For the tree fruit grower, an abundance of spring blossoms can translate into a lot of work thinning the load. If blooms are left alone, one risks having a large crop of small, low quality fruit. A heavy crop load may break branches, cause a lesser bloom next season and potentially damage tree growth and health.

“Thinning is the really challenging, really costly and really necessary part of growing high-quality fruit,” said Daniel Ward, associate Extension specialist in pomology and director of Rutgers Agricultural Research and Extension Center.

Why thin?

When thinning the crop, the primary goal for commercial growers is knowing “when and how to thin to get larger size and better quality, and how to get return bloom,” Ward said. Crop load management and “decreasing the crop enough so it doesn’t break the branches on the tree,” is one reason to thin. Disease control is “not a direct consideration,” although pest and disease concerns can be elevated when fruits are crowded, as sprays can’t reach areas where they are touching.

Return bloom refers to next year’s crop load. With many fruits, such as peaches and some apple varieties, trees tend to carry the heaviest crops biennially. Thinning at the proper time, in the proper amounts, will encourage repeat bloom and keep the crop loads on an even keel year to year.

Thinning needs vary among species of fruit. For peaches, which tend to overbear, thinning is primarily used to “get fruit size to quality, and not break” the branches. For pears and apples, return bloom is crucial and promoted via appropriately timed thinning. In Asian pears, the taste of the fruit is highly correlated to size, and early thinning is imperative for achieving the most flavorful fruit.

“Taste is definitely affected by thinning,” Ward said. “Size and taste are related in some species much more than others.”

Market considerations may play a role, too. Peaches are often tastier when larger. Getting larger sizes through thinning brings more value. But larger peaches are more difficult to store, and bruise more readily.

Some apple growers, targeting the bagged apple market, want smaller sized apples. Although cultivar selection is important, managing the crop load primarily via thinning to achieve a smaller sized apple is a management strategy. In apples, smaller fruits can be as high quality as their larger counterparts, Ward said.

“Growers aren’t actually focused on this,” but thinning helps to size the fruit load to the availability of carbohydrates, Ward said. “It’s important to have a balance between fruiting and vegetative growth.”

Thinning, whether done by hand, with mechanical means or via chemical sprays – and the options vary with species of tree – isn’t a one-size-fits-all prospect. The species and size of the tree, its age and its overall health will dictate thinning requirements.

Timing and methods

“Thinning has a narrow window of time when it must be accomplished,” Ward said. Early thinning has a greater effect on fruit size, quality and return bloom than does post-bloom.

But thinning at bloom runs the risk of loss, particularly for early-blooming varieties. Aggressiveness of bloom thinning has to be weighed against the potential for frost damaging the remaining blooms, resulting in severe yield reductions.

Once fruitlets are greater than 1 inch in diameter, the effectiveness of thinning for size as well as impact on return bloom, which is already established for next year, are minimal. If a grower is facing an overcropping situation, simply knocking fruits off of the tree decreases stress and can serve as a “rescue” method if thinning was not able to be completed in a timely manner.

Apples and pears are most often chemically thinned, at or shortly after bloom. Different available chemicals work via a variety of modes of action and are used at different times. When chemical thinning, the weather during spraying is important and will impact results.

Most chemical thinners are synthetic plant growth regulators. Their effectiveness is linked to the disruption of photosynthesis or the prevention of carbohydrate movement to the fruits. Warm, cloudy days enhance the effectiveness of chemical thinners, as chemical thinners that are applied at times of greatest carbohydrate stress are most effective. Carbohydrate availability is dependent on photosynthesis, which is limited under these conditions.

Using the correct materials, at the correct rates and during the correct time, considering weather both pre- and post- application, is the key to chemical thinning success, Ward said. The Cornell Apple Carbohydrate Model guides growers in proper timing for applying thinning sprays, based on real weather data and growing conditions. Access the model here at the Cornell news website.

“It’s not an exact kind of thing. It’s subject to the vagaries of weather condition and plant response,” Ward said of chemical thinning. “There are ways to thin from about the time of petal fall until the fruits are 1 inch in diameter.”

Lime sulfur and fish oil options are available for certified organic growers at bloom, but not post-bloom. These work to damage the pollen tube, preventing pollination, but don’t impact already fertilized flowers.

Chemical thinning is not often used in peaches or plums, due to inconsistent results and risk of over-thinning. Growers rely on hand thinning or use mechanical methods, such as the Darwin String Thinner or various shakers. See more at the Penn State University Extension website.

Additional considerations

“Thinning is one of the most expensive operations in the orchard,” Ward said. Labor costs for hand thinning can be significant, particularly with peaches where “other than pruning, there is no commonly used thinning method other than by hand, or mechanical thinning.”

Water stress at any time can impact fruit size. Weed management is often overlooked, but weeds compete with the trees for moisture, limiting fruit size. Apples grow over a long period of time, so unless water stress is prolonged, they typically recover. Peaches undergo rapid growth, follow by a lag, then another growth period. Lack of water when growth is minimal isn’t a problem.

But during rapid growth, water stress can be “terrible for fruit size,” Ward said. “Water is so, so important. It is the single biggest factor that we can control.”

The best growers keep accurate records. Bloom size, frost events, details on thinning methods – dates of thinning, rate of application, chemical type and chemical strength, weather conditions, and ultimate crop size are monitored, Ward said. Growers should refer to these year to year, and use these records to guide thinning decisions for their individual operation, increasing consistency.

Healthy trees are the basis for optimizing fruit size. Thinning manages the crop load to maximize fruit quality and value and regulate repeat bloom. While this season’s thinning window is closing rapidly, next season provides another opportunity to optimize thinning protocols and harvest a crop of high-quality fruit.