Katy Lesser Clowney, while working at the Adams County extension office, found the Darwin at a show in Europe and suggested it be tried out. She has been involved in testing it.

Katy Lesser Clowney, while working at the Adams County extension office, found the Darwin at a show in Europe and suggested it be tried out. She has been involved in testing it.

Steve Hollabaugh

The Darwin mechanical string thinner, developed in Europe for thinning organic apples at blossom time, has been tested in Pennsylvania as a peach blossom thinner and is now being adopted by growers, who find it saves them labor and gives them larger fruit. Orchardists are finding it can pay for itself within a year or two.

Three Springs Orchard at Bendersville, Pennsylvania, has one Darwin machine co-owned with a Maryland grower 50 miles south. Since their seasons are offset a bit, sharing one thinner works for them, says Dave Wenk, who operates Three Springs Orchard with his brother John.

“We bought a Darwin fifty-fifty with another grower, Henry Allenburg at Smithburg, Maryland,” Wenk said. “He bought, I paid half, and I store it. He uses it first and finishes a week before I need it.

“For us, it saves a week of green peach thinning by a crew of 12 people,” he added. “At $4,000 a week for labor, the machine paid for itself in the first two seasons.”

Large fruit

Dave’s son, Ben, has come into the business to develop direct-to-consumer fruit sales through seven farmers’ markets. These markets are upscale, located in suburbs of cities like Philadelphia, Baltimore, and Washington, D.C., where customers pay premium prices and want excellent quality fruit. One big element of peach quality is size.

“We have gone to earlier and earlier peaches, and it is so hard to get size on these early season semi-clingstone varieties,” Dave said.

He thinks it’s essential to thin early varieties early—before the green peach stage. There is probably some location effect to size of peaches, he says, but he sees a whole-tree effect as well. While it may be somewhat important to space out peaches along the tree limbs, he says, it’s also critical to reduce the tree load earlier so the tree doesn’t use its resources to grow fruit that will be removed during thinning.

“We have bigger fruit because we use the Darwin,” he said. “We reduce competition right from the start.”

When the Darwin was first brought to the United States, the rotating spindle was used along sides of the trees, with the brush in a vertical ­position. It was first tried on peach trees grown in the perpendicular vee design, which only a few growers used.

The latest design can run the rotating brush either horizontally, over the tree tops, or vertically, along the sides, or both. Their spindle is ten feet long. The Wenks use the Darwin on their open-vase trees in the horizontal position only.

“I drive up one side of the row and down the other, doing the top of each tree from both sides,” Dave said. He figures he can do five acres a day on the open-vase trees planted on a 12- by 22-foot spacing.

Like fingers

The strings on the rotating spindle are not really strings, he added. They’re more like two-foot-long, soft fingers. While the early models had weed-trimmer-type plastic strings, the newest model’s strings are thicker, firmer, and an eighth-inch in diameter. The plastic bends easily but will not wrap around limbs—an important feature that greatly reduces limb damage, Dave said. These fingerlike bristles penetrate into the canopy, reaching into the fruiting zone, knocking off the blossoms.

After using the Darwin, supplemental blossom thinning is done by hand using a brush to remove blossoms. The goal is to blossom-thin everything, but especially all varieties that mature before Redhaven, following up with green fruit as needed.

Dave has monitored wear on the Darwin’s strings. After two seasons of use, covering about 100 acres a year, the strings were about four inches shorter than new strings. None had broken off; the wear occurs at the ends from brushing against the limbs, and there is no noticeable thinning of strings as they wear. The ends don’t get sharper either. He plans to change them for next spring.

Three Springs uses half the number of strings the machine will hold. Instead of six rows of strings, they use three. They move about two miles per hour rotating the spindle about 200 revolutions per minute.

The machine mounts easily on the front of a tractor, held by forks used to handle bins. The spindle can be tilted using hydraulic controls, and a digital read-out telling speed and rpms is mounted on the tractor.

“We run it, stop, assess what we’re doing—looking at how many blossoms are on the ground and how many left in the tree will probably fall off,” Dave said.

“We have a rope thinner, too,” he said, “but we don’t use it any more. It beats using nothing, but ropes wrap and damage limbs. That doesn’t happen with the ­Darwin.”

Bear Mountain

Joy Cline, farm manager at Bear Mountain, Aspers, Pennsylvania, said the biggest advantages she sees are more consistent fruit size and a labor savings of at least 40 ­percent.

She manages 1,000 acres of fruit—280 of them peaches—for owners John and Sheila Lott. The company operates a large packing operation, selling its own fruit and that of several other growers.

Bear Mountain was an early testing site for the Darwin when Penn State’s Extension people in Adams County found out about the European machine and bought one to test. The first one Joy Cline used was the vertical version that would flex somewhat but would not thin in a horizontal position.

The orchard has mostly open-vase trees, and the two newer machines work in the horizontal position. “We thin across the top,” she said. “We could do it along the sides, but there’s no need to do that.”

Because of the premium that is paid for large peaches, thinning is very important, Cline said, and she’s looking for ways to cut labor needs, because of the cost and uncertain availability in the future. The Darwin paid for itself the first year, with a combination of labor savings and improved fruit quality, she said.