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Mechanical thinning tested on pears

Pear growers will need to consider redesigning their orchards to take advantage of labor-saving technology.

When Josh Koempel, a pear grower in Washington’s Wenatchee Valley, heard that a mechanical fruit-thinning machine was being evaluated in Washington, he asked if it could be tested in his orchard.

Karen Lewis, Washington State University Extension educator for the Columbia Basin, has been conducting replicated trials with a couple of different mechanical thinners as part of a research project funded through the federal Specialty Crop Research Initiative.

One machine is the Darwin, which has one upright spinning spindle with plastic cords that whip around somewhat like a string trimmer, removing buds or blooms from the tree. The Darwin, which was developed in Germany, is available commercially.

The other is the Bonner, which was developed at the University of Bonn in Germany. It has three rotating arms with cords that can reach into the canopy at various angles. It is not yet in commercial production.
For the past three seasons, Lewis has been testing mechanical thinning in apple, peach, apricot, and nectarine orchards. Two seasons ago, she began testing it in cherries. The test in Koempel’s orchard this spring was the first in pears.

Reduce labor

Bartlett pears grown for the fresh market are usually hand thinned to improve fruit size. Koempel has his workers thin the crop down to one pear every eight to ten inches. His interest in mechanical thinning stems from a desire to reduce his labor needs, not just because of the cost savings, but also because of the difficulties of the potential liability of employers who hire illegal workers.

Koempel is convinced that pear growers will have to look at ways to reduce their dependence on labor, since Congress has not addressed the immigration issue to make it easier to find legal workers. “I see labor as a huge Achilles heel for our industry,” he said. 

Lewis and her assistants used the Darwin to thin two rows of Bartletts at his orchard during bloom. The amount of thinning accomplished depends on both the revolutions per minute of the spindle and the speed of the tractor. Koempel had them speed up the spindle and slow down the tractor to maximize the amount of thinning, figuring that even if he lost some production volume it would be worth it if he didn’t need to hand thin.

He found, though, that the machine was not well suited to the style of his planting, which is on an 8 by 15-foot spacing. Large limbs interfered with the passage of the spindle along the row and he ended up with some places where the fruit was overthinned and others where it was underthinned. He still had to have a crew of workers go through and hand thin.

To accommodate the thinner, he would need to do a lot more pruning and change the tree structure, he said. However, if he just pruned the large limbs out of the existing trees, he would lose production because the rows are too far apart.

The experience got him thinking about what his future plantings might need to look like in order to take advantage of future mechanization and technology. He thinks the plantings would need to be tightened up, with only 10 to 12 feet between rows, and the trees would need to be pruned differently to rejuvenate the wood. A trellised planting, where the trees form a fruiting wall, would be ideal, he said.

Bonner

Dr. Michael Blanke, who developed the three-arm Bonner, said his machine has been tested in Germany and other countries for several years on soft fruits, cherries, apples, and pears. It is expected to go into commercial production soon.

Some of the cords on the rotating arms can be removed, depending on the desired amount of thinning, which makes it very versatile, Blanke said when he presented the machine during Washington State University’s research field day near Wenatchee in July. It can be used with any spindle type training system—slender spindle or super spindle—where the arms can access the branches.

In apples, the objectives are to improve fruit size and color and break alternate bearing pattern. Compared with chemical thinning, there’s a wider window during which mechanical thinning can be done, and the results can be seen immediately, whereas it might take three days to assess the results of chemical thinning, Blanke said. This means that the mechanical thinning can be repeated if necessary.

Mechanical thinning could be a good alternative as the number of chemical thinning options decreases, he said. It is one of the few methods that organic growers can use to thin their crops.