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Washington’s Olympic Peninsula is on the edge of being able to successfully ripen many traditional wine grape varieties. And when you combine a marginal heat unit region with unusually cool springs and summers, like the growing seasons of 2010 and 2011, the uncertainty of harvest can test even the most determined of growers.

Dr. Tom Miller, retired veterinarian, researcher, horticulturist and owner of Dungeness Bay Vineyard in Sequim, Washington, has devised a way to envelop his vines in warmth to give them an extra month of growing time and hasten maturity. After experimenting for several years with wrapping his vines in plastic to push shoot growth and advance flowering, this year he enveloped his entire vineyard.

In 2012, he harvested the white variety Madeleine Angevine at the end of September—several weeks earlier than he ever has, even compared to the abnormally warm 2009 vintage, he told Good Fruit Grower. At his small Sequim vineyard, he also grows Pinot Noir, Pinot Noir Precoce, Pinot Gris, German varietals Regent and Siegerrebe, and the French-American hybrid Leon Millot.

Miller got the idea of using plastic while attending a grape conference in Vancouver, British Columbia, Canada. During the meeting, he tasted a local Pinot Noir wine that was “tented,” a concept that piqued his interest, especially considering the wine was good.

“We’re (western Washington) right at the limit of getting sufficient heat units to grow cool-season varieties like Madeleine Angevine, Siegerrebe, and Regent,” he said. “But Pinot Noir requires an even higher number of ­growing degree-days (above 1900 growing degree-days to fully ripen) than those three. I’d always wanted to grow a little Pinot Noir to enjoy with my ­farm-raised lambs, so I decided to try this tenting thing.”

He learned the Canadians were using plastic to enclose the vines early in the season to hasten bud break, tenting over the top of the canopy and anchoring down on each side. “What the Canadians were doing—coming over the top and anchoring on the side—wouldn’t work for me because there’s too much wind here,” he said.

The tenting idea was different than growing vines under hoop tunnels, a technique that is being tried in tree fruit and wine grapes in the Midwest and East Coast for frost and weather protection.

Miller doesn’t have frost risks, just cool temperatures. His vineyard is in the Olympic Mountains rain shadow, and he annually receives 12 to 16 inches of rain.

Granted, Miller’s one-acre vineyard is tiny compared to commercial vineyards in eastern Washington that produce most of the state’s wine grapes. But his plastic-wrapped vineyard, with potential to increase the number growing degree-days, is generating interest among western Washington grape growers, and several growers are trying the technique in their own vineyards.

Pallet shrink-wrap film

Miller uses rolls of plastic shrink-wrap film—the type that pallets are wrapped in—to envelop his vines. The rolls are 30 inches wide by 6,000 feet long and sell for around $130, including delivery. He attaches the film, folded double, to 1- by 2- by 32-inch wooden strips nailed onto the sides of the vineyard’s wooden trellis posts. Deck screws are used to attach the plastic into the strips. At the top, the film is secured to the trellis wire with wooden clothespins. Both sides of the vine row are enveloped in the plastic.

He devised a tractor implement to carry the roll and dispense it down the row. A crew of three people can put up the plastic in his small vineyard in a day. It takes about the same amount of time to remove and dispose of the plastic.

Growing degree-day bonus

Miller has collected a wealth of data from enveloped and control vines, tracking temperatures inside and ­outside the plastic, growing degree-days, Brix, pH, and titratable acidity of harvested grapes.

“When the sun is out, it can reach 90°F inside the plastic,” he said, noting that temperatures inside were typically 10 to 15 degrees higher than outside on sunny days. “Even during overcast days, the vines are still warmer inside the plastic than without, averaging about 5°F higher.” Additionally, nighttime temperatures were 2° to 5°F warmer than the nontented.

Growing degree-days are a measure of heat accumulation, calculated from April 1 through October 31. The number is used as a guide when matching grape varieties to sites to ensure there are enough heat units on average in a growing season to ripen the variety. Because little shoot growth occurs at temperatures below 50°F, 50° is the baseline in determining growing degree-days. Average May and June high temperatures in Sequim are 61° and 65°F, respectively. Average May and June low ­temperatures are 43° and 48°F.

In 2011, Miller gained up to 200 growing degree-days for vines enveloped in plastic for 40 days. Enveloping for 60 days could add 500 to 700 growing degree-days to a season, he said, depending on the length of time the ­plastic was up and the variety.

Flowering dates for vines inside the plastic were from eight to 21 days earlier than for control vines; flower-­capping dates were 13 to 27 days earlier than the control.

Early harvest potential

Miller sees great potential to advance harvest by two to four weeks in a cool year when using the plastic, and in a warm year, still advance harvest and produce higher quality grapes.

Harvesting earlier could also help reduce yellow-jacket damage and avoid feeding from migratory bird flocks.

“In a bad year, you’d get something of value instead of 12 months of work for zero benefit,” he said.

In the past, Miller has sold his grapes to western Washington wineries. But he said that this year, he plans to keep his grapes to make wine for private enjoyment.