Surviving the cold
Recovering from winter damage requires planning and close management.
These grapevines await their winter pruning. Mechanically pruning vines that have severe bud damage from cold may be a cost-effective option, says Vincor's Frank Hellwig.
Reports of vine death in British Columbia, Canada, following last winter's bitter cold spells may have been greatly exaggerated. Initial reports from Vincor Canada's field staff indicated that some blocks were wiped out, prompting conservative estimates of the 2009 crop. However, while the potential for disaster existed, the truth wasn't as ugly.
Speaking to growers at the B.C. Wine Grape Council's annual viticulture and enology conference in Penticton last summer, Vincor's Frank Hellwig estimated that production from Vincor's 1,300 acres of vineyard in the Okanagan Valley would be off 20 percent this year. It's a far cry from the 70 percent hit Vincor took when cold ripped through Ontario vineyards in 2005. Hellwig, who oversees vineyard operations for Vincor Canada, said the optimistic outlook is thanks to basic site characteristics, as well as pruning, irrigation and fertilization strategies that in some cases have surprised even him with their effectiveness.
"I thought for sure that would be dead," he said, showing growers a slide of one vineyard on the Black Sage Road near Oliver that's being nurtured back to health. "It's not [dead], and it's coming back really well."
The key is helping vines prepare for cold weather, and knowing how to help them recover when damage happens.
Washington State University's Dr. Markus Keller, who preceded Hellwig during a sequence of three presentations on winter damage to vines, noted that vines need to experience temperatures of 5°C (41°F) or less in order to begin senescence. The development of cold hardiness accelerates once temperatures hit 0°C (32°F) and reaches its maximum at -5°C (23°F).
But the actual degree of cold hardiness varies throughout the winter, reflecting recent temperature trends in the vineyard, Keller said. Two or three days of warmer weather followed by a blast of cold weather could hurt vines, which had been acclimatizing to the warmer temperatures. The steadier the temperature in the vineyard, the more resilient the vines will be.
Unfortunately, many vineyards hadn't even started to enter senescence last fall when a hard frost hit in early October. Subsequent blasts of cold, arctic air in late December and January took temperatures in some vineyards to -26.8°C (-16.2°F), compounding the damage.
Vincor's properties, which are concentrated in the southern Okanagan Valley between McIntyre Bluff (just north of Oliver) and Osoyoos, saw some of the coldest temperatures on sites where cold air couldn't drain.
"Air drainage is everything," Hellwig said. "It can be -26°C, but if you're on a slope where the air's continually flowing as water does down a slope, then it seems to make a hell of a difference. If that air sits, then that's where you get the worst damage," Hellwig said.
When workers went into affected blocks this spring, the varying degrees of damage required different levels of attention. When up to 15 percent of buds are damaged, there's typically no major impact on yields, Hellwig said.
Damage of 20 to 50 percent requires growers to increase bud numbers by 30 to 50 percent in order to compensate, and even then the vine may yield 30 percent less than normal. Bud damage above 50 percent usually means damage to the vine's xylem and significant crop losses, while the crop that does appear will be low quality. Hellwig said experience in Ontario suggests that pruning the vines mechanically can help keep costs in line with the prices grapes from these vines fetch, allowing growers to maintain margins despite the damage.
Severe damage means allowing for suckers, which can facilitate the recovery of vines by providing initial growth in the wake of a damaging winter.
"Suckers are your friends, although you've been taught completely the opposite—that you've got to sucker everything, and it's got to look neat," Hellwig said. "A recovering vineyard does not look neat. It looks terrible, but you've got to get over it."
Weed management is also important because the recovering vines should have as little competition as possible for nutrients.
The same principle applies to water. Too much in the wrong place raises the risk of infection, Hellwig said, while too little can stress the vines and prompt collapse.
Keller said vines that have had cold damage would not tolerate drought stress during the following season. "They're apt to collapse," he said. "So make sure you have enough water available."
Hellwig believes these strategies have helped Vincor pare its losses in the coming year to just 20 percent from a 25 percent minimum loss expected earlier this year. Slightly more than 100 acres may have to be replanted, but many of those sites will be modified to reduce the risks posed to the new vines. "It's a long-term investment, planting a vineyard—you want to make sure you do it right," he said.