Peach trees produce more flowers than the tree can bring to full size fruit. Only about 5% of flowers are required to set for a commercial crop. Flower or young fruit thinning is required to get larger fruit. Fruit number is reduced, but the increase in fruit size of remaining fruit, and thus value, makes the trade off with total weight acceptable. Fruit are usually thinned when they are the size of a nickel to a spacing of about six inches apart. This must be done within four to six weeks of blooming. Timing is critical because after pit hardening the effect of thinning is diminished. Although hand thinning is preferred, it is costly and there have been various mechanical devices developed to reduce the labor requirements. Unfortunately, most of the devices thin in a random way and require a narrow upright canopy for best results. Chemical thinning also has not been very successful and there are currently no chemicals registered for this use.
Dr. John Cline at the University of Guelph Ontario wanted to use something non-chemical, and tested high pressure water for peach flower thinning. He tested two cultivars trained to a central leader spindle system over two years. The treatments applied at full bloom were based on the amount of time that he sprayed with high pressure water. Trees were sprayed for 45 seconds, 60 seconds, 75 seconds, or thinned by hand.
To avoid injury to the shoots and bark, apply at a distance of about five feet from branches. Uniform application of the water is critical to proper thinning, and was a problem in this research. Tree architecture in a single plane, such as a “V” trellis would be best suited to this type of treatment. The treatments removed between 40 and 57% of flowers and resulted in larger fruit. Total yields were similar to hand thinned trees although the weight of fruit in the larger categories was increased by treatments. Results were similar for the two cultivars, although there was variation in the degree of the treatment response. The longer the water was sprayed on the trees, the greater the thinning effect. The ideal rate of thinning using high pressure water was in the range of 60-70 seconds per tree. Hand thinning may still be required afterwards, but labor time required will be reduced. This method of flower thinning is promising, but needs refining and will be most useful on uniform trees with a single plane architecture.
The full article may be seen at the Journal of the American Pomological Society 71(4): 203-213. 2017.