Losing grapes to frost can be an expensive proposition — but as Napa County grower Jim Lincoln will tell you, providing frost protection when it’s not needed can cost some real money, too.
It can also waste water. That’s why he relies on phenology of his vines, along with temperature and dew point, to determine when to start and stop sprinklers and wind machines.
Lincoln manages grape production for Beckstoffer Vineyards, Rutherford, Calif. His vines are at risk of frost damage for a two-month period beginning in mid-March.
“The easiest way to save water during frost season is to prolong starting the sprinklers,” he says. “We don’t start our sprinklers until we have to.”
Young shoots with only an inch or two of green tissue are pretty resilient, he explains. Based on his experience, he figures buds in the popcorn stage are insulated against frost down to 32 degrees.
When the dew point is in the high 30s, he won’t start watering his vineyards until the temperature drops to 32 degrees. On the other hand, if the dew point is in the low to mid 30s, he’ll start the water at 34.5 degrees. “I won’t play it too close because of the evaporative cooling that occurs when you start the sprinklers.”
Delaying the start of sprinklers by just an hour or so can pay off, Lincoln notes. “When we’re applying water at the rate of 50 gallons per minute, that represents a significant savings of water.”
Rather than using a wet bulb to determine dew point, he relies on a meteorologist’s 3 p.m. forecast for the dew point and other weather factors for that night.
“Withholding water until the last minute can be a dangerous game to play,” he admits. “But, if you’re short on water, it can be the safest game to play. You want to make sure you have enough water for the last frost event of the season. Even if you’re not short of water, in this day and age, we can only do ourselves a favor by saving water.”
At the same time, Lincoln cuts the time he starts his 12 propane-powered wind machines even closer, waiting until the temperature drops to 32.5 degrees to turn then on. The fuel cost of running all the machines at once, he says, is $10 a minute.