The Darwin mechanical thinner makes a pass down a row of dormant high-density sweet cherries during field trials at Hanrahan Orchards north of Buena Feb. 12. It was the first use of the machine in this application. Richard Burger photo

By RICHARD BURGER

A mechanical thinner successfully employed in apples, pears, and soft fruit was tested for the first time on dormant cherries near Buena Feb. 12.

Field trials took place at Hanrahan Orchards in a block of high-density sweet cherries, as part of the Yakima Valley Soft Fruit Day hosted by Washington State University and the Washington State Fruit Commission at Buena Grange.

The Darwin mechanical thinner, which is now available commercially, has proven itself in trials over the last four or five years in orchards in several states, where it has been used to remove fruit blossoms.

Karen Lewis, WSU Ephrata, who presented the results of mechanical bloom thinning trials in nectarines and apricots, said thinning while the cherry trees are dormant gives the grower a larger window of opportunity for the thinning process.

She said the thinner had been used in cherries in bloom, and effectively broke up the large clusters of blossoms.

“We’re excited about cherries,” said Lewis. “We need something for cherries.”

Mechanical thinning works best when booms are fully open, but it can be difficult for growers to complete thinning between full bloom and the onset of petal fall.

Thinning the buds while the tree is still dormant gives the grower more flexibility to carry out the work, Lewis said.

In the Feb. 12 trials, initial results indicated that about 10 to 20 percent of the buds were removed.

The mechanical thinner incorporates a rotating spindle to which are attached strings much like that used by string trimmers for lawns, or strings that are molded poly.

The latter is a new type of strings that were used in the Hanrahan trials.

The amount of buds or blossoms that are removed is controlled by varying the speed of the tractor on which the thinner is mounted, by the speed of the spindle, and by the arrangement of the strings.

Jim Schupp, tree fruit horticulturist at Pennsylvania State University, who also presented at the Soft Fruit Day, said that in the trials he’s conducted, what seems to work best is to set the tractor speed as fast as possible, usually between two and three m.p.h., then adjust the spindle speed and string pattern for optimum results.

He also said it is best to get the spindle and strings as far into the tree as possible, and well-pruned trees enhance the thinner’s effectiveness.

Because uniformity of the trees in the orchard block is important to the effectiveness of mechanical thinning, orchard systems have been developed to take advantage of the equipment.

Schupp said mechanical thinners have been used in angled and vase tree arrangements.

He said it has been demonstrated that mechanical thinning reduces labor, increases fruit size, and in some cases, has resulted in a positive impact of $1300 per acre.

Schupp said the average impact is $500 to $700 per acre.

In addition to the Darwin thinner, Schupp and Lewis also reported on the results of trials with the Bonner string thinner and a drum shaker that is used to thin green fruit.

The Bonner machine can be operated in the vertical or horizontal position, and is “by and large a success,” but is not yet available commercially, Schupp said.

The drum shaker shows some promise, but “will take a lot more design time,” Schupp said.