Thinking through strategies for peach crop thinning

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Peaches are thinned to prevent limb breakage, increase fruit size and quality. Thinning is a time-consuming, expensive job. Thinning techniques are used before, during and after bloom to reduce peach crop load. Early season thinning techniques can enhance fruit size by reducing competition. Early ripening varieties and varieties with less genetic potential for large fruit should be thinned first to provide the best opportunity for size enhancement.

Thinning by pruning

An efficient way to reduce final numbers of fruit per tree is aggressively prune excess or poor-quality fruiting wood in the dormant to pre-bloom pruning window. Aggressively eliminate limbs in the low center area of the tree where poor light and insufficient foliage will cause small fruit size. Prune to remove all fruiting shoots shorter than 8 inches long, as these limbs tend to produce smaller fruit. Crop load can be also be reduced by making heading cuts to shorten fruiting wood (limbs that grew last year), although do not cut shorter than 12 inches.

Crop reduction by blossom thinning

Various mechanical aids have been used to remove excess blossoms from peach trees. These methods include dragging large diameter ropes across canopies, rubbing with a gloved hand or brush and high pressure water systems. Chemical methods such as Wilthin disrupt pollination by damaging the flower parts. Of these, rope thinners have been the most consistent technique under Michigan conditions.

First assessments of crop load

An experienced orchard manager can look over a peach orchard shortly after bloom to estimate the fruit thinning job to come. After looking at 10 to 20 "typical" limbs in a peach block, a rough pre-thinning crop load estimate can be made. This is only a crude estimate – fruit size is greatly affected by weather following bloom. Mild conditions following bloom increases cell division numbers and the potential for fruit size. Higher rainfall, especially during the few weeks before harvest, can also increase final fruit size.

Preliminary thinning

With very high fruit densities it is efficient to go through the orchard with loppers to adjust crop load by cutting out entire fruiting limbs and remove the ends of many fruiting limbs longer than 12 inches. In the two to three weeks after bloom, the very short fruiting limbs less than four inches are brittle and can be quickly snapped off completely by hand with little damage to the supporting limb. At this early stage, running a hand along the bottom of a fruiting limb can remove half or more of the fruit. These strategies are much faster than plucking off individual fruit by hand later when the fruit are larger and held tighter to the limb.

Target final fruit densities

A large mature tree of a mid- to late season variety can produce four or five bushels of 2.5-inch diameter fruit. A bushel of 2.5-inch diameter peaches contains approximately 150 fruit. So a target fruit load is 600 to 750 fruit per mature tree – nowdays the target fruit load should be less for larger fruit required by today's markets. For a typical 4-scaffold open center tree this works out to approximately 150 to 190 fruit per scaffold. For smaller, closer-spaced trees, or where the goal is slightly larger fruit, the number of fruit per scaffold could be as low as 60.

If working with a crew, thin several trees in advance and mark these with colored ribbons to serve as examples. A general rule of thumb is to leave an average of 6 to 8 inches between fruit (the larger spacing for earlier or hard-to-size varieties). Two or three peaches can be left clustered if there is enough additional limb space to support their growth. Keep the largest fruit on a limb, even if they are clustered.

Getting the job done

Hand thinning can start before various striking and shaking methods, which require fruit large enough to be dislodged by the vibration. If the fruit thinning has not been completed earlier, rubber-tipped poles, padded bats and plastic "wiffle" bats can be used to strike limbs to remove excess peaches and is faster than hand thinning. Wiffle bats are preferred in Michigan for their combination of speed and safety to the tree. In addition, portable hydraulic or pneumatic limb shakers are available and do a somewhat satisfactory job on those limbs rigid enough to transmit vibrations to the fruit. If limb shakers are to be used, the trees should be pruned carefully to remove willowy growth that prevents good shaking action. Both striking and shaking strategies generally require follow-up hand thinning. Hand thinning provides greater control and causes less limb damage than limb shaking and striking.

In summary, the proper strategy for fruit thinning depends on many factors such as the variety characteristics, pruning style, the crew, fruit set and fruit growth rates. Each grower will develop the strategy that works for them. The key is to do what can be done earlier, quicker and more efficiently while there is time to benefit by improved fruit size growth.

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