Fruit Thinning for High Quality
By G.A. Moulton and J. King, WSU – Mount Vernon
A fruit tree in spring, covered with flowers, is a beautiful sight. Yet most people don't realize that if just 5% of all those spring flowers set fruit, it will be enough to provide a full crop. Too many fruits on a tree means fewer cells for each fruit, which translates into smaller fruit that is often of poor quality. Timely thinning of excess fruit increases the number of cells per fruit and maximizes the potential fruit size. Thinning also improves the tree's productivity in the year to come.
- The earlier the better for thinning fruit. In the 30–40 days immediately after flowers are pollinated, the newly set fruit undergoes rapid cell division and growth. Since the total number of cells determines the potential size to which the fruit can grow, it is important to thin fruit early, so that the ones that remain will have more cells and can grow bigger as they mature.
- Early thinning promotes the development of fruit buds for the following spring's bloom. The fruit buds that develop during this summer will determine next year's crop. The presence of seeds, even the immature seeds in the current year's fruit that is just forming, will inhibit the formation of flower buds for next year. By thinning early and heavily, the total amount of hormone produced by immature seeds is greatly reduced.
- Thinning helps to even out crop load from year to year. Trees can get into a cycle of alternate bearing, overloaded with fruit one year, and cropping very poorly in the year following. Some varieties such as Gravenstein are very prone to this. In a heavy bearing year, removing half or more of the blossom clusters at bloom time can help reduce the problem in varieties with this tendency.
- Remove the smaller fruits and leave the larger ones, because the smaller fruit have fewer cells and will remain relatively smaller even after thinning.
- Remove fruit with disease spots, hail damage, or other defects.
- Aim for an even spacing as much as possible. Keep in mind the size that fruits will be at maturity and leave enough room so that fruits won't crowd each other along the branch. Some varieties, called tip-bearing, often have fruit clustered at the ends of long shoots. In this case it may be necessary to keep two fruits together in the end cluster if the rest of the branch is bare.
Apples, pears, and Asian pears almost always need heavy thinning. Apple varieties that bear heavily year after year can be thinned at the bloom stage. The king bloom, in the center of the blossom cluster, is the first to open and produces the biggest fruit. Remove all the other flower buds on that spur, then after fruit has set, check back and thin again where spurs are too close together. A good spacing for apples and pears is one fruit per 6" of branch. Asian pears should be spaced at one per 6–8".
Peaches and nectarines should be spaced at one per 6–8" and fruits that are joined together should be removed.
Plums, especially European plums such as Italian or Stanley, often need thinning when fruit set is heavy. They can be spaced somewhat closer depending on the size of fruit.
Apricots in our area do not need to be thinned in most years.
Cherries don't need thinning
Fruit thinning is the key to producing good sized fruit of high quality, but other factors are also important. Be sure to provide plenty of water during the season when fruit is ripening, particularly if it is a dry year, as good watering helps increase fruit size. Serious stress on the tree can adversely affect fruit quality or even cause some to drop. Fruit trees repay good care by providing the gardener with an ample harvest of ripe, tasty fruit – just as beautiful, to the fruit enthusiast, as the flowers of early spring.
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