Science

Late frosts: for winegrowers, a puzzle with devastating effects

The late frost episode which has just hit French vineyards will leave its mark. In certain areas such as the south of the Gironde, where temperatures have dropped to -6 degrees two nights in a row, professionals are anticipating losses of around 80%. “Champagne, Burgundy, Bordelais, Loire were very severely affected. Some vineyards in the northern Rhône as well. It is one of the most important spring frosts since the start of the new millennium, given the number of regions affected. and temperatures that have dropped severely, ”notes Jeremy Cukierman, MW, Director of KEDGE Wine School.

Unfortunately for the winegrowers, the years go by and look the same. “Over the last six vintages, there has not been a single one without a spring frost or alert. These are not new phenomena. In 1991, for example, we had late frosts with dramatic consequences, but the frequency of this type of event has increased sharply, “notes Anastasia Rocque, engineer at the French Institute of Vine and Wine.

It must be said that winegrowers find themselves stuck between two phenomena. On the one hand, the vine wakes up earlier than before. Bud burst – the phase when the first buds appear – occurs two weeks earlier than thirty years ago. A direct consequence of global warming. “The bud break date is conditioned by the sum of the average temperatures during the winter period. However, climate change is reflected in a rise in these temperatures”, confirms Philippe Stoop, director of research and innovation at Itk, and member of the academy. French agriculture.

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But paradoxically, global warming has not put an end to late cold spells. “Scientists are still working on the explanations of this phenomenon”, explains Serge Zaka, agro-meteorologist and researcher at Itk. For example, the theory of the “Jet Stream” states that because of the increasingly smaller temperature differences between the North Pole and the equator, cold air masses descend more easily over Europe, chased by hot air rising. “It will take time to confirm this hypothesis. But scientific studies anticipate the persistence of late cold spells until 2050”, warns Serge Zaka.

“These episodes, which produce frost advective, combine more and more with the gel radiative, more localized which happens more when the sky is clear, not very cloudy and there is not too much wind. In fact, the problem is becoming more complex. Thus, areas that we thought were safe are not necessarily. In 2016 in Burgundy, we had areas affected by frost without any logic “, confirms Jeremy Cukierman.

“Let’s get on well, climate change is at work. In fact, the temperatures reached during frost phases are becoming less low,” said Serge Zaka. But that does not prevent the damage from being substantial. The more the vines grow in vegetation, the more sensitive it is to temperature. “A grapevine that is widely budded will be sensitive to around -1 degree. Those we have at the moment, less advanced, can go down to around -2 degrees”, explains Anastasia Rocque. Unfortunately, temperatures have dropped much lower in recent days.

The winegrowers have tried to limit the damage caused by frost, for example by lighting braziers all night long to warm the vines. “However, you are never sure to protect your plot 100% even if you have invested in protection systems”, assures Anastasia Rocque. “The frost cannot always be fought. If the temperature drops too much, there is nothing to be done,” confirms Jeremy Cukierman. And then, each solution has its advantages and disadvantages “.

No quick fix

The heaters scattered in the vines offer a maximum thermal gain of two degrees, on condition of placing a lot of them: more than 500 per hectare. But arranging them takes a long time and is expensive in labor. Sprinkling, which consists of watering the vines when the temperature drops to zero in order to cover them with a protective film, is more effective. But this technique requires significant water resources because it is necessary to sprinkle during the entire period of risk. “If we water too much, we can alter the structure of the soil, create erosion. So there are side effects,” warns Jeremy Cuckierman.

Another means of struggle: the stirring of the air using antifreeze towers. By pushing warmer air back to the ground, these wind turbines can prevent many fields from freezing. Problem: in France, winegrowers do not have the right to install them as they wish because of visual and noise pollution. And above all, they have proven ineffective in recent times because there is not enough hot air to blow back to the ground. They must therefore be combined with burners. The helicopters, regularly mobilized to stir the air above the vines, suffer from the same constraint. In addition, they do not fly at night for safety. However, it is there, a little before dawn, that the risk of frost is the highest.

“Today there is a real awareness of the problem of late frost among winegrowers. But there is not always an ideal solution to put in place to solve the problem”, summarizes Anastasia Rocque. There is a lot of research carried out on anti-freeze bacteria, but it is not yet sufficiently advanced “, specifies Jeremy Cuckierman. In fact, some estates take out insurance, others rely on past wine stocks to cross the crisis. Still others do not have the means to react and pray that the frost will spare them. One thing is certain: “for the winegrowers, these episodes have become more stressful than the harvest”, assures Anastasia Rocque. half a degree, they can lose everything or save part of their production, and in the space of an hour, everything can change.


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