By Renee Stern
Contributing Editor

Thinning machines offer tree-fruit growers boosts in efficiency and yields at a reduced cost, but they come with a few caveats.

Two types—string thinners to remove blossoms and drum shakers for green fruit—compare favorably with hand-thinning in speed and labor costs, researchers and growers say. But they perform best with V-shaped and planar tree architecture, and still require touch-up hand-thinning.

Mike Robinson, general manager of Double Diamond Fruit Co. in Quincy, Wash., has worked for the past two years with researchers in a four-state project to develop and test mechanized thinners.

The Darwin string thinner from Germany-based Fruit-Tec “is visibly a winner in V-trellis apricots,” he says.

Despite having to hand-thin green fruit, costs still are lower than hand-thinning alone, he says. Robinson also sees larger green fruit where machines have thinned blossoms. “And it’s so fast you can go in and thin blossoms early,” he says.

The waiting game

Growers generally prefer waiting as long as possible before thinning, says Scott Johnson, Extension specialist at the University of California’s Kearney Agricultural Center in Parlier. Thinning too early risks additional yield reduction from freezes, poor pollination and other factors.

But reducing crop loads early gives the remaining fruit a size boost, he says. Not all varieties set fruit well, Robinson says. Any type of thinning there may not be the best approach.

Johnson is working with other researchers in California, Pennsylvania, South Carolina and Washington state. Shakers currently seem to create less-uniform results than string thinners, he says.

“The Darwin string thinner is the most uniform I’ve seen,” Johnson says.

Robinson says machine-thinning’s main benefit is reducing labor management needs. One tractor driver replaces a 10- to 15-person crew, with job time dropping from days to hours.

Labor is the driving force

Labor costs drove the decision at Family Tree Farms to purchase a Darwin thinner, says Eric Wuhl, director of research and development.

Labor accounts for 80 percent of the Reedley, Calif.-based company’s farming expenses.

Hand-thinning alone can hit $2,000 per acre. “That’s a brutal expense,” Wuhl says.

The string thinner doesn’t tackle everything, but eliminates enough hand-thinning to pay off quickly. “Right now we’re not seeing it as a finishing tool but as a thinning aid,” he says.

The project focuses mainly on soft fruit, particularly peaches, where chemical thinners either aren’t available or produce inconsistent results. But researchers also are looking at apples, pears and cherries.

Expanding the scope

Karen Lewis, a Washington State University Extension educator in Ephrata, last year used a machine for dormant thinning to break up clusters in sweet cherries. The tests were “relatively successful,” she says.

Future tests will work out timing and other details to avoid overdoing machine-thinning at the later stages of bloom, however.

Apple trials for the past three years have incorporated the Darwin thinner and a prototype developed at the University of Bonn in Germany that provides more flexibility and better suits a wider ranger of orchard training systems. King bloom survival averaged 71 percent across all varieties tested, Lewis says.

One concern is the machines’ potential to spread fire blight from tree to tree. The Washington team is testing ways to add protective applications of antibiotic during thinning operations.

The most important precaution, Lewis says, is to sideline thinning machines during an active fire blight infection period.

Modifications boost efficiency

Wuhl’s experience showed the need for additional orchard preparation to increase machine efficiency. Shifting to a trellis or other system that creates more uniform branches would allow greater speed and more accurate contact with blossoms, he says.

“The most critical thing is the shape of the tree,” Johnson says. Most stone-fruit blocks still are trained with open-center trees, making internal blooms harder to reach.

Modifications that run the rotating spindle over the tops of trees are one solution. Clemson University researchers also added separate hydraulic systems for the tractor and thinner spindle, says Greg Henderson, county Extension agent for commercial fruit in Edgefield, S.C.

That’s made the string thinner more responsive to the operator as it moves in and out of trees without sacrificing spindle speed, he says. The changes also save fuel and reduce operator fatigue.

Paul Heinemann, head of Penn State University’s department of agricultural and biological engineering at University Park, is overseeing a graduate student’s project to better position the string thinner’s spindle unit during operation.

The prototype shifts the thinner to the rear of the tractor to give priority to new positioning sensors in the front that guide up-and-down and in-and-out movement among branches. A spring field test in peaches is planned, and Heinemann says if all goes well, the improvement could be available to growers within two years.

Heinemann also is tackling a more longterm project on selective thinning that will require adding vision sensors to map blossoms, and end effectors to remove specific blossoms without damaging anything else.

“Selective thinning is extremely challenging,” he says. “What we’re trying to do is take human thinking and put it in a machine and put that in an environment that’s highly variable.”

Bottom-line considerations

Until then, the optimum approach adapts both orchard and equipment to work together, Henderson says.

“The operating cost of this compared to handthinning is minuscule,” he says. But any difference in returns depends on the market value placed on various fruit sizes. “Our observation is that the machines, where we’ve used them, have made us money,” Wuhl says. “Or at least we’ve lost less.” Another consideration for the bottom line is how much acreage requires bloom thinning. Henderson suggests growers focus mechanization on varieties where thinning makes a critical difference in fruit size and quality, freeing up workers to hand-thin the remaining blocks. The key to thinning is determining the minimum percentage of blooms to remove to achieve the desired effect. “The less fruit you have to remove by hand the better,” he says.