If you have never tried thinning your peach crop at bloom, this may be a good year to evaluate the benefits in your orchard. There is a small window of opportunity for thinning peach trees at bloom, but labor savings and increases in fruit size can be substantial. Many growers are hesitant to thin peaches at bloom because they fear the chance of bud loss to frost at this early stage of development. Our studies show that any bud loss is compensated by a reduced natural drop. If you are considering blossom thinning, start by setting up a trial in one of your peach blocks.
A tractor-mounted string blossom thinner was designed for peach trees based on field trial experiences in Pennsylvania. The height and angle of the spindle are adjustable to conform to the height and form of the tree canopy. The intensity of thinning is adjustable by changing the rotation speed or the tractor speed. Growers who already have a tractor-mounted string blossom thinner are sometimes willing to cost-share with a neighbor. On a smaller scale, try rubbing off blossoms from the sides and undersides of branches by hand or with the aid of a brush. With practice, hand removal may take only 5 minutes on a detail-pruned tree. Hand-held string blossom thinners are also commercially available.
Peach orchards are typically hand-thinned at around 30 to 40 days after bloom, but this practice is labor-intensive and costly. While apples can be chemically thinned to reduce labor requirements, there are no effective fruit thinners for peaches. We have tested candidates for chemically thinning peaches at bloom, but none have been registered, and our research shows mechanical thinning of blossoms is more predictable.
Although it will be tempting to thin the entire block, be sure to leave some untreated trees so that you can compare differences in follow-up hand-thinning labor requirements at the green fruit stage. In our research trials, the thinning time has been reduced by as much as 50%. Most of our commercial-scale studies also showed an increase in the percentage of fruit in larger fruit size categories that have the highest market value, so take time to make these comparisons at harvest. Grower observations indicate another benefit is an improvement in the seasonal distribution of labor-intensive work. Net economic impact in uniform, multi-year trials conducted across four major peach producing regions was $250 to $850 per acre.
Fruit size comparisons of White Lady peach at harvest. Peaches on the left were hand-thinned. Peaches on the right were blossom-thinned with a string thinner. Photos: Tara Baugher, Penn State
It’s important to avoid over-thinning, however. A good gauge is to calculate how many fruit a block should bear per acre and work backward to how many per tree and how many per fruiting shoot. Then leave twice as many as flowers per fruiting shoot.
Penn State Research on Peach Blossom Thinning