Grape growers don't have to buy pricey windmills or fancy sprinkler systems to protect their vines from winter freezes and spring frosts, says a grape-growing expert at SIUC. A little oil will do the trick.
"No, it's not snake oil," says state viticulturist Imed Dami. An assistant professor of plant, soil, and agricultural systems, Dami came to SIUC in 1999 to assist the state's grape growers through the Illinois Grape and Wine Resources Council, headquartered here.
"Soybean oils and mineral oils, applied to the canes while they're dormant, will delay bud break," he says. "With a continental climate like ours, if you can delay bud break by a week, you can significantly minimize the chance of frost damage."
Buds on a vine cane hold the season's new shoots, leaves, and the immature flower clusters that will become summer grapes.
"What happens is you will have this really beautiful weather in late February to early March that tricks the vines into thinking it's spring," Dami says. "The dormant buds begin to swell and leaf out, and then the temperature drops below freezing. That happens very frequently here in Illinois.
"When it does, you can lose between 5 and 80 percent of your crop, depending on how much has leafed out and whether you have a low-lying site, where frost damage is worst."
Fruit growers have long coated tree branches with oils during the dormant season as a way of keeping down the bug population. Along the way, they noticed that the leaves and blooms of treated trees took a little longer to emerge. Dami wondered if he could make use of that side effect to protect grapes.
The answer, published this past spring in Wine East, a trade magazine, is yes.
"I started this work in Virginia with European-type grapes and had such interesting preliminary results that I wanted to continue it here," says Dami, who reported some of his findings in 2000 at scientific meetings in Seattle and in Melbourne, Australia. His second study, underwritten by a grant from the Illinois Council on Food and Agricultural Research, involved hybrid grapes grown in southern Illinois.
Dami and Brad Beam, a master's student in plant and soil science, tested two types of oil: soybean-oil-based adjuvants (treatments used commercially to improve the effectiveness of herbicides) and mineral-based oils. After mixing the oil with water and an emulsifier, they used a backpack sprayer to spray one-year-old canes with the solution. Many adjuvants already contain an emulsifier, which would make them easier for grape growers to use.
Beam tried solutions containing 2, 4, 6, 8, and 10 percent soybean oil. He found that the 8 and 10 percent solutions delayed bud break by several days, but that the 10 percent solution might be deleterious to some grape varieties.
"There is a very small window of effectiveness," says Dami. "Eight percent seems optimal."
Although some grape varieties were a little more sensitive to oil than others, the treated buds were generally hardier than the untreated ones and leafed out later with no loss in yield or fruit quality.
"It's an easy and cost-effective method with a huge effect," Dami says.
Petroleum-based oils also work, but they should be applied at much lower rates—no more than 2 percent, he says. A rate higher than that can be harmful to the vines. High levels of emulsifiers can be harmful, too, so Dami recommends using soy-based adjuvants that have low percentages of emulsifier. With a lot of products out there, growers may need to test several.
Treatments should be applied when vines are dormant—mid-December to mid-January—when the outside temperature is at least 45°F.
More research is needed to study the differences in how different varieties of grapes respond to the treatment, says Dami. Meanwhile, he's advising growers to treat just a few vines at first and see what the results are.
—K. C. Jaehnig, Media & Communication Resources; Marilyn Davis, ed.