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2.8.2.d What preventive measures can be taken against frost?

Indirect methods

Indirect methods applied well in advance of frost risks, sometimes as early as planting, are likely to be the most economical and effective. These measures include:

  • Plot selection: planting vines in frosty areas, especially in hollows or valleys where cold air collects and stagnates, is to be avoided. The presence of compact hedges or strips of woodland can increase the risk of frost by preventing the flow of cold air. However, surrounding vegetation can also sometimes reduce the risk of frost, depending on its position in relation to the plot. Choosing a well-drained plot or carrying out drainage work can also help reduce the risk of frost.

  • Cultivation and agronomic practices: to combat spring frosts, late budding vines can be planted and the height of the vines can be raised. Grassed plots should be mown before budburst. Late pruning can delay budburst. It is possible to gain up to a week by pruning in March instead of November. There is an old saying, "taille tôt, taille tard. Rien ne vaut la taille de Mars" ("Prune early or late. Nothing beats pruning in March"). It is also possible to delay the folding and attachment of canes which have been pruned long, the acrotonic principle then delays the opening of the base buds which will then be protected in the event of frost.

Direct methods

  • Hilling: only used to protect against winter frosts, this consists of covering the base of the vines with soil. This practice is widely used in Canadian vineyards (Ontario, Quebec), and in Central Europe and China.

  • Water spraying: this technique consists in continuously spraying the vines during the critical period, with sprinklers every 15 to 20 metres, so that the temperature of the buds and the herbaceous organs does not fall below 0°C. This is a non-polluting method but requires a lot of water (about 50 m3 per hour, per hectare). Continuous spraying creates a pocket of ice around the bud without the water in the bud tissues freezing, this is the principle of supercooling. This means that the state of a material, in this case water, remains in the liquid phase while its temperature is lower than its solidification point, in this case 0°C. The state is a metastable state, i.e. a very small disturbance can lead to the solidification of the water. This is the reason why the spraying is continuous. This system is very effective, regardless of the type of frost.

However, it is expensive and requires a degree of technical expertise. If not implemented at the right time, it can be worse than not intervening at all. It also requires the presence of a substantial water reserve near the plot. It is advisable to start spraying at -2/-1°C wet temperature. (Reminder: the wet temperature differs from the dry temperature of the air and is obtained by taking into account the humidity). The initial spray tends to evaporate (conversely, it gives off heat as it turns to ice), causing violent cooling that can destroy the buds if the starting temperature is too low. There are wet/dry temperature conversion tables that enable spraying to be started at the right time.

Water spraying method. Source: Mon-viti.com
  • Wind machines: this method consists of mixing the air to replace the layer of cold air in contact with the vines with the warmer air higher up. The equipment and the implementation are very expensive for a temperature increase from about 1 to 4°C. This often quite noisy operation can be done by helicopter or by propellers (each propeller protecting an area of about 4 ha). This method also has the indirect effect of drying out the air and thus lowering the humidity at ground level and can be combined with a heat source (candles for example) in case of advective frost.
Propellor at Château Latour
  • Burning of liquids: an underground pipe containing fuel is connected to burners with between 180 and 200 units/ha. This system is very effective against heavy frosts but presents soil pollution risks and is highly polluting to the air.
  • Burning of solids: candles can heat the air with some effectiveness at temperatures down to -4 to -5°C, they are therefore suitable for white frosts but ineffective for heavy frosts (-6°C to -9°C). They can be blocks of pure paraffin or a mixture of paraffin and combustible materials (sawdust, wood chips). They burn for between 7 and 12 hours and about 500 candles per hectare are required. Paraffin candles are labour intensive to set up and light. This protection system is very polluting with a carbon footprint of up to 15.8 tons of CO2 per hectare per year. Paraffin is a mineral fuel derived from oil production. Candles are now available from renewable and animal sources (stearin). Another possibility is heat logs, but their effectiveness is limited to mild frosts and they are also highly labour intensive.
Candles to counteract frost in Burgundy
  • Gas combustion: this heats the air by burning propane and by infrared radiation. The burners are placed between vine rows at a rate of 150/ha. This system is efficient and enables a high thermal gain (5°C) and can be used on wet frosts of -7°C and dry frost of -9°C. This system causes relatively little pollution (0.7 tons CO2 ha/year), and is easily automated and less expensive than a water spraying system. There are also propane gas burners coupled with a fan that project hot air at 360°C. These machines can be fixed or placed on a tractor and are effective for all types of frost.

  • Heating wires or cables: these heat the air near the buds within a radius of 5 to 10 cm. The installation cost is high. This system is effective down to -5°C and automatic. However, it must be adapted to the type of pruning and decreases in efficiency the further it is from the vine.

  • Alternative methods: other experimental methods include anti-freeze tarpaulins (not authorized by the INAO) and spraying of elicitor substances (PEL-101-GV) which reinforce the vines' defences against frost. This spraying must take place 6 to 12 hours before the frost, the accumulation of glucose in the leaves then lowers the freezing point. This approach requires the vines to be at the unfolded leaf stage and is therefore not suitable for buds that have just emerged, for example. The use of antifreeze bacteria is also being developed. A new technique using thermal fogging is arriving in France. This consists of creating a mist of ultra-fine droplets at a temperature of 40°C in order to reduce heat loss from the soil by creating a blanket of warm fog.

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