A SPECIAL GROWERS’ REPORT OF AG ALERT ® CALIFORNIA Trees & Vines ®
University of California experts say mowing cover crops or vegetation on vineyard floors a few days ahead of a frost reduces shading and allows the ground to absorb more radiant heat from the sun.
Growers ponder ways to curb vineyard frost damage By Vicky Boyd
“But it doesn’t matter when it’s so cold. You can’t do anything about it,” Fry said. There are two types of cold snaps: advection and radiation. Advection freezes occur when a cold air mass comes in and replaces warm air. These are associated with strong winds, no inversions and low humidity. Fortunately, Tanner said, these typically don’t occur in the state’s winegrape regions. The more common radiation frost occurs under clear night skies and calm winds, when heat absorbed during the day is lost as radiant energy into the atmosphere. As a result, temperatures drop faster near the surface, causing a temperature inversion with a layer of warmer air atop the cold air layer. After harvest, grapevines transition to dormancy during the fall. They’re in their deep- est sleep between December and February. During this period, vines can withstand temperatures below zero without damage. As the vines break dormancy in early spring, leaves begin to emerge as part of bud break. Frost damage occurs when leaf cells, which are mostly water, freeze and burst. The further leaves are past bud break, the more sensitive they are to cold. Last year, the Lodi region experienced 90-plus degree temperatures in early April, which promoted early bud break in many vineyards, Tanner said. See FROST, Page 8
Although California’s winegrape-producing regions don’t experience many spring frosts, they can be devastating when they occur. Such was the case in April 2022 when the mercury dipped below 32 degrees after vines had leafed out in the Lodi area. If growers know ahead of time, they may take steps to help minimize the effects of cold temperatures, said Justin Tanner, University of California Cooperative Extension viticulture advisor for San Joaquin, Sacramento and Stanislaus counties. “Late spring frosts occur so infrequently that it doesn’t justify active measures like wind machines,” he said. “Focus on passive avoidance measures.” By that, Tanner means vineyard site evaluation, cultivar selection, trellis systems, delaying pruning, promoting cold air drainage and vineyard floor management. He offered his insights during the 71 st Lodi Grape Day this month. Bruce Fry, who farms with his father as part of Mohr-Fry Ranches near Lodi, said even using some of those tools did little to minimize the impact of the April 12, 2022, frost. “It was just a phenomenon that this happened,” he said. “Usually, when you have a freeze like this, you have streaks of warmer air that flow around. But there was just so much cold air from the top to the bottom layer that you couldn’t do much.” Preparing the ground during bud break may raise the temperature in a vineyard a degree or two, and irrigating the ground may warm it up another half degree, he said.
February 22, 2023 Ag Alert 7
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