Thinning Stone Fruit

Unfortunately, most PGR formulations are not available for post-bloom thinning of stone fruits such as peaches and cherries. Application of some mildly caustic materials during full to late bloom has been used successfully in the past to reduce fruit set in both apples and stone fruits. Examples of these are ammonium thiosulfate (ATS), a foliar fertilizer; and lime sulfur, a fungicide. The mode of action is to allow pollination to occur on early blossoms and then damage the later blossoms with the caustic material, preventing further pollination. Blossom thinning with these caustic materials in cold and frost-prone areas of the Intermountain West is extremely risky. Trials in Utah indicated that application of caustic bloom thinners to peaches, followed by cold but non-freezing temperatures, resulted in complete crop loss.

The one exception to the PGRs available for stone fruits is ACC for peach thinning.

Flower Bud Prevention in Tart Cherry

Application of gibberellic acid (GA3) can be used in stone fruits to reduce the number of flower buds formed for the following season. This has been used successfully to prevent over-cropping of weak tart cherry trees, to delay fruiting in young tart cherry orchards, and to “thin” processing peaches where hand thinning and detailed pruning are not justified by the value of the crop.  

As older tart cherry trees begin to decline, the natural tendency is for these trees to produce too many flower buds. If fruit are produced at lower nodes on one-year-old wood (which happens often in Montmorency), blind wood results because there are no vegetative buds to produce spurs or branches. Reducing the number of flower buds relative to vegetative buds allows for spur formation and greater long-term productivity. 

Using ProGibb to Reduce Tart Cherry Bud Formation

Rate of ProGibb Timing Notes
4-18 grams a.i./acreUse higher rate on older trees.

See label for specific rate.
In general, 2 to 4 weeks after bloom.

Optimal is when 3 to 5 terminal leaves have fully expanded, or at 1 to 3 inches of terminal shoot growth.
Similar applications can reduce flowering in non-bearing tart cherries.