Public Release: 

Mechanical thinning increases fruit size, reduces labor

Thinning technology can produce economic benefits for peach and apple growers

American Society for Horticultural Science

BIGLERVILLE, PA--Hand thinning is a necessary but costly management practice in peach and organic apple production. Mechanical devices designed to help with thinning have been developed, but none has proven highly effective and capable of completely replacing hand thinning.

J.R. Schupp and a team of researchers from the Department of Horticulture at Pennsylvania State University, Penn State Cooperative Extension, and the U.S. Department of Agriculture's Agricultural Research Service, published a study in the American Society for Horticultural Science journal HortTechnology that evaluated two mechanical thinning devices on peach and organically grown apple trees during 2005, 2006, and 2007.

The first machine was a rotating string thinner designed by H. Gessler, a German grower, to remove apple blossoms in organic orchards. The second machine was a vibrating direct-drive double spiked-drum shaker designed for harvesting citrus fruit.

In 2007, the spiked-drum shaker, designed by the U. S. Department of Agriculture and slightly modified for these tests, was used to thin pillar (columnar) peach trees at 52 to 55 days after full bloom. The drum shaker was driven at two different speeds in the orchard, and reduced crop load an average of 58% and follow-up hand thinning time by 50%.

The spiked-drum shaker increased fruit size by 9% at harvest compared with conventional hand-thinned or non-thinned control trees, a result appreciated by growers and consumers. As consumer demand for larger-size peaches increases, fruit brokers pay premiums for peaches measuring 2.75 inches in diameter or larger. Hand or mechanical thinning resulted in average fruit sizes within the desired size range

Peach and apple producers can see multiple economic benefits from mechanical thinning technology. According to the research, the "net economic impact of mechanical thinning versus hand thinning alone ranged from $175/hectare to $1,966/hectare".

Schupp explained that the study results have other implications for growers, stating; "Mechanical thinning has greater predictability than chemical thinning. Because the effects of physical removal are immediately visible, the level of crop removal can be determined by comparing pre- and post-thinning flower or fruit counts. Therefore, a grower can assess the level of crop removal and adjust the machinery to increase or reduce thinning as needed."

The researchers summarized that mechanical thinning appears to be a promising technique for supplementing hand thinning in apple and peach trees. The application of mechanical thinners offers a solution to critical components of fruit grower profitability: the premiums paid for large fruit in the fresh fruit market, growing labor costs, and a potential shortage of farm labor.


The complete study and abstract are available on the ASHS HortTechnology electronic journal web site:

Founded in 1903, the American Society for Horticultural Science (ASHS) is the largest organization dedicated to advancing all facets of horticultural research, education, and application. More information at

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