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Below-freezing temperatures that can damage Sonoma County’s $575 million grape crop before it even develops are prompting some growers to launch an unusually early campaign to protect their vineyards from frost.

Pockets of vineyards across the county have been awakened early from their winter slumber by mild, dry weather over the past week, leaving them vulnerable to frost damage.

Most are concentrated in the Russian River Valley and Carneros wine regions, said Karissa Kruse, president of the Sonoma County Winegrowers trade group. Growers in both areas have detected signs of bud break, a fragile stage that marks the beginning of the annual growing cycle and culminates with harvest in late summer.

A little less than 5 percent of the county’s 60,000 acres of vineyards have experienced bud break, Kruse said.

As the tiny buds on the vine begin to swell, they will soon yield tender shoots that will transform the county’s brown, woody vineyards into green, leafy vistas.

To protect them, some vineyard managers brought out wind machines and overhead sprinklers as temperatures dropped to a low of 25 degrees at the Charles M. Schulz-Sonoma County Airport early Tuesday morning.

“We saw it turn on a dime and get really cold,” said John Bucher, owner of the 55-acre Bucher Vineyards in the Russian River Valley that is planted with pinot noir and chardonnay grapes. Some of his vines started bud break around last weekend, about two weeks earlier than normal.

Workers at Bucher Vineyards started turning on fans at 10 p.m. Monday and then followed with overhead sprinklers that began spraying water on the vines at 2 a.m. Tuesday, specifically targeting the valley floors of the property. Growers use fans to mix warmer air above the vines with the colder air near to the ground to stave off frost.

If not successful, they use sprinklers to provide an icy coat on the vines, which keeps the newly green tissue safe at a constant 32-degree temperature.

At 5 a.m. Tuesday, it was 24 degrees in his vineyard, Bucher said. “If we didn’t have that (fans) on, it probably would have been in the high teens,” he said.

Growers also used sprinklers over at the Huichica Hills Vineyard that straddles the border of Sonoma and Napa counties in Carneros, freezing some chardonnay and pinot noir vines.

“Everyone is still on high alert for the next week,” said Alison Crowe, director of winemaking at Plata Wine Partners, which sources fruit from the vineyard.

The chilly weather will continue for the rest of the week, driven by a cold front from British Columbia, with another freeze possibility by Friday morning, said Brian Mejia, a meteorologist for the National Weather Service. A warming trend should begin by Saturday afternoon.

The cold spell will slow down bud break across the region, pushing it closer to its traditional onset in mid-March, said Kruse.

Most growers should benefit by the delay because it will mean a shorter season to worry about frost, which in some cases can linger into late May and early June throughout parts of the region.

“The shorter we can make frost season, the better we are,” she said.

While growers cannot control Mother Nature, they can take steps to mitigate damage without resorting to fans and sprinklers. For example, some have delayed pruning, which triggers when bud break will eventually occur. Others have more actively mowed their cover crops between the vines, a practice that brings in warmer air to the buds and leaves the colder air closer to the ground, said Garrett Buckland, president of the Napa Valley Grapegrowers and owner of Premiere Viticultural Services in Napa.

Even though Napa County Airport reached 22 degrees on Tuesday, Buckland said none of his clients suffered any damage.

Bud break has only occurred in about 2 percent of Napa County vineyards, mostly in Carneros, he said.

Still, he is still concerned, following last year’s eventful $1.5 billion North Coast grape harvest that featured a massive Labor Day heat spike followed by the October wildfires.

“We could have a frost season that lasts into June,” Buckland said. “In the back of our minds, there’s always the threat.”

You can reach Staff Writer Bill Swindell at 707-521-5223 or bill.swindell@pressdemocrat.com. On Twitter @BillSwindell.

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