With juice grape prices so low, it’s hard to imagine growers would even consider spending money to thin their crop. But the practice paid off last year for some who turned their mechanical harvesters into thinning machines.

b748e33168b5c89241fdcfaf64416010.jpeWashington’s 2005 Concord grape crop will be remembered by many for a long time as “huge and long,” said Mike Concienne, senior regional manager at National Grape Cooperative, the processing arm of Welch’s grape products. The record crop, unofficially pegged at 280,000 tons, was so large that processors ran out of storage space and had to turn extra loads away.

The crop was demanding for growers, as well, he said, due to concern about the grapes reaching high enough sugar. “At National, every grape came in at sugar, but it was challenging for many growers. Mother Nature was kind to us during the fall, and we didn’t have serious frost problems.”

At the annual Washington State Grape Society meeting, three Washington State growers discussed thinning their Concord crops last summer. All agreed that although they were nervous at first, thinning the crop with mechanical harvesters was successful.

Larry Marchant, a grape grower from Prosser, said they saw they were in trouble with high yields about a month after bloom. They annually estimate crop yields in their blocks by counting and weighing clusters.

“We were projecting 15.6 tons per acre in some blocks,” Marchant said, adding that in the control block, where they did nothing, they harvested 15.3 tons per acre. “We turned the harvester loose in the field and were hoping for averages around 10.2 tons per acre. We picked 12.5 tons per acre, with sugar at 16.5 percent, and we’re happy about what we did.”

Sugar percentage in the control block was 15.1, which they blended with other blocks to bring up to 15.7.

“I’m really happy that we thinned,” he noted. “It made harvest less stressful.”

Count clusters

At Wyckoff Farms in Grandview, clusters are always annually counted, explained Albert Don, operations vineyard manager. “We saw potential for some huge crops in the 15- to 18- ton-per-acre range, with some even higher.”

Don took part in a field day held for Concord growers in early summer to demonstrate how to mechanically thin with grape harvesters.

“We concentrated our thinning on the lower third of the vertical canopy and ran the harvester at a slow ground speed,” he said, adding that the closures on the harvester were removed, and they turned off the conveyors. Tarps were laid under the rows to collect and measure the grapes being knocked off.

“We were fairly successful,” Don said, though he was nervous about thinning with a machine and not knowing if the vines would be damaged.

He noted that they thinned some Concord blocks trained to a Geneva Double Curtain by hand at a cost of $100 per acre. Thinning with the harvester cost around $25 per acre.

“Using the mechanical harvester for thinning was a very viable option,” Don said. “My goal is to maximize production on an annual basis. You need to leave some buds out there in the beginning of each year so you can adjust the crop if needed.”

Dr. Ron Grow of Grandview has been in the Concord business since 1948. He remembers well the large crop years of 1983, 1987, and 1993, and didn’t want to go through a similar harvest in 2005. After cluster count numbers showed potential for yields up to 18 tons in some blocks, Grow decided they, too, would use a mechanical harvester for thinning.

They took the conveyors off the harvesting machine, took the upper striker rods out, and ran the machine at two miles per hour, 250 revolutions per minute, targeting the grapes below the cordon wires. He reduced his yields to an average of 11.64 tons per acre at harvest, with sugar at 17.3 percent.


All three growers collected cluster weight and number statistics about 30 days postbloom, which is near the end of June. Within days of determining the estimated tonnage, the vines were then mechanically thinned.

“The timing was good at 30 days postbloom to weigh the clusters,” Don said.

Marchant added that it’s important to wait the 30 days, “but once you make the decision, time is of the essence because everything in the vine is going into the energy of making fruit.”

Don added that he noticed the smaller berries were harder to shake off than larger ones.

“We’re on a very steep learning curve in Washington when it comes to mechanical thinning,” Concienne summarized. East Coast growers have some experience with mechanical thinning, but the practice is not done in Washington. Chemical thinning with nitrogen-based materials was done years ago, with some research coming from Michigan State University, but there were problems with inconsistency and burn.

More research may be needed, he suggested, to fine-tune the timing, as it may be more related to growing degree days. Health of return wood the year after thinning is also a consideration.


“The important thing is that we definitely don’t want to get into the situation we were in the 1990s where the crop was up and down. Thinning is a tool that can help growers when we know vines will be overcropped.”

Concienne added that thinning should be thought of as a last resort, but may be useful in maximizing sustainable yields at the highest quality.