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Be Nice to a Wine Grape Grower: It’s Frost Season!

While home gardeners can wait until the frost season has passed before planting certain crops – tomatoes, corn, peppers, for example – grape growers, whose vines are in the ground year-round, don’t have that luxury. From the moment budbreak occurs (or even as the buds begin to swell) -- usually sometime in March -- until the last frost event of the season -- usually in May although sometimes into June -- California’s wine grape growers live in a state of high alert. And on very little sleep.

“It’s a stressful time,” says Bill Pauli, who along with his wife and three sons runs Mendocino County’s Pauli Ranch, which farms 1100 acres of vines in the Ukiah, Potter and Redwood Valleys.

As with tomatoes and corn, frost can severely damage new growth on grapevines. When temperatures dip below freezing for several hours at a time -- usually in the early morning hours before dawn -- moisture is drawn out of the plant cells, rupturing them and turning the young shoots and leaves brown and/or black. When this occurs, the size of the crop is dramatically compromised. While some California vineyards, because of their geographic location  and/or orientation, rarely experience frost, most are susceptible, especially in the early months of spring.

In the last several decades wine grape growers have devised several methods for protecting their vines from frost. One such method involves the use of wind machines, essentially windmill-like contraptions that sit in the vineyard and (when activated) stir up what’s known as the “inversion” layer, mixing the colder air that tends to settle near the ground around the vine with the warmer air that hovers above the vine. This action can raise the overall temperature around the vines a crucial one to two degrees – enough to save them from the damaging effects of frost.

Another common frost protection technique -- used by growers like Pauli when a warmer inversion layer isn't in place – is the activation of overhead sprinkler systems. When a frost event is imminent, Pauli triggers permanently-installed sprinklers which spray the vines with a protective covering of relatively warmer water (the sprinklers have other uses as well, including irrigation and germinating cover crops). The amount of water Pauli uses varies according to the length and severity of the frost event (the lower the temperature, the more water is needed); the water pressure and sprinkler heads are both finely attuned to the specific needs of each vineyard block. One of the keys, according to Pauli, is to make sure that the ice which forms around the plant is continually dripping so that the plant tissue doesn’t freeze inside its icy casing.

No matter which method is used, the most important factor in frost protection is getting the timing right. And that can be complicated. Not only do farmers need to know what the weather is doing on any given night, they need to be able to predict what’s going to happen, i.e. how quickly will the temperature drop, what will the lowest temperature be, what is the humidity and predicted dew point, will it be cloudy or clear, etc.? All of these are key factors in the grower’s decision to activate frost protection measures.

Fortunately, the tools available to California’s wine grape growers are much more sophisticated than they used to be. Many vineyards – including Pauli’s – have individual weather stations which provide up-to-the-minute data including ground temperature, air temperature, wind direction and velocity, humidity, etc., as well as predictive information about what might happen in the next few hours. Some vineyards even have automated alert systems that dial the grower’s cell phone when the temperature reaches a certain critical point.

“We used to call in to the National Weather Service and they’d give you a pretty good nightly forecast that included the temperature, predicted dew point, etc.,” says Pauli. “Now we can get all that information on our cell phones.”

Of course the weather – and frost – is still driven by Mother Nature and is never entirely predictable. Growers like Pauli spend years getting to know their vineyards, learning which blocks are most susceptible to frost events and adjusting pumps and sprinklers so that just the right amount of water is released with none wasted. And in the springtime, growers are inevitably prepared to lose a lot of sleep. Even the most automated systems need to be checked on; no grower can afford to lose a crop because a wind machine or pump failed to activate.

“You’re always looking out the window,” says Pauli, who’s seen more frost seasons than he cares to count. “You go to bed with one eye open.”

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Wind machines mix the colder air with the warmer air decreasing the chance of a freeze. Photo credit: Sonoma County Vintners


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