Winter Protection of Grapevines
Cold Hardiness in Grapevines
Grapevine cold hardiness is a dynamic process and changes throughout the dormant period. Beginning in early fall as the vine prepares itself for dormancy, the tissues begin to acclimate. This is a gradual process and, in grapevines, acclimation is in response to shorter day length and cooler temperatures. It is complex in nature and involves many factors and mechanisms. As temperatures drop to sub-freezing temperatures, the vine becomes more cold tolerant and achieves maximum cold hardiness just prior to the coldest periods experienced mid-winter, and is maintained until external temperatures begin to climb at the end of winter.
Acclimation
Cold acclimation is the process leading to the development of freezing tolerance in grapevines. Grapevines, like other woody plants, gradually acclimate to low temperatures in the autumn in three stages.
Deacclimation
Deacclimation occurs as the vines prepare to leave cold winter temperatures and begin to resume active growth as daylight periods lengthen and temperatures rise in the spring. During deacclimation, progressively warmer temperatures enable the vine to begin to have water redistribute back to proximity of the bud cells.
Assessing Cold Hardiness
Cold hardiness is the ability of a grapevine to withstand low temperatures. “Cold hardiness” is a vague and often misleading term because low temperature injury can vary depending on when the low temperatures occur (early vs. mid- or late-winter), how fast the temperature drops, what the temperatures were during the previous few days, and how long the low temperatures are sustained.
Low Temperature Exotherm
The ability of grape buds to resist injury from winter temperatures from dormancy (leaf fall) through to bud break in the next spring varies during the winter period. Bud hardiness is based on Low Temperature Exotherm (LTE), which is the temperature at which damage is likely to occur.
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