(file photo, TJ Mullinax/Good Fruit Grower)

(file photo, TJ Mullinax/Good Fruit Grower)

Tight water supplies in California are driving research that promises to give wine grape growers a better understanding of the factors contributing to frost ­damage, and alternative approaches to reducing the risk of damage.

Mark Battany, an advisor to growers in San Luis Obispo and Santa Barbara counties with University of California Cooperative Extension, presented the results of recent research to growers attending the annual B.C. Wine Grape Council meeting in Penticton, British Columbia, Canada, this summer.

Battany discussed work regarding the strength of temperature inversions, the performance of wind machines as frost-control tools, and the importance of trellising in ­preventing frost damage.

Speaking afterwards, Battany explained that showering the vines with water to create an insulating coat of ice has been a simple, but increasingly inefficient way of protecting California vineyards from winter damage.

“Water was relatively available and cheap, so sprinkler frost protection was the default tool that they would use,” he said. “But now we’re starting to get a lot more competition for that water, or the water’s simply not available. So, growers really have to look at what other options they can use.”

In order to make the right choices, growers need to understand a site’s specific climate, how vines respond to it, and the management tools that will improve vine ­performance, Battany said.

“[We’re] helping to understand what the weather conditions are like during frost events, and those weather conditions will then tell growers how they need to respond to frost.”

His goal is to help people understand that the height of the vines has an impact on what the vines will experience during cold weather.

Site

Wise site selection is the basic way for growers to manage climate risks such as the potential for frost damage. Robert Evans, recently retired from the USDA-ARS Northern Plains laboratory in Sidney, Montana, told Good Fruit Grower earlier this year that site selection was “the most effective passive risk avoidance strategy.”

A site with good air drainage allows cold air to flow out and prevents the inversions that occur when cold air pools, pushing warmer air (which rises) to higher altitudes. Those altitudes can be significant.

“The size of the potential cold air pond will likely be four to five times greater than the height of a physical obstruction,” Evans said.

Sometimes, however, a particular site has many desirable traits, and growers want to find a way to work with local conditions to make use of the parcel. This is where trellis height can play a role, ­Battany said.
Since cold air pools closer to the ground, a vineyard might be designed to raise the canopy to a level that protects sensitive tissues from frost damage in winter.

“Knowing that, maybe we can design vineyards from the beginning to place the frost-tender tissues up higher to avoid frost damage in the first place,” he said.

On the other hand, in some cool-­climate vineyards, those same tissues may need to be close to the ground to garner the maximum benefit from the reflective power of the earth in summer.

He showed British Columbia growers a slide from Argentina where very short vines are trellised close to the ground to maximize exposure to heat units.

“[Those growers] feel that in order to get the fruit to ripen, they need to plant the vines very low to the ground so that fruit and those vines are enjoying the warmest air possible,” Battany said. “Of course now, that then predisposes those vines to spring frost damage, so in that vineyard they have sprinklers for frost protection.”

But with water supplies under pressure in California, alternatives are needed. Wind machines aren’t always the answer, however, because temperature inversions are sometimes too weak or nonexistent because of factors such as wind that prevent ­inversions from forming.

“If growers are going to make the shift, or if we’re trying to encourage growers to make the shift, they need to have some data that yes, wind machines are going to work for them,” Battany said. Using 35-foot towers that track air temperatures at five-foot intervals above the vineyard floor, researchers have been seeking to understand the conditions under which wind machines will be most effective.

The machines work by drawing down warmer air, dispersing the cold air surrounding the vines. Research has found that they’re most effective when there’s at least a two-degree difference between the air temperatures at 35 feet above the vineyard floor and 5 feet.

“The presence of that temperature inversion is going to be a key factor in permitting a wind machine to provide a good degree of protection,” Battany said. “Otherwise, a wind machine may not be a good investment for that area.”

Growers with sites where wind machines tend not to be effective, and where water is in short supply, might need to consider adjusting trellising or replanting the vineyard to varieties more suitable for local conditions.

“We [might] need to rethink some of the varieties that we’re trying to make ripen in certain areas,” Battany said. “If we have to train vines very low to the ground in order to get them to ripen, that might tell us that variety isn’t the most suitable for that climate.”

Many growers in southern British Columbia faced just that kind of choice when a hard frost hit in early October 2008. Many vines still had fruit on them and hadn’t yet entered senescence. An arctic front in late December then dumped snow and extreme low temperatures on the vineyards; temperatures in one of Constellation Brands’s blocks near Oliver recorded a temperature of -26.8 degrees Celsius (-16.2 degrees Fahrenheit).

Air drainage

Ironically, it was in vineyards further north, where temperatures were warmer but site conditions allowed cold air to pool, that damage was the worst.

“Air drainage is everything,” Frank Hellwig, then national director of vineyard operations for Constellation, told growers attending the grape council ­conference the following summer.

“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 a level site with good air drainage, as well as appropriate vine management, can significantly reduce the risk of damage regardless of variety.

But for Battany, if trellising for specific varieties increases risks, then growers need to think twice.

“Why not plant a shorter-season variety? Then it might ripen perfectly adequately when trained to a taller height, and then you avoid the whole frost-risk problem or at least reduce the risk,” he said.
Battany’s research is ongoing, and with advances in data loggers and the analysis of the information they gather, he expects future seasons to yield additional information that will help growers fine-tune vineyard management.

“Some of the electronic measurement devices—the little data loggers—are really improving, greatly increasing what we can measure,” he said. “We’ll have so much more information to base our decisions on, and in real time.” •