Fruit trees almost always produce more flowers than needed for a full crop. Some flowers do not get pollinated and drop. Some flowers are pollinated and fertilized but the seed(s) fail to develop. In apples and pears, fruits that have more seeds are more likely to persist to maturity, while those with fewer seeds tend to drop from the tree in late spring and summer. Even with this fruit drop, trees tend to produce more fruit than can be adequately ripened. When trees carry too many fruit, the fruit tend to be small, and lack desired sugar, color and flavor. You have a choice, lots of little fruit of low quality or fewer larger, higher quality fruit. Adjusting the crop load to the appropriate level requires thinning.
Diagrams of fruiting wood for apples and peaches.
Flowers on apple trees are produced on short lateral shoots called spurs. Fruiting spurs can produce for multiple years, but produce the best fruit in the second and third year. Peaches and nectarines fruit only on 1-year-old wood. Cherries fruit on side shoots of both 1- and 2-year-old wood, with most of the production on 2-year-old wood.
Fruiting wood formation on sweet and tart cherry (from Westwood, 1993)
In the case of apples and pears, allowing the tree to carry too many fruit has the added side effect of reducing the crop for the following year. When trees produce a large crop one year, there is often a very small crop or no crop the following year. Seeds developing in the fruit produce plant hormones that prevent the nearby development of flower buds. Large crops also deplete the tree’s carbohydrate reserves which can further interfere with flower bud development, and in some cases can reduce the cold hardiness of the buds and shoots, increasing susceptibility to winter injury.
Thinning apples and pears maximizes fruit size and quality, and ensures return bloom the following spring. Timing of thinning is very important. Optimum timing is usually in the first 2 to 3 weeks after bloom. Later thinning has less effect on fruit size, and will not improve flower bud development for the following year, resulting in “biennial” or “alternate” bearing. The basic concept of apple thinning is to remove all of the fruit from at least half of the fruiting spurs, so that these non-fruiting spurs can produce flowers for the following season. In practice, leaving one fruit for every three spurs is ideal for both fruit size and return bloom.
Peaches, nectarines and apricots need to be thinned to improve fruit size and quality, but typically produce more than adequate bloom with or without thinning. A combination of inadequate or improper pruning and lack of thinning can result in limbs breaking off due to crop load, a common problem in home fruit trees. As mentioned above, peaches fruit on 1-year-old shoots. For ideal fruit size, limit the number of developing fruit to 1 to 2 fruits per shoot, and space fruiting shoots during dormant pruning. Cherries and plums are typically not thinned, but fruit crop also can be reduced during dormant pruning by removing more of the fruiting wood.