Removal of flowers or young, immature fruits early in the spring can lead to increases in fruit size by limiting the number of fruits that continue growing to harvest. It increases the leaf-to-fruit ratio and removes smaller fruit that would never reach optimum size or quality. Apples, European and Asian pears, apricots, peaches, plums, kiwifruits, and persimmons are almost always thinned until the leaf to fruit ratio is favorable for supporting growth of adequately sized fruits. Typically, nut crops and cherries are not thinned.
Large fruit come from strong flower buds that grew in full sunlight, and on trees that have a favorable leaf to fruit ratio; not too many fruit per branch. In order to manipulate the tree into having just the right amount of crop that will size well and develop adequate flower buds for next year’s crop the fruits are hand thinned, removed in the dormant pruning process, or both. Apples, European and Asian pears, apricots, peaches, plums, kiwifruits, and persimmons are almost always thinned. Fruits on non-thinned trees are still edible, just smaller.
The earlier that fruits are thinned and the leaf to fruit ratio is increased, the larger the fruits will be at harvest and the greater the effect on next year’s bloom. The home fruit gardener needs to use judgment regarding spacing of the fruit as well as removal of small and damaged fruit. Spacing fruits evenly along a branch or leaving only one fruit per spur is a good practice, but leaving the largest fruit is more important. The small ones even well spaced will never become as large as the big ones closer together or in clusters. Leave the same number of fruit per branch, but leave the big ones no matter how they are spaced. This will lead to larger fruit. Most home fruit producers do not thin enough fruit off. It hurts to drop all that potential fruit on the ground. It usually takes 2–3 years of experiencing small fruit from inadequate thinning to get it right.
Blossom thinning also increases the leaf to fruit ratio because competition among developing fruits and elongating shoots and roots is relieved early; however, risk can be associated with blossom thinning because bad weather during bloom and postbloom may reduce fruit set even more.
Standard apple and peach cultivars require leaf to fruit ratios between 40:1 and 75:1 (40 to 75 leaves per single fruit) to reach typical size at harvest. Early varieties need a larger ratio of leaves to fruit. Spur-type apple varieties require a smaller leaf to fruit ratio of 25:1 to attain good crop size. This result seems to be related to the fact that photosynthates and stored food reserves are distributed more for fruit growth relative to vegetative growth in trees of this growth habit. Leaves of dwarf trees seem to be more efficient in exporting photosynthates because they are exposed to direct sunlight for more hours of the day than those on standard trees.
Nut crops are not thinned because kernel size is not an important factor.