An understanding of alternate bearing, premature fruit drop, and hand thinning of fruit trees will help you produce the best apples and other tree fruits in your garden.
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 but is particularly common in apple and pear.
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.
Thin Fruit to Prevent Alternate Bearing
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.
Fruit Drop in Otherwise Healthy Trees is Not a Problem
Gardeners are often surprised when small apples and other fruit drop prematurely to the ground. However, premature fruit drop is relatively common on apples and other fruit trees. The home gardener shouldn't be alarmed if the fruit tree appears healthy. The fruit drop may simply be nature's way of reducing a heavy fruit load.
Premature Fruit Drop Because of Poor Pollination or Poor Weather Conditions
Two fruit drop periods commonly occur on apples. The "first drop" occurs shortly after petal-fall and may continue for 2 to 3 weeks. The fruit that falls during this period is pea-size and may be the result of poor pollination. Most apple varieties are considered self-unfruitful. These fruit varieties will produce little or no fruit when pollinated with their own pollen. Another variety (cultivar) is required for cross-pollination and fruit set. 'Jonathan' and 'Yellow Delicious' are two apple varieties that are notable exceptions. Each will produce a fairly good crop without cross-pollination.
A lack of pollination may also be due to poor weather. Most fruit trees are pollinated by bees. They are most active on sunny, warm days. There is little bee activity during cool, rainy weather. Cool, rainy weather during the bloom period reduces bee activity, results in poor pollination, and may lead to fruit drop. Exposure to freezing temperatures during flower bud development and bloom may also cause fruit drop.
Premature Fruit Drop Because of Heavy Fruit Set
The "second drop" usually occurs in early June. (This is commonly referred to as "June drop.") The fallen apples are approximately 1/2 to 1 inch in diameter. The second shedding of fruit is often due to the competition among the developing fruit for food, water, and nutrients. This natural thinning removes excess fruit and allows the remaining fruit to develop properly. Hot, dry weather in late spring will contribute to fruit drop.
While the number of fruit that falls to the ground as a result of natural thinning may seem quite high, additional thinning may be necessary. Trees overloaded with fruit need additional thinning to
- obtain large, high-quality fruit at harvest;
- allow the development of flower buds for next year's crop, thus overcoming the tendency for some fruit trees to bear fruit in alternate years;
- prevent limb breakage.
Hand thinning of apples should be done within 6 weeks of full bloom. Leave the largest apple in a cluster unless it is damaged. After thinning, the apples should be spaced about 8 to 10 inches apart on the branch. Pears, plums, and apricots may also require hand thinning. Fruit should be spaced about 6 to 8 inches apart on the branches following thinning.