Winetech Technical Yearbook 2022

DECEMBER

THE COLD FACTS ABOUT MANAGING VINEYARD FROST

BY HARRISON DAVIES

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REPUBLISHED WITH PERMISSION FROM GRAPEGROWER AND WINEMAKER , ISSUE 702, JULY 2022.

PREPARING VINES FOR COOLER MORNING CONDITIONS The onset of climate change means different conditions than what have historically been recorded. Harrison Davies explores the increasing threat posed by frost conditions in vineyards and how producers can mitigate the impact on their vines. Everyone remembers mornings as a kid when their lawn would be coated in a thin, white layer of frost. It always seemed a magical moment when one was rugged up in a blanket with a mug of cocoa on hand, but the harsh realities of frost become clear with age. Frost can be a significant threat to

of Meteorology (BOM) as: “a deposit of soft white ice crystals or frozen dew drops on objects near the ground; formed when the surface temperature falls below freezing point”. Frost can form in two circumstances when the temperature is low enough: • The first and most common form is known as radiation frost; the type of frost that occurs when the air and ground temperatures cool due to the loss of heat in the atmosphere. It begins at ground level and then rises as the temperature persists. Most of the time this type of frost will occur under clear skies. • The second way frost can form is called ‘advection frost’ and occurs when a mass of very cold air moves over an area, replacing the warmer air, however, this rarely happens in Australia. A spokesperson for BOM said conditions for frost had to be still, dry and with clear skies. “In Australia, frost is more likely to form under a clear sky, with low humidity and light surface winds,” they said. “Clear skies favour the escape of radiation (heat) from the earth’s surface to space. Clouds reflect the outgoing radiation, slowing the cooling at the surface. When the air is more humid, internal processes relating to the change of state of water between vapour liquid-solid causes the release of heat (latent heat). This slows

vineyards if not prepared for properly and wine regions the world over have been experiencing greater periods of frost in recent years. Producers in Bordeaux, Burgundy and Champagne all tore their hair out due to frost conditions persisting longer into the year than what was normal, and many lost part of their yield due to it occurring during budburst. Frost in Australia is commonplace in cool-climate regions (refer to map) and is defined by the Bureau

down the cooling, decreasing the likelihood of frost. At night, wind can act to mix the cooler air near the surface with warmer air just above it. This slows the radiative cooling at the surface, making frost formation less likely.” FROST IN VINEYARDS Frost poses a significant threat to grapevines, especially when it occurs around the same time as budburst. Vines affected by frost can experience lowered yield and even eliminate the crop itself in extreme cases. Injury from frost occurs due to the freezing of plant cells as ice forms outside or inside the cell itself. According to the Australian Wine Research Institute’s (AWRI) fact sheet on frost; young or immature plant tissue is the most prone to damage and air temperatures of -3.4°C or less will damage 50% of vine buds at the woolly bud stage.

Frost probability in Australia. (Map: Bureau of Meteorology.)

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WINETECH TECHNICAL YEARBOOK 2022