Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Fruits such as apples are prone to alternate bearing. A heavy crop as seen above may lead to a light crop the next year.
Biennial or alternate bearing, refers to the tendency of perennial fruits to put on a heavy crop one year and then little or no fruit the second year. This is most common in tree fruits such as apples.
Next year’s flowers are initiated this year. If there are too many fruits on the plant in the current season, most of the energy goes to fruit development and less to flower initiation for next year. As a result, a fruit tree often produces a small number of flowers and fruits the year after a heavy crop.
Alternate year bearing is often variety related, so choose varieties that do not have alternate bearing habits when possible.
The main tool that we use to prevent alternate bearing is fruit thinning. Apples are commonly chemically thinned during bloom. If the chemical thinning was incomplete, hand thinning may be required if fruit loads are too high. In peaches, fruit thinning starts with dormant pruning to remove fruiting wood in vigorous varieties. This is followed by mechanical blossom thinning and the hand thinning to reach the optimal 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.
In the past, chemical thinners were extremely variable in stone fruits, unlike apples. However, a new thinner for peaches is now available from Valent BioSciences. Accede is the first PGR based on a naturally occurring compound developed specifically for thinning of stone fruit, including peaches and nectarines. This will reduce or eliminate the need for blossom thinning. However, with the late spring freezes we have been having, many growers will limit early thinning until a final fruit load is determined.
This will require thinning when green fruits are the size of a nickel to a quarter. 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.
Hand thinning of apples should be done within six weeks of full bloom. Leave the largest apple in a cluster unless it is damaged. After thinning, apples should be spaced about 8 to 10 inches apart on the branches.