Photo by JP Tremblay
Chantale Gagne of Arterra Wines
Chantale Gagne of Arterra Wines is busy collecting Chardonnay cuttings at the Thomas Ranch Vineyard in Okanagan Falls, for the grape bud hardiness project.
When it is 40’C in an Osoyoos vineyard at the beginning of August, “cool climate viticulture” doesn’t seem to be an accurate description, but the winter temperatures in BC grape regions tell a different story.
When thermometers across the Okanagan dropped below -20C this February, it was the third winter in a row that temperatures were low enough to cause bud damage to grape vines. Luckily, Summerland researchers have your back.
The grape bud hardiness index run by Carl Bogdanoff at AAFC Summerland gives grape growers an indication of how a cold spell will affect their vines. Every two weeks, AAFC staff, as well as technicians from Sebastian Farms (Mission Hill) and Arterra Wines collect cane samples of the most popular varieties at select sites across the Okanagan valley.
Grape buds are carefully cut from the canes, placed on temperature monitoring trays and gradually cooled in specialized freezers. As the moisture in the bud freezes, it gives off a spike of energy, and that temperature is recorded as the “lethal temperature exotherm 50” (LTE50), the temperature at which 50% of the buds sampled have frozen. AAFC graphs the results for 17 cultivars from 13 different Okanagan sites and distributes the data to the industry.
“There are a number of reasons why we do this work,” says Bogdanoff. “The index gives growers an indication of the risk of freezing damage that cold temperatures will have on their vines, and that in turn dictates the sort of mitigation they will need to take in their vineyard.”
Growers that have wind machines will know at what temperature to turn them on during a freezing spell, or if they are automatic, what temperature to set them for. “The machines are expensive to run, so they need good temperature data,” Bogdanoff points out.
If temperatures at a vineyard site are in the range of those shown on the index, then growers will have an idea how much bud or trunk tissue damage they might expect and adjust their pruning accordingly.
A pruner’s job is to leave just enough buds to produce a balanced crop, and that can depend on knowing how winter
temperatures might have affected buds. Troy Osborne, Director of Viticulture West, at Arterra Wines, leads the BC Grapegrowers Association’s annual pruning workshops, and suggests pruning be done as late in the winter as possible.
“You need to be able to inspect your buds for damage, and this will give you an idea of how many to leave,” Osborne says. “If you prune too heavily and there was winter damage, there may not be enough fruiting buds and you will have a light crop. If you leave too many buds, you will be summer pruning to cut back on vigour, or you will have a large crop of lesser quality.” Either way, he says, there is a risk.
If it has been cold enough (around -25C, Bogdanoff says) growers also need to check for trunk damage and cut vine trunks back to below the damage, to help vines give out new shoots in the spring.
Grape vines acclimate progressively through the winter starting in October and are usually at their hardiest in the dead of January-February, so the timing of a cold snap is an important factor. This last October, temperatures dropped to -10’C across the Okanagan, over the Oct. 23-26 weekend. Most of the grape crop was in, and a bit of frost will not usually affect grape quality, but growers were initially worried about vine damage. Bogdanoff says vines gain hardiness very quickly in October and the 24th was late enough that there should not be any long-term effects. “I measured grape bud hardiness throughout the valley that week and the buds were hardy to about -16C,” Bogdanoff says.
But that was not the case at Thanksgiving in 2009, when temperatures of -10C resulted in widespread bud damage across the valley. “October 12 was too early and the vines had not yet had time to acclimatize,” Bogdanoff says.
2008 and 2009 were particularly tough on grape vines in BC, with December and January temperatures dropping to -25’C. “We got off pretty easy in the early 2000’s, and growers had gotten complacent.” Bogdanoff recalls. “But -25 is a big deal.” He says growers had to pull out blocks of dead vines the summer of 2009 and then suffered the Thanksgiving bud damage. “That is when we decided that growers needed to know about hardiness and we started this work.”
Given the Okanagan’s differences in micro-climates, Bogdanoff says that it is extremely important for growers to know their own vineyards. “One of the first things I noticed when we really started looking at winter temperatures and grape vine hardiness, was the huge variations in temperature up and down this valley.” he says. If an artic front settles in and there is no wind, Bogdanoff says there can be a 10’C difference between vineyards even half a kilometer apart. “One vineyard is going to have no winter damage and the one next door is going to be practically wiped out,” he explains. “You have to know the cold spots on your own site. That’s where you need to check on things.”
