In spring, winegrowers have only one word on their lips: frost. More and more frequent with the climatic disturbance, it is known to wreak havoc in the vineyards and to jeopardize their harvest. Last year (2021) black frosts, devastating for the buds, largely affected the vineyards of the whole of France.
What is the effect of a late frost?
The weather that is conducive to frost corresponds to a well-cooled climate at night, with negative temperatures. Spring frosts can go down to -9 degrees.
Two types of frost:
There are two types of frost. The advective frost which is a polar mass that crosses the region, it is uniform over a significant height. And, the radiative frost, a cold air, heavier, which accumulates in the bottoms of valleys, there is a temperature inversion and a possibility of stirring the air to bring down warmer air to the level of the vines.
2021, the year of the black frost
Last year, we faced a black frost. This is the passage of a polar air mass over a significant height making the air mixing inefficient. Moreover, the presence of wind limits the possible warming. The vine becomes sensitive as soon as the young leaves appear, which are rich in water. The buds just out can be burned by the cold from -2 degrees. They will turn brown and lead to the death of several parts of the plant.
What are the main solutions for wine growers to protect their vines?
To avoid the late frost and its consequences, winegrowers have developed some techniques to warm up their vines and save their harvest.
When a black frost is expected in the coming nights, the winegrower decides to install candles in his vine rows. The candles are made of kerosene and have a life span of about 8 hours, all in metal boxes. The candles will warm up the air around the vines and prevent the temperature from dropping. This technique increases the temperature by about 2.5 degrees so that the leaves do not freeze. However, the supply is complicated because when there are forecasts of frost, many winegrowers buy candles and stocks are limited. Manual lighting is often done in the middle of the night. Unfortunately, beyond -4 degrees, the candles do not protect the vines anymore. They offer a magnificent night show but they are not respectful of the environment.
The spraying of the vine
This method may be surprising, but it is very effective in periods of extreme cold.
To fight against the spring frost, the wine growers can also "cool" the vines.
This method consists in sprinkling water on the vines so that the buds are caught in an ice pack without the water they contain freezing. The bud is protected by a type of igloo. The plots must be watered until the temperature becomes positive again. The temperature of the bud is at 0 degrees while the outside temperature continues to drop. This is the most efficient and environmentally friendly technique because it does not pollute.
Flying over the vineyard: the helicopter and the wind turbine
Some estates go even further to protect their vines from spring frost and use an unusual technique. The low-flying helicopter stirs and warms the air above the vineyard. The fact that the air is stirred up by the blades brings the warmer air directly to the ground and warms up the vines instantly. The method reduces the humidity on the ground, which hits the vine during the spring frost episode. This operation is dangerous and quite expensive but this method could effectively protect a vineyard.
Other techniques have been invented such as air blowers, small windmills of 11 meters high, powered by a small engine, which, by mixing, make the air a little warmer located at 11 meters high come to "warm up" the vine stocks.
Most of the active control techniques represent a financial investment and a rather heavy work force for a result which is not always optimal and which can be very variable according to the type of frost and the temperatures reached. Each of these techniques does not provide 100% protection against frost damage.
Globally, the winegrowers will have as much work in the vineyards for less harvest. It will be necessary to wait for the flower to make a precise evaluation of the damage. Indeed, as long as the vines have not grown, the winegrowers remain in the dark. Today, it is impossible to evaluate the percentage of damage. The economic consequences for the wine industry will be quantified in the weeks and months to come.
This wave of cold and frost has affected in 2021 almost all French vineyards, both in Burgundy, Bordeaux, Mâconnais, Languedoc-Roussillon, Rhone Valley, the Loire ... Alsace and Provence seem to have been spared. A wave that can remind the one of 2016. According to the FNSEA (National Federation of Farmers' Unions), a third of French production would be lost and would result in a loss of earnings of at least 2 billion euros. Faced with this situation, the government has announced an aid of one billion to help farmers and winemakers.