Answer: Under most circumstances, mature peach trees have a natural tendancy to produce more fruit than the tree can adequately support. Thinning is an attempt to reduce the crop load at some advantageous point in time during the year. There are three primary times of the year when peach thinning can be done. These include the dormant season, during bloom, and prior to pit hardening when fruit are in the “green fruit” stage.
The earliest thinning in the dormant season can be accomplished using fall applied ethephon or winter applied soybean oil to reduce the number of living flower buds on the tree. Another practice to “thin” at this time is actually pruning out and reducing the number of fruiting shoots on the tree.
During bloom, flowers can be reduced by chemically burning and destroying them using chemicals such as ammonium thiosulfate or tergitol (a surfactant). Also during bloom, flowers can be removed by hand or using a “toilet”-type brush or even mechanically using “Darwin” string thinner developed in Germany. The latter is being tested at various sites in the U.S. for mechanical bloom thinning but uniformity of tree architecture is vital to the success of this device.
Finally, the latest stage to do thinning is while the fruit are green and prior to forming hard pits. At this stage, green fruit removal is by hand, using “wiffle ball” bats, a “Kentucky bumper”, or using a mechanical spiked drum shaker. The latter is a prototype being developed and tested through scientists at the USDA-ARS in Kearneysville, WV. The prototype green fruit thinner is based on a design originally created for mechanical harvesting of citrus.
The best time to thin to maximize the impact on final fruit size is as early as possible. Because the tree has a finite supply of stored carbohydrate reserves to support growth of new fruits at a time when the leaf canopy has not developed sufficiently yet, the fewer fruits present, the more reserves that can be supplied to individual fruits that remain (less competition). Initial fruit growth is primarily cell division. Fruits with more cells initially can then have those cells expanded in the final swell phase with water and solutes (sugars, acids, etc.) resulting in larger fruit at harvest. This is the ultimate goal, large fruit size that will attain the highest market price possible to the grower.
The one drawback of early thinning, however, is that there may be an unexpected spring freeze event that could substantially reduce the flower crop or developing fruit crop. If this were the case, it would be best to have as many flowers/developing fruits on the tree as possible. This is part of the reason that risk-averse growers often practice green-fruit thinning. Their rationale then is that they “know what crop they have” by that point. However, later thinning minimizes the impact on final fruit size, potentially resulting in smaller fruit that attain a lower market price usually.