The Electroflor looked promising for mechanically thinning cherries in tests that Karen Lewis, WSU extension specialist, conducted.

The Electroflor looked promising for mechanically thinning cherries in tests that Karen Lewis, WSU extension specialist, conducted.

After success with mechanical thinning of stone fruit, researchers have turned their attention to cherries and are experimenting with several tools to help cherry growers reduce the crop load and increase fruit size. Karen Lewis, ­Washington State University extension specialist, has been running trials.

Two years ago, she began trials in cherries with the Fruit-Tec Darwin thinner, which has a single spindle with cords that can be operated horizontally or vertically, and the Bonner thinner, from the University of Bonn in Germany, which has three ­independent spindles. Lewis did bud-removal trials during the dormant season (mid-­February or mid-March) in Rainier, Early Robin, and Santina cherries. The dormant treatment reduced fruit set by 23 to 49 percent, with the March treatment thinning the most. Lewis said more research needs to be done on mechanized dormant bud removal.

She also tested the thinners during bloom in Sweetheart and Selah cherries. One of the goals was to determine the most effective spindle rotation speed, and she tested a range of revolutions per minute from 275 to 350. The ground speed was kept at a constant three miles per hour, a speed that Lewis thinks is necessary for treating large, western orchards. Bloom treatments gave 29 to 52 percent reductions in fruit set. More flowers were removed at the lower spindle speeds.

Problems resulting from the Darwin bloom treatments included overthinning, blind wood, and leaf damage. Lewis said thinning cherries is different from thinning peaches and apricots, because cherries have leaves ­during bloom that can be ripped up by the thinner, whereas other stone fruits do not.

The Bonner thinning results clearly reflected the limited cord-to-bloom contact, which is directly related to tree architecture, Lewis said.

One of the big issues with these two mechanical thinners is that the tree fruit industry in the Pacific Northwest has what Lewis calls four-dimensional trees. They are three dimensional, and the fourth dimension is the variability from row to row and tree to tree. Trees can have ­different bloom intensity and different thinning ­requirements.

Handheld mechanical thinners allow a more targeted thinning approach to suit each individual tree and crop load goals or strategy. They can be used from the ground or from a platform.

WSU has been working on a handheld thinner that was adapted from commercial  weed trimmers. The first prototype had a gasoline engine on the pole, but it proved to be too heavy and noisy. It also would power up or down, depending on the load, and might come to a stop when it reached a tight cluster, Lewis said.

WSU agricultural engineers developed a version with a battery on the pole, but the tool lacked power and didn’t work very well, she said. A later version had two batteries in a backpack worn by the worker, which was more ­successful.

Lewis said one grower who tested the thinner believed that wasn’t the best configuration, either. He took two batteries out of a couple of wind machines, put them on an all-terrain vehicle, and plugged the thinners into those.

Last year, in Lapins on Gisela 5 rootstocks, Lewis compared the handheld thinner at 70 to 90 percent bloom with green fruit clipping or no thinning. More than 50 percent of the mechanically thinned fruit was nine row, compared with less than 20 percent for the clipped fruit and less than 5 percent in the control. Trends were less clear with Sweetheart on Mazzard.


WSU collaborated with Matías Kulczewski of Asesorias K & R  to conduct trials in Chile. The WSU handheld overheated early in the project so the group tracked down a handheld thinner made by INFACO. The Electroflor is a battery-operated handheld which has a 10-foot ­telescoping pole and an optional 48-volt battery system.

Kulczewski tested the Electroflor in Lapins on Maxima rootstocks on a central leader system. Electroflor treatments at 20 to 50 percent bloom and 70 to 90 percent bloom were compared with hand bud removal, hand bloom removal, and hand thinning of fruitlets. While hand bud removal and hand bloom thinning gave the greatest boost in fruit size, the mechanical thinning was as good as hand fruitlet thinning and much less labor intensive than any of the hand thinning treatments.

For example, hand bud removal took 189 hours per acre, hand bloom thinning took 207 hours, and hand fruitlet thinning took 437 hours per acre, compared with only 42 hours for the Electroflor when used in early bloom. Lewis noted that fewer hours would be required in Washington State, where workers are typically more efficient.

In terms of net income, based on the price of the fruit as well as labor costs, hand thinning of buds was the most profitable followed by hand bloom thinning, then the Electroflor treatments and the control. Hand fruitlet ­thinning was the least profitable.

Results were different in Sweetheart cherries, however. Hand fruitlet thinning was the most profitable and mechanical thinning the least profitable.

Lewis concludes from these preliminary results that both the WSU and Electroflor thinners are effective in cherries. Lapins appears to be a better candidate for mechanical thinning, or thinning in general, than is ­Sweetheart.

“We really like the results we’re getting in Lapins,”she said.

Lewis said economics, and the growers’ cost structure or business model, will determine if they thin cherries and what with. Growers will probably use a combination of methods. They might use the Darwin first and then ­follow up with a handheld thinner or even chemical ­thinning.

“These are tools,” she said. “Our job is to bring tools to the toolbox and the more full your tool box, the more options you have year in, year out. We think there’s an appropriate place for these in your crop load ­management toolbox, based on our data.”

To see photos and videos of the Electroflor, go to