Handbook for Irrigation of Wine Grapes in South Africa

Chapter 9

9.4.4 OTHER METHODS Smoke can be generated by burning oil, old rubber tyres or wood to create “clouds” above vineyards. However, the smoke particles are too small to prevent outgoing longwave heat irradiation (Perry, 1994). In fact, this practice may only add some heat to the environment and vineyards. Another problem is that smoke may delay warming in the morning since the smoke particles are big enough to prevent incoming shortwave solar radiation. Furthermore, generating smoke can be labour intensive, environmentally unfriendly and difficult to control, particularly in windy conditions. Man-made fog holds promise, but similar to smoke, the size of artificial fog particles is generally too small to reduce longwave heat irradiation (Perry, 1994). It is also difficult to apply and control man-made fog if the wind blows. Apparently, using heaters to warm the air in and around crops works well (Perry, 1994). In addition to convective mixing, heat radiated from the metal stack will reach any solid object if there are no obstructions. Consequently, heaters may provide some protection against windborne freezes. Unfortunately, this practice is becoming more expensive due to the increasing cost of fuel. Wind machines, i.e. large power driven fans, are effective to protect vineyards against cold damage, particularly over short distances. However, if the altitude of the layers of warm air is too high, the fans cannot mix the air effectively. Similar to heaters, wind machines are expensive, but require less energy. Wind machines, combined with heaters, provide a more energy effective means of protection against cold damage (Perry, 1994). Windbreaks may reduce wind speeds and conduct cold air away from vineyards, thereby decreasing the risk of cold damage (Zabadal & Andresen, 2004). However, dense windbreaks could also trap cold air within vineyards, which will increase the risk of cold damage (Fig. 9.11). Covering the soil surface by means of organic mulches or foam could reduce heat losses during the night, but are too expensive to apply over large areas (Fig. 9.12). A full surface mulch will probably be more efficient, but will increase the cost. Depending on the durability of the organic material, mulches may decay or weather rather rapidly and need to be renewed almost annually (Moffat, 2017). Overhead plastic or glass covers are also efficient, but too expensive to install extensively.

FIGURE 9.11. Cold air flowing downslope may collect in frost “pockets” if obstructed by windbreaks, thereby increasing the risk of cold damage in adjacent vineyards.

IRRIGATION OF WINE GRAPES 297