2.8.2.d What preventive measures can be taken against 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.

• 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.

• 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.

• 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.