Just before dawn on a clear May morning in Joching, Austria, acrid smoke begins to rise. About 30 people walk through the vineyards and set fire to damp heaps of straw and foliage that were prepared the previous night.
In the southern English county of Sussex, two men are out in the overnight hours in Ridgeview’s vineyard. They light hundreds of “candle” heaters between the vine rows that give off heat and bathe the landscape in an eerie, beautiful light.
A few months earlier in Central Otago, New Zealand, helicopters circled above the Akitu vineyards as sprinklers began to bathe the vines.
These differing techniques share the same aim: to save the tender new shoots on the vines from deadly spring frost.
Spring frosts can be devastating. During winter, vines are dormant. They are frost hardy and can easily survive temperatures from 14°F to as low as 5°F. However, the situation changes dramatically once buds have sent out their first green shoots.
Buds and shoots contain water, and when they freeze, it bursts their delicate cell walls. Even if it’s warmer, a few critically cold minutes can destroy an entire crop. Some plants can recover, but vines can’t. Their secondary shoots are never as fruitful. Wine growers in frost-prone areas have learned to counteract this, but they need to know the kind of frost they fight.
Frost Basics: Advection versus Radiation
Not all frost is the same. Meteorologists distinguish between radiation frost and advection frost. We’re all familiar with advection frost, which occurs when temperatures drop below freezing in winter. Radiation frost, however, can occur on clear nights when advection frosts have long passed.
Cold air, heavier than rising warm air, flows downward like water. The ground radiates warm air. On cold nights, especially those without cloud cover, that warmth can rise up as the cold air pushes down, which creates a so-called inversion layer. Lower-lying parts of vineyards can become dangerous frost pockets. The early morning hours are most critical, as temperatures can sink low enough to freeze new shoots.
Fighting Vineyard Frost Traditionally: Smoke, Fires and Smudge Pots
“The main frost-fighting methods rely on either producing heat or recirculating warm air,” says Brad Greatrix of Nyetimber, a sparkling wine producer in England. “But as you can imagine, the wind that accompanies an advection frost negates all of them. One is almost defenseless.”
Advection frosts are rare once buds have burst in spring. To fight frost, it’s all about understanding radiation frosts and the inversion layer.
Dr Herwig Jamek, of Weingut Jamek in the Wachau, was one of those men out there in the dawn light in Austria. With a weather forecast that showed radiation frost a distinct possibility, the village’s growers got together to lay out straw and dead leaves in lower-lying vineyards. Just before temperatures dropped to a critical level at sunrise, straw fires were started that created vast plumes of smoke.
“During a radiation frost, that smoke acts like artificial cloud cover,” says Jamek. The smoke doesn’t allow the warmth from the ground to rise, he says, nor can the freezing air sink down and damage the vines. The fires create just enough smoke to cover that critical period.
At Ridgeview in England, Marketing and Communiucations Director Mardi Roberts says, “We lit our candles [fuelled heaters] for eight nights in total in 2017, with two people on duty. Working nightshifts, they light around 750 candles each night. It made a huge difference, and we believe that we probably saved 50 percent of our crop.”
The candles generate enough heat to stave off the frosty air and also create protective smoke. They are slightly more sophisticated versions of so-called “smudge pots,” diesel-fueled orchard heaters once common in vineyards and citrus orchards. Today, biofuel candles are available.
Advanced Vineyard Frost Fighting: Fans, Choppers and Sprinklers
While frost is associated with freak weather in many European regions, in Central Otago, on New Zealand’s South Island, frost is a recurring problem in some vineyards. Mobile or permanent frost fans are a common sight. Their propellers mix up the inversion layer. Helicopters can do this, too.
“We are vulnerable to both spring and autumn frosts here,” says Andrew Donaldson, owner of Akitu in Central Otago. “We’ve always used choppers for frost fighting. Wind fans are problematic, because we are in an area of ‘outstanding natural beauty,’ and we’d need consent.”
“The chopper pilot finds the inversion layer and hovers around just under it, forcing warm air to circulate back down to the ground, keeping the frost at bay. We have temperature sensors in each vineyard block [that] change color. The pilot watches these sensors and moves across the [30-acre] vineyard. It’s very effective for us.”
Donaldson needs the helicopters “an hour or two before sunrise,” and he only calls on them when his main strategy needs support.
“In spring, our primary defense is water sprinklers,” says Donaldson. “If a spring frost is particularly potent, we’ll bring a chopper in on standby overnight and fly, if need be.”
These sprinklers exploit the latent heat developed through freezing. Water is sprayed over the vines, which freezes in a clear film around the newly formed shoots. In the frosty air, this change from liquid to solid releases heat and protects the shoot. Encrusted in ice, the shoot is safe.
“The sprinklers need to go on before frosts set in, as the first period of adding very cold water to a frost that’s already established is detrimental,” says Donaldson. Accurate weather forecasts and precise timing are crucial for all frost fighting.
Protecting Vines from Advection Frost
In some wine regions, winters get cold enough to damage even dormant vines. At Hinterland Wine Company, in Prince Edward County in Ontario, Canada, vines are covered in soil each winter.
“We hill up the vines, because it provides an insulating layer to protect against winter-chill damage,” says Hinterland partner and president Vicki Samaras. “It isn’t unusual for our winter lows to drop to minus-22°F, and even colder. Even if it is to protect the vines for just one day, it’s worth it.
“After harvest, we select our most suitable fruiting canes for the following year’s vintage and tie them to a trellis wire close to the ground,” she says. “After cultivating the soils, we use a custom-made V-plow to layer soil on top of the canes, providing an insulating layer. In April, if the soils are dry enough, we slowly begin to remove the soil and expose the canes.”
Again, timing is everything. “The spring unearthing poses several risks,” she says. “You need to wait long enough to pass the risk of late-spring frost, but not too long, because you don’t want to lose those swelling fruiting buds while unearthing the canes.”
It’s a labor-intensive process and adds mechanical stress to the vines. Samaras is experimenting with geotextiles in her Riesling vineyards this winter that should provide similar insulation.