Based on experience in West China, physiological responses to pruning of orange trees are quite different from apples. While winter pruning in suitable but not excessive amounts stimulates the vegetative processes of apples, it enhances the reproductive processes of citrus and summer pruning does the opposite. As a supplementary practice to winter pruning, summer pruning of oranges at the right time with specific techniques will invigorate neglected and old trees and modify the alter nate bearing habit of healthy trees. During the on-year, trees respond to moderate summer pruning by producing moderate amounts of summer flushes which increases the shedding of young fruits. Thus, the on-year crop will be reduced and the next year crop increased. This is “physiological thinning/’ as compared to chemical or hand thinning. Excessive summer pruning before the time of “June drop” can result in considerable fruit drop.

Growth patterns and endogenous processes of fruit trees differ during the season and from season to season. Therefore, the responses to pruning practices in different seasons also may differ distinctly. These phenomena have been found in apple orchards, for example, in Germany, Lange (ca. 60’s reported that “Summer pruning (of apples) promotes reproductive development and winter pruning promotes vegetative growth” (Der Sommerschnitt fordert.. .der Fruchtbarkeit; der Winterschnitt fordert … die Triebkraft). However, for citrus fruits little work has been done on the relation of pruning to fruiting habit.

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C. K. Kiang and R. H. Biggs
University of Florida, I FAS, Fruit Crops Department