The reasons for thinning fruit are as follows:

1. Increases annual yields of high quality fruit.
2. Improves uniformity of fruit size at harvest.
3. Improves color.
4. Improves eating quality.
5. Reduces limb breakage.
6. Promotes tree vigor and maintains a balance between vegetative growth and fruiting.
7. Minimizes the handling and storage of low grade or cull fruit.
8. Permits more thorough spraying of fruits during late season applications.
9. Expedites all handling operations at harvest and reduces their respective costs.
10. Prevents trees from biennial production.

A fruit tree commonly produces many more blossoms than should be permitted to set as fruit. Dwarf trees, especially, have a strong tendency to produce more fruit than necessary or even proper for the tree. Ten percent of the flowers on apples and pears are sufficient to produce a full crop; with cherries, peaches, and plums may be allowed to set 20 to 30 percent of their blossoms. Although the natural competition between fruits may reduce the crop somewhat, this reduction in fruit set will not be enough. Fortunately, there are practices that will control the number of fruits on a tree more accurately. The earlier in the season these are performed, the better the result.

Trees should not be allowed to fruit too heavily at too early an age. This will depend upon the rootstock and the variety. If a dwarf type tree is permitted to over produce, the leader of a central leader type tree may be bent severely, and a low, scraggly tree will develop. In the case of dwarf apple trees, it is considered wise to remove all blossoms the first year.

The effect of blossoming and fruiting early in the life of the tree was demonstrated in studies from England by Preston, in which trees were deblossomed for the first seven years after planting and compared with trees not deblossomed. The first crop from the trees that had been deblossomed for seven years was almost equal to the accumulated crops of the trees that were not deblossomed. On the other hand, the deblossomed trees did become biennial following that first heavy crop as all blossoms were allowed to set fruit.

When apple and pear trees bloom and set a heavy crop of fruit, they may become biennial bearers; that is, they bloom and fruit good one year and then bloom and fruit poorly the next year. This habit continues year after year, producing a crop every other year. Dwarfed apple and pear trees can easily become biennial. Therefore, it becomes important to prevent too heavy a set of apples and pears. Blossom thinning or thinning of the young fruits very shortly after bloom is the best way to avoid alternate bearing. Thinning done in midsummer is too late to affect next year's crop materially, since the time is then past when next year's blossom buds are initiated.

The first opportunity to thin is during the dormant period. Pruning is one type of thinning commonly used on peach and nectarine. These trees fruit on one-year old wood, thus excess buds can be reduced by cutting back some of the shoots. See the fact sheet on pruning stone fruit at http://www.coopext.colostate.edu/TRA/PLANTS/pruningstonefruittrees.html. It is best to delay this type of thinning until all danger of winter injury to buds has passed, and after examination of the buds has been made to determine how many are alive. Pruning to thin only removes a certain percentage of blooms and does not control the distribution of the fruit.

The next opportunity to thin is at blossom time. If the bloom is potentially heavy some of it can be literally knocked off with a rubber-tipped pole or a clean toilet bowl brush. This has proved an efficient and practical method for peaches, plums, apricots, and cherries. It can also be used for apples and pears, but is less satisfactory because of the spur-bearing habit of these fruits. The blossoms that remain will set a high percentage of fruit.

Chemical Thinning