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Grape harvest decisions ahead of frost or freeze

Author: Annie Klodd, Extension Educator - Fruit and Vegetable Production.

Some areas of Minnesota are forecast to experience a frost or freeze tonight. This leaves grape growers debating whether to harvest today or leave the fruit on the vines. The goal of this article is to provide growers with information that they may use to help make their decision.

University of Minnesota Extension is not responsible for an individual farm's decision - this information is for educational purposes only.

A conservative estimate from Jorgensen et al. (1996) suggests that frost damage can be expected when temperatures dip below 31 degrees F.We do not have definitive data showing whether the berries of MN-bred grape varieties can withstand lower temperatures than other varieties. Therefore I am using this general, conservative threshold.

There are two different types of frosts, advection and radiation. Just because a frost is predicted for a certain region of the state, that does not necessarily mean that the frost will impact each vineyard the same. A vineyard in a low-lying "frost pocket" has a higher risk of frost than others. 

Frost and freeze cause leaves to fall prematurely, which reduces the flow of nutrients and sugars into the grapes. Grapes do not ripen much more after the vines are defoliated (i.e. once the leaves are frozen off the vines). Brix may increase slightly after a freeze or frost, but this is most likely due to desiccation (drying out) of the berries.

Grapes deteriorate quickly after they freeze and re-thaw: If grapes freeze, the skin of the grapes can become damaged. Once the temperature goes back up to the 50s and 60s, the damaged skin is susceptible to infection by fruit rot pathogens like botrytis grey mold. 

If the grapes' brix level is slightly target when harvested, the issue can be corrected during the winemaking process by adding sugar. Low pH or high acid can be corrected by adding water. This is not cheating!


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