A. When Should Thinning Be Considered?
Excess fruit Set on Young Trees: Sometimes young trees set to much fruit. Failure to thin early and adequately will adversely affect tree growth. This characteristic is especially noticeable in dwarf apple trees.
Heavy Fruit Set: Thinning is particularly important when fruit is set in clusters. Fruit should also be thinned if only part of a tree has a heavy crop.
Older Trees: As trees age, fruit size tends to decrease. Thinning the crop, even if set is only medium, may be essential to give good size.
Trees Low in Vigor: Non-vigorous trees need thinning more frequently
than vigorous trees to attain good fruit size.
Poor Soil: Whatever the reason for poor soil -- low fertility,
poor drainage, etc., fruit will not attain large size when compared to
trees in good soil. Thinning will help increase size of remaining
fruit.
Varietal Variation: There is a difference in the size crop that different varieties may set and a difference in the ability of different varieties to size fruit.
In addition to increasing fruit size, there are other reasons to thin. They are:
To Attain Annual Production: Thinning a fruit crop during the heavy production year in an alternate bearing cycle will tend to reduce the severity of alternate bearing.
To Improve Quality: Thinning should increase fruit quality both in terms of appearance and taste. Fruit color will be better and taste will be improved when a heavy crop of fruit is thinned.
To Avoid Tree Breakage: Breaking or splitting of limbs may be severe when a heavy crop of fruit is not thinned.
B. How is Thinning Done?
Hand Method: Hand thinning is the surest way of doing a good job of thinning fruit. It involves removing the excess fruit by hand. Clusters of fruit should be broken up and fruit should be spaced on the limb. Damaged or weakened fruit should be removed in the thinning process.
Pole Method: A piece of hose attached to a stick may be used to strike the limb to knock off fruit. This method does not give the precise thinning job which may be obtained by using the hand method; however, it is considerably faster. Best results with this method are obtained in early morning or late evening, as this is the time that fruit stems are the most firm and crisp. Hand thinning may have to be used as a follow-up procedure to obtain a satisfactory thinning job. This type of thinning is most frequently used in peaches, plums and pears.
Chemical Thinners: These materials are most often used by commercial orchardists. Chemical thinners are not practical for the home orchardist. Hormone-type materials and a pesticide (Seven) are most commonly used to thin apples. The correct material and time and rate of application depend on the variety to be thinned. Peaches do not respond in a reliable manner to chemical thinners.
C. Correct Spacing of Fruits When Properly Thinned:
1. Apples - 6 to 7 inches apart
2. Peaches - 6 to 8 inches apart
3. Pears - 20 to 30 average-sized leaves per fruit.
D. Timing of the Thinning Operation:
Chemical thinners are applied based on days after full bloom or in regard to fruit size. The larger that fruit get, the harder they are to chemically thin.
Thinning by hand or with a pole is done later than chemical thinning and is, therefore, not as effective in increasing fruit size and return bloom. The later thinning is done, the less pronounced the results will be.
Fruit crops go through a natural drop period during which many of them will be shed. The magnitude of this drop will vary among years. Manual thinning is frequently delayed until after the natural drop, which should occur within four to six weeks following bloom. Fruits that will drop naturally stop growing and appear yellowish when compared to other fruits as the time of drop approaches.