When your wine grapes get frosty…

What does it mean for your wine?

 


One only has to look to Burgundy this spring to understand the ramifications of frost damage in vineyards. According to Chris Mercer of Decanter Magazine, it is being called “the worst frost for the region in the past 30 years”. Frost is among wine growers’ worst nightmares, the literal overnight death of thousands of fragile, baby shoots which can decimate the vintage. Frost is a common risk to grape growing that occurs worldwide in the spring and late fall. However, there are many different ways to mitigate the risk as well as protect the vines during an event.
There are two different types of frost events; advective frosts and radiation frosts. Typically the type of frost that affects grapevines are radiation frosts which occur due to an inversion layer in the atmosphere where cold air sinks and warm air rises. These generally occur in the spring and fall when the ground temperature is warmer than the air above, the wind is calm, and the nights are clear. These three conditions allow the warm air given off by the earth to rise above the sinking cold air mass which creates the inversion layer. Cold air is denser than warm air so the cold air sinks to the lowest possible area. This is why mild frosts can only impact a few vines in lower areas of the vineyard and not touch others. The physiology of frost damage is interesting because it is not the ice itself that causes damage to the vines but the cold internal temperature of the cells that causes cell membranes to rupture.

This can be particularly harmful in the early stages of growth where the shoots are small and developing. This can lead to stunted shoot growth or loss of apical dominance leading to multiple shoots on the same bud which can lead to a crowded canopy increasing the risk for fungal infections. Developing flower clusters can also be affected which directly results in a loss of crop potential. This was the case in 2011 on the Central Coast of California where up to 50% of the crop was lost during a particularly bad spring frost in mid-May of that year.  

End of season frosts can also be problematic for growers. These hard frosts may occur in the fall before the vineyard has had enough time to fully ripen to the desired level. When killing frosts occur at this time of the year, growers must move quickly to harvest the fruit. This is due to the vines ability to load Potassium into the fruit after severe damage to the leaves which results in a higher pH from to the buffering capacity of Potassium. This imbalance in the chemistry of the wine can cause problems for winemakers since the total acidity (TA) is not affected. This makes adjusting the pH of frost affected fruit very challenging for winemakers without affecting the mouthfeel and acid perception of the resulting wine.  

Not all hope is lost however. There are several ways a grower can reduce the risk of frost. The largest impact on frost formation is the site selection itself. If growing in a cool to moderate climate with a known history of frosts during the vine growing season, growers should try to choose a site which is uniformly graded and free of dips and swales which can be frost pockets during the frost season. A slight to moderate slope is advantageous to drain off the cold air away from the vine area. Growers should make sure to allow room in the surrounding vegetation to let cold air continue to drain away and not become trapped with low lying brush or thick tree lines near the vineyard. The Mosel vineyards of Dr. Loosen are quite dramatically sloped, some as steep as 50% or more and this aids in the drainage of the frost forming cold air down to the river level and away from the vines.  

Once the vineyard has been established and it has been determined that a frost risk is present there are a few ways that the vineyard can be protected. One of these involve physically protecting the vines through overhead sprinklers. Sprinklers work because the process of water freezing gives off heat. If one thinks back to their high school chemistry class one may recall a concept called the latent heat of freezing. This is when the energy possessed by the liquid water is lost during the process of freezing which in turn protects the tissue the water has frozen around. This method does use quite a bit of water to adequately protect the vines. This may prove prohibitory in areas such as Australia or California where water has become a scarce resource.    

Other methods of frost protection involve disrupting the air layers through air movement or environmental heating. Wind or air drain machines are also very popular ways to disrupt the inversion layer but they are expensive to purchase and may only be cost effective in areas of very high value fruit or for larger growers since one fan can provide protection for 5 acres of grapes. Environmental heating is another way growers can reduce their frost risk through the use of smudge pots or tractor mounted heaters. These devices also break up the inversion layer by creating additional heat to move up through the cold air and warm the air around the vines.  

Frost can be a very costly issue for grape growers however, through careful site planning and the implementation of appropriate protection measures these risks can be mitigated. Unfortunately nothing can completely eliminate the risk of frost but the techniques above can reduce that risk to some extent, keeping the crop whole and undamaged for wine drinkers to enjoy after harvest.

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