Types of frost event
To ascertain ways of managing frost, a review of the types of frost events is required. The type with the fewest options to manage is the movement of below-freezing air into an area, or advection frost (Cornford, 1938). In areas with surrounding mountains or hills covered with snow, the freezing air moves down from the force of gravity (cold air is heavier than warm air) in a katabatic flow, enveloping the lower areas.
A second type of frost is a radiation frost, which occurs on clear and windless nights as heat from the surface of objects is lost to the atmosphere (see Plate 32). If there is little or no mixing of air, the heat lost from the surface collects such that a layer of warmer air (the inversion layer) develops above.
Since, in this case, there is warmer air above the freezing air, a strategy for reducing the freezing temperatures near ground level is to mix the two layers.
Passive control strategies
The best way to deal with frosts is to avoid them through site selection, which has been dealt with in an earlier chapter. Cultivar choice can also have an effect. Late budbursting cultivars such as ‘Riesling’, ‘Muscadelle’ and ‘Dolcetto’ are a better choice in a frost-prone area than early-budding cultivars like ‘Chardonnay’ and
‘Pinot noir’.
Given that the coldest air is closest to the ground, a simple solution is to increase the height of the buds from the ground. Data from New Zealand suggests that a cane/cordon height of 2 m (for example, as may be found in a high GDC system) could provide an extra 4°C of warmth than the fruiting buds on a vine trained to typical VSP, at 90 cm from the ground (Trought et al., 1999).
Active control strategies
The type of freeze event determines which control strategies are most useful.
Radiation frosts have more options when it comes to preventing damage. Many take advantage of the inversion layer that develops over the vineyard by mixing the layers, for example by the use of helicopters. These have the advantage of not requiring a huge capital investment, but the disadvantage of being relatively expensive to run when they are needed. Therefore, these are best used in areas where frost is not a regular concern.
Air can also be mixed with engine-powered fans mounted high on a support pole (see Fig. 6.31). These are fixed high enough to draw down the warmer air from a typical inversion layer and mix it with the colder air.
Other innovations for mixing air include (i) portable, ground-based fans that blow cold air at ground level up into the inversion layer, resulting in
mixing (see Fig. 6.32) (Augsburger, 2000; Guarga et al., 2003); (ii) tractor- mounted fans (heated or unheated); and (iii) forcing air through the drip irrigation system, which causes it to heat as well as cause air to move through the vineyard (Annabell, 2001).
Other methods are available that add heat to the vineyard environment which are useful for both radiation and advective frosts. Burning straw bales is one such technique, or the use of fuel burners, also known as smudge pots, in vineyards (see Plate 33). However, for a number of reasons these are no longer favoured. The method by which they work is sound, however. Aside from providing radiant heat, they also create a mixing of the inversion layer as the hot air rises, meets the upper layer, cools and then settles back to the earth, effectively circulating the air. Point sources of heat, if too strong, can cause such an upward rush of hot air that it travels right through the inversion layer (Troughtet al., 1999), causing complete loss of the warmer air, with potentially catastrophic results back on the ground.
It may not seem intuitive, but the addition of water to the vineyard system can also add heat in a useful way. The most effective systems apply water over the vines, adding heat (the heat of fusion) as the water freezes. As long as there is liquid water freezing around the plant tissues, their temperature will not fall below ⫺0.6°C (Evans, 2000), which even green grapevine tissues can tolerate.
The result of this can be spectacular (see Plate 34). Application of water should continue until the ice is melting because, if it is not, there is no heat of fusion, and the temperature of the ice and vine tissues can drop precipitously (Evans, 2000).
Fig. 6.31. Two frost fans are visible in this picture of a pre-budbreak vineyard. The snowy mountains in the distance are a source of cold air and potentially frosty conditions.
Water can be applied over the whole vineyard area, or be targeted towards the vines. Micro-sprinklers (see Plate 35) limit the amount of water used compared with that of impact sprinklers; alternatively, in-row sprinklers can be used, which saves further on water consumption (Trought et al., 1999).
Use of the methods mentioned so far is fairly simple in concept, but complex in execution. Those interested in learning more about these frost protection systems are referred to Rieger (1989), Snyder et al. (1992), Trought et al. (1999), Evans (2000) and Poling (2007).
Other methods
There are a number of other methods of note for decreasing the occurrence of freezing temperatures. Already mentioned in this book are techniques such as keeping cover crop height to a minimum and practising delayed pruning.
Another makes best use of the heat that the soil absorbs during the day by maximizing the amount that is released at night. To do this the soil must be free of weeds or other plants and also be moist, as this darkens the soil and also increases its heat-carrying capacity. The soil surface was traditionally packed, but at least one study has found that cultivated soil releases slightly more heat at night (see Fig. 6.33; Creasy, 2004).
Fig. 6.32. A forced cold air system called Selective Inverted Sumps, developed in Uruguay. The fans pictured are portable, but others can be built in vineyard low spots to access the coldest air.
Fig. 6.33. Examples of cultivated (upper) and packed (lower) soils used to assess their effect on fruiting cane temperatures during frost events (from Creasy, 2004).
The influence of frost cloth tents over the canes was also evaluated, but was found to make little difference. Note how dark the moist soil is, contributing to the amount of energy that can be absorbed during the daytime.