FARGO -- The record-shredding warm weather in North Dakota could be trouble for grape growers in the state this year, officials said.
Lorna Bradbury, a horticulture specialist with North Dakota State University's Williston Research Extension Center, said grape production could suffer in 2012 if early-budding vines start to leaf out and are hit with a killing frost.
"This is really early," Bradbury said of the warm weather. "Lots of times, grapes will start breaking bud in mid-April, and you hope you don't get that late spring frost." Oftentimes, a spring frost isn't that bad, or the plants aren't susceptible, if it's been a cold spring and the vines haven't broken dormancy. But if the leaves are out, then it's trouble.
"The leaves are very susceptible," she said. "Sometimes you get through and sometimes you don't. That's one of the problems of growing grapes in the northern climate."
Grape development, as with other crops, is predicted based on growing degree-days (GDDs). There is a formula for each crop based on adding the nighttime low temperature to the daytime high temperature. For grapes, if the sum is more than 100, then anything over that is divided by 2. For example, if the minimum of 40 is added to a maximum of 68, the total is 108. The GDD for that day is 8 divided by 2, or 4 GDDs.
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"The first warm day that I had was March 10," Bradbury said. "We've had five days of that since then, so I've accumulated 22.5 GDDs for grapes at Williston."
There are at least 15 operations in the state that produce grapes for a commercial market. Most grow a half acre to six acres. The industry has been working with the state Legislature to improve marketing prospects for the grape industry.
Frost damage cost
Much is at stake for a frost-kill. One rule of thumb is that grapes can yield up to 3.5 tons of fruit per acre. At that yield, one acre could produce 525 gallons of wine. One gallon produces five bottles. At $11 per bottle for an average retail price, the gross revenue associated with an acre would be $31,500. Of course, the cost of making wine varies greatly, and not every producer calculates they are making money.
The Williston research station plots have 20 grape varietals under testing for winter hardiness and other characteristics. "I'd been working with the NDSU horticulture department on other projects and suggested they bring some (grapes) out here, as a good test for survivability. We can get cold, but also can have high winds and dry weather here."
"Most of the grapes that are bred for this area are bred to withstand the really cold winters," she said. "Once they come out of dormancy and start to put out leaves in the spring, they're just as frost-susceptible as anything else."
Lost to frost
One Minnesota grape grower who comments on the North Dakota Grape Growers Association listserv reported they lost 90 percent of their grape production in 2010 because of a Mother's Day frost. A Montana grower lost 80 percent in an untimely spring frost.
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Some producers on the organization's email listserv have been talking about ways to slow down the budding, or otherwise reduce frost damage. Some are thinking about spraying with a vegetable oil, or even sugar water. "People are saying that even if you did all of these things, we're so early that we're likely to get more cold weather," Bradbury said. "We had snow last year on April 30. We've all been on edge all winter because we keep thinking about last winter, last spring. Those of us who have lived here all our lives know anything can happen."
If a "first bud" is killed in the vineyard, the vines can go on to a second or even a third bud, but it will likely cut the yield. "It takes more energy to produce more buds," she said. Severe frost after leafing can result in more than usual vine death.
Northern grape growing is so new that it isn't clear what will happen with the crop, she said. "Our crop is 30 or 40 years (old) while the grapes grown in California are from European varietals that are hundreds of years in America."
