The tendency of fruit trees to bear fruit in two-year cycles, consisting of a large crop followed by a small crop, is termed alternate or biennial bearing. Alternate bearing occurs in almost all tree fruits.
The flowers that produce next year's crop are initiated during the development of the current season's crop. When a fruit tree is producing a large fruit crop, most of the tree's energy is utilized for fruit development, little energy remains for flower initiation. As a result, a fruit tree often produces a small number of flowers and fruits when preceded by a heavy crop the previous year.
To discourage alternate bearing, home gardeners should thin apple and other fruit trees when fruit set is heavy. Thinning also promotes the development of large, high quality fruit at harvest and prevents limb breakage on trees heavily laden with fruit.
Hand thinning of apples should be done within six weeks of full bloom. Leave the largest apple in a cluster unless it is damaged. After thinning, apples should be spaced about 8 to 10 inches apart on the branches. Pears, apricots, and peaches may also need to be thinned. Fruit should be spaced 6 to 8 inches apart on the branches. Plums will generally thin themselves.