Conditions for frost to damage the vines

The most sensitive part of the vine is the bud which, when newly exposed, can’t survive at temperatures less than -2/-3°C. The bud is also the most important part of the vine as it from where the fruit and harvest for the new year will grow. That’s why the frosts that cause the most damage are the late spring ones, because it is at this stage that the buds have started to flourish and are more vulnerable.

The vineyards most at risk from frost are those situated at the bottom of the hillsides, in hollows, or on the valley plains, because that is where the cold air gathers. Plots that have grass between the rows are also more at risk because the vegetation holds humidity and cold. And if the vine varietal or the year is early in exposing the buds, the risk of damage from spring frosts is greater.

It’s worth noting that an early autumn frost before the leaves have fallen, or a prolonged winter freeze below -15°C can also harm the vines.

How does the vine freeze in spring?

As the temperature rises in spring, the sap starts to rise in the vine, and we can see the “vine tears” form from the wood exposed during pruning. The buds appear, burst and then the first leaves unfurl. The buds and the first leaves are rich in water.

When it freezes, this water cools so much that it destroys or burns the cells in the bud or the leaves. There are two types of frost. The white frost burns the moist vegetative matter through the sun rays, and the black frost which occurs when the temperature drops below -7°C in dry and windy conditions.

What is the impact from frost?

Damage caused by spring frosts is more frequent but less serious for the longevity of the vines compared to the autumn and winter frosts. Even if there is a direct result on the year’s harvest, they don’t cause the vine to completely die.

If the primary buds are burnt, there is always the hope that the secondary buds will appear after the frost has passed. These buds burst later, but are also fruit-bearing. Months of working and caring for the vines can be ruined in just a few hours due to frost.

Protecting the vines from frost?

The most common solution is to light candles or torches to protect the vines using smoke. The winegrowers place them every few metres in the vineyard, which is enough to rise the temperature by 2-3°C, and often avoid frost from forming. In the past they also burned wood or fuel in old oil drums, but that gave off lots of carbon dioxide, so now large blocks of paraffin are preferred. The smoke that they emit stops the early morning sun rays burning the buds that have been thinly covered in frost.

Another solution, but more costly, is to spray the vines with water. An ice cocoon forms around the buds which stops the water inside the buds from freezing. The vines are sprayed frequently until the temperature rises above freezing to avoid the ice from melting too fast, thus protecting the buds from the sun rays. This method is generally reserved for vine plots that are most at risk because the installation of the water pipes and sprinklers is very expensive.

More and more often now, you can also find wind turbines being used nowadays in the vineyards. These small turbines cause the air to circulate and the warm air to come back down and warm up the vines. A few degrees gained can often be enough to avoid the buds from freezing.

Heated wires which run along the training wires have also started to appear in the past few years. Of course, this can only be used in vineyards where the vines are trellised. Apparently this system has proved to be very effective in the plots that it has been tested in, and can be set to automatically activate below a set temperature. Running costs are fairly reasonable, but the purchase and installation costs are high given the kilometres of vine rows that have to be equipped, so explaining the small take-up for the moment.

Some winemakers even call in helicopters to fly low level over the vines and circulate the air as the turbines do. It’s fairly dangerous as the pilot must fly very low, most of the time at dawn when the light is not necessarily the best, and the pilot perhaps not the most awake!  It’s fairly expensive, but is worth the cost for some of the better quality vineyard plots.

And then sometimes, as our partner winemakers at Château Coutet reminded us, it’s simply nature that offers protection. Having dense vegetation or trees around the vineyards can help stop the temperature form dropping too far and can protect the vines from wind and the morning sun rays which can be so fatal for the frost covered buds.

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