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One way to get consistent crops of large, high quality apples is to hand thin to size. So if you need to thin by hand, the best idea is to instruct workers about how many apples you want per tree and tell them to take off the smallest ones.

That’s not the way most growers do it, says Steve McArtney, the southeast apple specialist at the Mountain Horticultural Crops Research and Extension Center in Mills River, North Carolina.

After actually measuring the size of the apples on the ground compared to the size of those still on the trees after thinning, he concludes that growers rely more on spacing than on size. There was no size difference between apples on the ground and apples still on the trees.

“One problem with the traditional approach to hand thinning is that in the process of reducing the number of fruit per spur and spacing the remaining fruit at intervals along a branch many of the largest fruit may be removed, and many of the smallest fruit may remain on the tree,” he said.

Removing the smallest apples when thinning does some desirable things:

First, it may take off more apples, and that reduces the number of apple seeds left on the tree. Apple seeds produce gibberellin, which signals the tree to produce fewer flower buds the next year. Fewer apples help assure a good return bloom.

Second, it takes advantage of a basic physiological fact about apples. From bud break to mid-June, the number of cells an apple will have has been determined. Mature apples have anywhere from 26 million to 120 million cells each, but the number is fixed early. After that, apple size is determined by how big the cells get.

The number of cells in an apple is determined by the amount of sunlight available early, so warm, sunny springs lead to larger apples.
“You can make a big apple small, but you can’t make a small apple big,” he said. An apple with lots of cells, given sunlight and water, can grow large, but an apple with fewer cells will be limited in size.

“What this means is that a fruit that is small in June will still be small at harvest,” he said. “If you want big apples at harvest, then you will need to make sure that any thinning you do, chemical or hand, removes only the smallest fruit.”

“Cell division is certainly finished by the time of June drop,” he said. “After June drop occurs, the growth of fruit is due entirely to cell expansion.” Getting rid of unwanted apples early, by chemical thinning, helps increase the size of the remaining apples.

Apple fruits are weaker sinks for carbohydrate than are leaves and shoots, but some apples are weaker than other apples. “Fortunately, most chemical thinners do in fact result in drop of the smallest, weakest fruit within a spur,” he said.

McArtney advocates making size the primary consideration when hand thinning—and not the spacing of the fruits. “If you size thin, you can have fruit one count size larger by harvest without changing the number of fruit on the tree,” he told growers he spoke to in Hershey, Pennsylvania.

Growers need to decide first on the target load and figure out how many apples they want per tree. Count the apples on some representative trees to see how many are there and figure out how many apples must come off. Calculate a percentage. Then randomly select 100 apples and actually measure them, ranking them by size. From that, you can tell what sizes of apples must come off to meet that ­percentage.

Rather than counting fruit, workers should be given a “model size” of the smallest fruit to leave on the tree. “That will be a moving target,” he said. “An apple that’s 33 millimeters on Monday might be 37 millimeters by Friday.”