Tree response can vary when these aspects of pruning are varied. When peach trees are summer pruned properly, one can expect economic benefits, but economic losses result from summer pruning incorrectly. During the 1980s and 90s, several researchers evaluated summer pruning in apple and peach. Simultaneously, others elucidated the importance of light and carbohydrate partitioning during fruit development that helps explain the tree’s response to summer pruning. I will try to review some of this information to explain how summer pruning can be used to maintain fruiting wood in the lower canopy and to enhance peach fruit quality.
Peach growth habit
Peach trees exhibit strong apical dominance and tend to produce new shoots on the ends of branches and in the tops of the trees. As trees grow taller, the lower canopy becomes shaded and new shoots developing in the extreme shade will die. Once these fruiting shoots in the tree interior are lost, it is very difficult to regenerate new fruitful shoots. However, if moderately high light levels are maintained in the tree interior, it is possible to maintain fruiting wood for the life of the tree. Shoots developing in early-season moderate shade will initiate few flower buds. Shoots exposed to high light levels early in the season will initiate maximum numbers of flower buds.
Branch orientation influences the number and quality of new shoots arising from those branches. If a branch is too upright, then no new shoots will develop along the lower section of the branch, even when light levels are adequate. If a branch is too horizontal, then dormant buds in the bark will develop into strong vertical vegetative shoots that produce sylleptic shoots that produce few flower buds but shade the tree interior. As shoots elongate, a vegetative bud forms at the base of each leaf. These axillary buds usually remain dormant until they are exposed to a chilling requirement and then may remain dormant or develop into leafy shoots. Some buds on vigorous shoots do not become dormant and develop into leafy shoots shortly after they are formed. These shoots developing from non-dormant buds are called “sylleptic shoots.”
Importance of light
Fruit developing in the shade during the final swell will be small and soft, with poor red color and soluble solids. The primary reason trees are summer pruned is to enhance light penetration and distribution throughout the canopy. Relatively high light levels (40% full sum) are required for red color development on the fruit.
Over a period of several years, I shaded ‘Redhaven’ peach trees with various levels of shade cloth at different times during the summer to determine the critical level of light and the critical time that light is needed for flower bud and fruit development. I found that the critical time for flower bud development is mid-June to early-August for flower bud formation. Just three weeks of shade (20% full sun) during that period will reduce flower bud density (buds per meter of shoot length). A six-week period of heavy shade (20% full sun) from mid-August to early October did not affect the number of flowers per meter the following spring. Forty-five percent full sun during the six weeks before the harvest was adequate for high-quality fruit.
Depending on the objectives summer pruning should be performed at different times, and different types of cuts should be made.
Summer pruning to maintain fruiting wood
One reason to reduce shading is to maintain fruiting wood in the lower canopy. To enhance flower bud development in the lower canopy, summer pruning must be completed by mid-July. Summer pruning after mid-July will not improve flower bud initiation or development. For mature trees, remove the vigorous upright water sprouts that shade the lower canopy. If this is done early, before the base of the shoot lignifies, the shoots can be ripped out by hand with little damage to the bark. After shoot lignification, had pruners are needed to cut the shoot off at the base. Do not head these shoots or cut the shoot just above a sylleptic shoot because new shoots will develop below the pruning cut and the resulting shoot will shade the lower part of the tree, and these vigorous shoots will develop few flower buds for next year.
Summer pruning for tree training
Young vigorously growing trees can be summer pruned to develop fruiting wood in the lower canopy and also to help train the tree and develop the scaffold system. In early June, the upright water sprouts developing from the upper surface of scaffold branches can be removed to enhance light penetration. To encourage a spreading growth habit, scaffold branches can be headed just above a sylleptic shoot that is growing outward. This same type of cut can sometimes be repeated in mid- to late-July if the sylleptic shoot that was retained also develops sylleptic shoots.
Summer pruning to enhance red fruit color
Summer pruning about two weeks before harvest can improve fruit red color development without adversely affecting fruit size and sugar levels if leaf removal near the fruit is minimized. Mowing trees to remove a high percentage of foliage in the tree top does enhance light penetration and red color development, but it also reduces fruit size and sugar levels. Therefore, mowing or hedging should be avoided because they remove too much foliage near the fruit. The only beneficial effect of summer pruning on the fruit is improved red color development of some cultivars some years, but summer pruning will not improve fruit size or sugar levels.
Figure 1. Three-year-old ‘Cresthaven’ trees before (left) and after (right) summer pruning. Photo: Rich Marini, Penn State
Potential negative effects of summer pruning
In New Jersey, I found that summer pruning delayed leaf drop, the onset of dormancy and acclimation to low temperatures. Therefore, to avoid reducing early-winter cold hardiness, summer pruning should be completed by mid-August.
What summer pruning will not do
Although many claims have been made about various responses to summer pruning, few are supported by research. Other than a moderate improvement in fruit red color, summer pruning will have little effect on fruit size or quality. My research showed that summer pruning reduced root growth and trunk growth but shoot growth the following season was never suppressed by summer pruning. So, summer pruning will not suppress the vigor of the above-ground parts of the tree. Late-season summer pruning to enhance fruit color will not enhance flower bud formation or development, fruit set or affect the time of bloom the following year. Valsa canker was not influenced by summer pruning. Cold hardiness was not enhanced but was delayed, by summer pruning.
In general, the biggest benefits from summer pruning are the maintenance of fruiting wood in the lower canopy and developing the scaffold system on young trees.
– Rich Marini, Ph.D., Penn State University
Photo at top: Donald Seifrit, Penn State