For the past few years, vineyards frost has been a recurrent topic in the media. Spring frosts have always existed, but climate change and the ever earlier development of vines are a real source of stress for winemakers, and rightly so. Frost, although necessary to the vine cycle, can be destructive when it occurs at the wrong time. Many solutions exist, more or less effective depending on the type of frost.
In this article, you will find all the information you need to understand the problem of frost!
Frost in vineyards has always existed, so why is it a problem?
Frost is not a novelty in our regions, but year after year, it causes more and more damage. Why is it a problem? The end of winter and the beginning of spring have higher temperatures, which results in an earlier start of the vegetative cycle of the vine. Thus, when the frost rages in its usual periods (in April for example), the vine is more vulnerable because it is out of its protective sleep.
What is the impact of frost on grapes?
Spring frost occurs when the grapes are not yet formed. It impacts the buds and therefore compromises the formation of the grapes. When temperatures drop rapidly and reach the freezing point, ice forms inside the tissues and cells which has the effect of “burning” the bud from the inside.
When ice crystals form on the buds, the first rays of sunlight are also a danger. Ice has a magnifying effect that can also burn the bud.
The different seasons and the risk of frost
The risk of frost is not limited to one season, vines can be affected several times a year.
lAthough not very frequent, the autumn frost can jeopardize the following year’s harvest. This frost is said to be “early” and has the effect of accelerating the dormancy of the vine. This has the consequence of weakening the vine which will not have accumulated enough energy to carry out its future vegetative cycle.
The season which knows the most negative temperatures is winter. It freezes regularly in many wine-producing regions without affecting the annual harvest. The vine is protected since it is in dormancy. If there is a bud, it is protected by scales that prevent the frost from affecting it. As a general rule, in France, damage due to winter frost is not very frequent.
Winter frost can be problematic and lead to serious consequences if temperatures reach extremely low levels (below -15 degrees Celsius) in a hurry or after a mild spell. This situation can be fatal for the vine stock and thus call into question several years of harvest. The vine is then frozen in depth leading to the death of the plant. In the coldest regions of the world, it is necessary to hilling up the vines. This practice consists in bringing back the earth on the base of the stock to protect it from the cold.
The freezing in winter is necessary for the good progress of the cycle of the vine. Indeed, it allows the elimination of pests and diseases. The frozen water in the soils also helps to aerate them and facilitate the progression of the roots in the soil.
For the past few years, spring frosts have been a concern for the wine industry. To understand the problem, we must look at the vine cycle. After the winter dormancy, the softening of temperatures is the starting point for the vine to begin its vegetative cycle. After the dormancy comes the budburst. The budburst phase means that the protective scales of the bud are spreading to let the bud appear. It is from this young shoot that the leaves will emerge, then the inflorescences which will give birth to the future fruits. You will have understood that if something happens to the buds, no fruit can be harvested.
For several years, at the end of March/beginning of April, we have been recording average temperatures above 10 degrees Celsius. These mild temperatures last for several days, announcing the arrival of fine weather, which brings the vines out of their slumber. When the temperatures drop below zero again after this warm spell, at the beginning of April, the life cycle of the plants can be brutally stopped. The buds and young shoots are no longer protected and are easy prey to the redoubtable frost.
The different types of frost in the vineyards
We speak of frost as soon as the temperature drops below zero degrees Celsius. There are several types of frost, more or less damaging.
It is the advective frost that caused great damage in Burgundy, in 2021. This type of frost is linked to the passing of large masses of cold air and strong winds. The cold remains “blocked” at the bottom, towards the vines. It is very difficult to fight against this frost. The strong winds prevent the use of many antifreeze solutions, the temperatures are very cold and the destruction is often global.
This frost concerns the soil and its cooling during the night. In the middle of the day, the soil accumulates heat and loses it throughout the night. Two types of frosts can result, depending on the humidity level in the air.
White frosts cover the vines with a white coat. This frost appears when there is a high level of humidity in the air. This type of frost does not represent a vital risk for the vine since the temperatures remain close to zero. The humidity and therefore the ice that can result can even protect the buds.
Unlike white frost, black frost can be very damaging for the vine. When the air is dry and cold, it penetrates directly into the heart of the plant and can cause serious internal damages. This can lead to the necrosis of the vine and therefore to the death of the plant.
The protection methods of the vineyards against the frost
Among the means of protection against frost, there are active and passive solutions.
Active fight against frost
The active fight against frost consists in fighting it with external means.
The images of the vineyards lit by candles went around the world after the heavy frosts of 2021.
This technique consists in placing, at regular intervals, candles in buckets filled with about 6 liters of paraffin wax. Lit before sunrise, these candles can burn for several hours and warm the cold air that stagnates on the ground. Temperatures can then gain 2 to 3 degrees Celsius.
There are several disadvantages to this technique. The first one is the cost, since between 200 and 400 candles can be necessary per hectare, which represents a budget that can go up to 2500 euros per hectare and per night. The intensive use of these candles during frost episodes also releases a thick black cloud. There are now candles based on vegetable wax, but for economic reasons, paraffin is still preferred by many wine growers. This solution requires time and labor between the installation, lighting, extinguishing, replacement and removal of the candles.
To avoid the release of black smoke and obtain results similar to candles, there are wood heaters. These small braziers heat the air around the vines by burning pellets. Like candles, they must be placed at regular intervals (200 to 250 heaters per hectare).
Spraying water is one of the most effective ways to fight against frost (up to temperatures not exceeding -7°C). This technique consists of spraying the vines with water. With the negative temperatures, the water will freeze and thus form a protective shell around the vine and in particular around the buds. This may seem counter intuitive, but ice is a good way to fight against frost!
By turning into ice, water will produce energy and therefore heat. This heat will allow the buds to keep a temperature of zero degrees and thus to survive the negative temperatures.
One of the disadvantages of this method is the important consumption of water which requires to have large reserves or a source nearby. Indeed, the spraying must be done continuously until the return of positive temperatures. The installation of this system is also very expensive, the budget can go from 7 000 to 20 000€ per hectare.
During the spring frost episode of 2021, you may have seen pictures of helicopters flying over the vineyards at very low altitude. Helicopters, with the power of their blades, can stir up large amounts of air quickly. The warm air above the vines is then mixed with the cold air on the ground, which warms the vines and prevents frost.
This method is efficient but very expensive (about 200 euros per hectare and per hour). Sometimes, several winegrowers with neighboring plots of land hire the services of a pilot together to reduce costs. As for the other disadvantages, the helicopter cannot leave before daybreak for safety reasons, so the frost may have already set in. Finally, this method cannot be used against advection frost (which involves strong winds).
Just like helicopters, wind turbines allow hot and cold air to be mixed. This action warms the vines and limits the damage caused by frost. Less expensive than helicopters, they are effective when the frost is not accompanied by wind, especially for white frosts.
Jean Baptiste Duquesne, winegrower in Bordeaux (Château Cazebonne) testifies: “This year I bought windmills, which will warm the air by stirring it. I was not able to use them at the beginning of April because I had not foreseen that it would freeze so early. Last year it froze on April 8, this year it froze on April 4 … This fan is telescopic, it is powered by the hydraulic plug of the tractor. Air is heated and then sprayed by the fan. This method allows to cover about 2 hectares”.
This technique consists in running a heating cable along the trellis wires to heat the air around the shoots. The temperature of the cable can go up to 40°C, thus allowing to warm up the vines and to fight against a frost going down to -9°C.
This solution requires access to electricity, which can complicate its installation on certain plots. The installation of this system is also expensive, the budget varies from 5 euros to 12 euros per linear meter (that is to say from 15 000 to 40 000€ per hectare, excluding the electricity budget).
Covering the vines
The veiling of the vines consists in covering them to protect them from the frost, allowing to gain a few degrees compared to the outside temperature.
Jean-Baptiste Duquesne explains to us: “I would like to investigate and try to use market gardening veils. These are protective films that are used on the ground to prevent the plants from freezing. This white film keeps the heat of the soil (plus 4 degrees compared to the outside temperature). I would like to cover my vines on certain plots. But this technique has some disadvantages, the installation can be complicated, especially on large surfaces of vines like in Bordeaux. It is also important that the cover does not blow away in the wind.
As you can see from this long list, there are many methods to fight frost. As a general rule, these active control methods are quite expensive and their effectiveness is not always optimal. It may be interesting to investigate other solutions such as passive protection methods.
Passive control to anticipate vineyards frost
Passive frost protection methods makes it possible to prevent damage by taking the risk of frost into account as soon as the vines are planted. After several years of low harvests, Jean-Baptiste Duquesne is exploring other methods to preserve his yields.
“I had a few candles left over from last year’s frost episode, so I didn’t buy any this year because it’s very expensive. The first thing that works and that I investigated this year is the pruning strategy of the vineyard. I was able to save about half a crop with that,” says Jean-Baptiste.
Pruning the vines
“The principle is simple. The vine shoot is a creeper. Historically it is a plant that grows under the woods. Its objective is to climb to the canopy to survive. This vine will bring out the buds at its ends. If you leave a vine unpruned and you want to harvest on the lower part of the vine (towards the stump), the vine will start by pushing out the buds towards its extremities, and not where you want to harvest.
So you have to let the vine grow, prune the shoots high enough and wait for it to freeze. Once the two or three nights of frost have passed, we prune the part with the frozen buds and we start again lower, where we wish to harvest. The pruning is therefore done in two stages so we leave, in a way, something to eat for the frost
According to Jean-Baptiste Duquesne, this is not a perfect strategy because it tires the vine. “To illustrate, we can think of the vine as a battery. In the winter, this battery has starch reserves that allow the vine to grow and bring out the buds. Once the buds come out, the vine does not have much energy left and it recharges its batteries through photosynthesis (the leaves that come out of the buds transform the light energy into energy for the vine). At the time of the pruning, there is not yet any leaf and thus the energy in reserve is quickly exhausted. Pruning in two times, tires the vine by pumping the energy of the latter. But we have no choice, if we don’t do it, we risk losing most or all of the harvest each year”.
Planting more frost-resistant grape varieties
Jean-Baptiste Duquesne is a winemaker who has the particularity of growing forgotten Bordeaux grape varieties. He explains how these grape varieties can be a solution (among others) to accompany climate change. “In addition to active methods, we must also look in other areas, such as replanting grape varieties that can provide solutions. There used to be up to a hundred grape varieties in Bordeaux. The winemaker was like a painter with a palette of a hundred colors. With time, the wines of Bordeaux have been content to use the three basic colors. I think it’s a pity, there is a territory of new tastes, new aromas, there are earlier grape varieties, later grape varieties that will resist frost, drought, certain diseases etc. These grape varieties are not “new”. These grape varieties are not the “future” of Bordeaux, but they are a future for Bordeaux.
For example, I planted a grape variety called Jurançon Noir. Last year I lost 90 to 95% of my harvest because of the frost. The jurançon noir gave me half a harvest after the frost. In the past, the climates in the 19th century were more frosty, but grape varieties like Jurançon Noir provided solutions for the winegrowers. Over time, it was decided to prioritize certain criteria such as yield, quality, plasticity of the grape varieties, and these choices should perhaps be questioned to investigate new solutions. The climate is changing and it is changing very fast. The appellations are not moving fast enough and I want to shake things up and try to make a change. After all, why not try?
Frost in vineyards has always existed and will always exist. Multiple solutions exist and the combination of several of them can save part of the harvest. However, successive years of relentless fight against late frost episodes and increasingly early budbursts, with means that are sometimes very expensive and without real efficiency, are tiring the industry. Perhaps the time has come to break the codes and investigate other solutions to save the viticulture of tomorrow!