit. Trees are pruned to help maintain a balance between vegetative and reproductive growth throughout the tree and to maintain desired tree shape and size with an open tree canopy that allows penetration of sunlight and pesticides. The practices of training and pruning are not easily separated because the training of a young tree will determine how the tree will be pruned as it matures. Too frequently, the pruning required in mature trees involves the correction of mistakes made while training them as young trees. Proper training of young trees will save time and expense in future pruning and produce earlier profitable crops. The greatest pruning skill is required during early tree development.

   Maintaining suitable vigor of an apple tree is necessary to develop a balance between reproductive and vegetative growth throughout the life of the tree. Such vigor is established by thorough land preparation, proper selection of rootstock/variety combinations, weed control, adequate available moisture, effective pest control, and fertilization programs based on soil and foliar analyses. Protection against rodent and deer damage is also very important. Good initial tree vigor is necessary to establish adequate canopy volume for optimum profitable production as early as possible in the life of the orchard.

   As trees come into production, it is necessary to encourage a transition from vegetative to reproductive growth. Continued excessive vegetative growth may delay fruiting and result in low yields of large but soft, poorly colored fruit with limited storage potential. Trees with low vigor, however, may produce small, firm, highly colored fruit. Pruning is one tool that can be used to regulate tree vigor.

   Dormant Pruning Response

   A pruned tree is always smaller than a non-pruned tree. Certain pruning cuts stimulate shoot growth in the vicinity of the cut, creating the illusion of increased growth. However, such growth is less than the sum of the wood removed by pruning plus the growth it would have made. Trees tend to maintain equilibrium between the top of the tree and the roots. Pruning shifts this balance in favor of the remaining shoots so that more stored reserves are available per remaining bud.    Therefore, a pruned tree produces more shoot growth than a non-pruned tree to maintain the characteristic top-root equilibrium. Shoot growth the season following pruning is proportional to pruning severity.

   Summer Pruning

   Pruning during the summer had traditionally been thought to suppress tree vigor more than comparable pruning during the winter. The summer removal of leaves reduces the quantity of carbohydrate reserves in the trunks and roots, which, theoretically, should suppress shoot growth the following season. However, recent research results from Virginia and other regions of the U.S. indicate that shoot growth is not suppressed more by summer than by dormant pruning, and that summer pruning is not a viable method of suppressing tree vigor.

   Types of Pruning Cuts

   There are three basic types of pruning cuts: heading, thinning, and bench. Heading cuts involve the removal of the terminal portion of a shoot. Since heading cuts remove the terminal growing point that is the source of auxin, lateral buds immediately below the cut are no longer inhibited and develop into shoots. Both the number and length of shoots developing below the cut increase, as heading of one-year-old shoots becomes more severe. Heading into older wood causes a conversion of potentially fruitful spurs to vigorous non-fruitful vegetative shoots. Shoot growth developing on horizontally oriented limbs will be less than on comparably pruned vertical shoots. Thinning cuts involve the removal of a branch at its point of origin. Thinning cuts do not induce dramatic changes in growth pattern because hormone production is not drastically altered. Thinning cuts are preferable to heading cuts for maintaining tree size and shape because heading removes future fruiting wood. A bench cut is actually a special type of heading cut and involves removal of the terminal portion of a branch at a point just above a side branch. Bench cuts on young trees during the tree training and early fruiting years will tend to stiffen the portion of the branch below the cut and reduce the natural limb spreading caused by the weight of fruit. Bench cuts are often used to encourage outward growth of branches. However, limb spreading is preferable to bench cuts because water sprouts often develop at the site of a bench cut. In addition, the branch immediately below a bench cut is sometimes weak and may not support a heavy crop. See Figure 1.

   Pruning Affects Fruiting

   Any type of pruning always delays fruiting of young trees and always reduces yield on mature trees. Pruning tends to encourage vegetative growth, which is antagonistic towards flower bud formation. Additionally, pruning removes flower buds and wood on which future flower buds will be formed. Pruning usually improves fruit quality by improving light distribution throughout the tree and by reducing the number of fruits per tree. Fruits on pruned trees tend to be larger than on non-pruned trees because there are fewer fruits competing for carbohydrates. Therefore, to encourage fruiting, young trees should be pruned judiciously. To improve fruit quality and reduce overcropping, mature trees should be pruned annually.

    For more information or to view the entire article, visit http://pubs.ext.vt.edu/422/422-021/422-021.html.

   Source: Virginia Cooperative Extension