Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
I am currently pruning in my peach orchard. This is the first step in managing the fruit load. Most years, peaches and nectarines will set many more fruit than the tree can carry, and they will need to be thinned. Thinning is done to prevent limb breakage, increase fruit size, and improve fruit quality. Thinning techniques are used before, during and after bloom to reduce peach crop load.
Unfortunately, chemical thinners are extremely variable in stone fruits, unlike apples. However, a promising new thinner for peaches may be available soon from Valent BioSciences. They have announced U.S. EPA registration of a new plant growth regulator (PGR), which will be marketed under the brand name Accede. Accede is the first PGR based on a naturally occurring compound developed specifically for thinning of stone fruit, including peaches and nectarines. We will keep you posted as the product is introduced to our region.
Because chemical thinners are problematic in peaches and nectarines, the first thinning starts with pruning out excess or poor-quality fruiting wood in the dormant to pre-bloom pruning window (now). Eliminate limbs in the low center area of the tree where poor light and will cause small fruit size. Prune to remove all fruiting shoots shorter than 8 inches long, as these limbs tend to produce smaller fruit.
The next thinning is done during the blossom stage. Various mechanical aids have been used to remove excess blossoms from peach trees. These methods include dragging large diameter ropes across canopies, using rotating string thinners, rubbing with a gloved hand or brush and high-pressure water systems.
Then next thinning period is when small fruits have formed. With very high fruit densities 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.
Final thinning is done by removal of green fruit at the stage when they are the size of a nickel. The target fruit load is 400-600 fruit per mature tree. 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.
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. 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.
Early ripening varieties and varieties with less potential for large fruit should be thinned first to provide the best opportunity for size enhancement.
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.
This article was adapted from “Thinking through strategies for peach crop thinning” from MSU https://www.canr.msu.edu/news/thinking_through_strategies_for_peach_crop_thinning#:~:text=Peaches%20are%20thinned%20to%20prevent,fruit%20size%20by%20reducing%20competition