During irrigation for frost protection, as 1 gram of water freezes, 80 calories of heat energy are released. As long as ice is being formed, this latent heat of fusion will provide heat. Irrigation for frost protection, also called sprinkler irrigation, can be accomplished by sprinklers mounted above or below the trees.
Although some risk is involved, the advantages of irrigation are significant. Operational cost is lower because water is much cheaper than oil or gas, and the system is convenient to operate because it is controlled at a central pump house. In addition, there are multiple uses for the same system, e.g., drought prevention, evaporative cooling, fertilizer application, and possibly pest control.
There are also disadvantages. The most important is that if the irrigation rate is not adequate, the damage incurred will be more severe than if no protection had been provided. Inadequate irrigation rate means that too little water is being applied to freeze at a rate that will provide enough heat to protect the crop. The situation is made complex by another property of water—evaporative cooling, or the latent heat of evaporation. As 1 gram of water evaporates, 600 calories of heat energy are absorbed from the surrounding environment. When compared to the 80 calories released by freezing, one sees that more than seven times more water must be freezing than evaporating to provide a net heating effect. An ice-covered plant will cool below the temperature of a comparable dry plant if freezing stops and evaporation begins. Since wind promotes evaporative cooling, wind speeds above 8 kilometers per hour limit the success of irrigation for frost protection. Further, with overhead irrigation, ice buildup can cause limb breakage, soils can become waterlogged, and nutrients can leach out. Also, most systems have a fixed-rate design; i.e., the irrigation rate cannot be varied. This means systems are designed for the most severe conditions and so apply excess water in most frosts.
Artificial fog is based on the greenhouse effect. If a "cloud" can be produced to cover the orchard, it decreases the radiative cooling. There has been some experimental success, but a practical system has not been developed. The difficulties lie in producing droplets large enough to block the outgoing long wave radiation and in keeping them in the atmosphere without losing them to evaporation.
Wind machines capitalize on the inversion development in a radiation frost. They circulate the warmer air above down into the orchard. Wind machines are not effective in an advective freeze. A single wind machine can protect approximately 4 hectares, if the area is relatively flat and round. A typical wind machine is a large fan about 5 meters in diameter mounted on a 9-meter steel tower.
Wind machines use only 5 to 10 percent of the energy per hour required by heaters. The original installation cost is quite similar to that for a pipeline heater system, making wind machines an attractive frost control alternative. However, they will not provide protection under windy conditions. Wind machines are sometimes used in conjunction with heaters. This combination is more energy efficient than heaters alone and reduces the risk of depending solely on wind machines. When these two methods are combined, the required number of heaters per hectare is reduced by about half.
Helicopters have been used as wind machines. They hover in one spot until the temperature is increased enough and then move to the next area. Repeated visits to the same location are usually required.
Heating for protection has been relied upon for centuries. The increased cost of fuel has diminished the popularity of this method; however, there are several advantages to using heaters that alternatives do not provide. Most heaters are designed to burn diesel fuel and are placed as free-standing units or connected by a pipeline network throughout the orchard. Advantages of connected heaters are the abilities to control the rate of burning and to shut all heaters down from a central pumping station simply by adjusting pump pressure. A pipeline system can also be designed to use natural gas.
Heaters provide protection by three mechanisms. The hot gases emitted from the top of the stacks initiate convective mixing in the orchard, tapping the important warm air source above in the inversion. About 75 percent of a heater's energy is released in this form. Most of the remaining 25 percent of the total energy is released by radiation from the hot metal stack. This heat is not affected by wind and will reach any solid object not blocked by another solid object. Heaters may thus provide some protection under wind-borne freeze conditions. A relatively insignificant amount of heat is also conducted from heaters to the soil.
Heaters provide the option of delaying protection measures if the temperature unexpectedly levels off or drops more slowly than predicted. The initial installation costs are lower than those of other systems, although the expensive fuels required increase the operating costs. There is no added risk to the crop if the burn rate is inadequate; whatever heat is provided will be beneficial.
The objective of having an inexpensive frost protection material that can be stored until needed and easily applied has existed since the mid-1950s. Numerous materials have been examined. These fall into several categories but, in general, they have been materials that allegedly (1) change the freezing point of the plant tissue, (2) reduce the ice-nucleating bacteria on the crop and thereby inhibit ice/frost formation, (3) affect growth, e.g., delay dehardening, or (4) work by some "undetermined mode of action." To this author's knowledge, no commercially available material has successfully withstood the scrutiny of a scientific test. However, several products are advertised as frost protection materials. Growers should be very careful about accepting the promotional claims about these products. Research continues, and some materials have shown some positive effects.
The proper method of frost protection must be chosen by each grower for the particular site considered. Once the decision has been made, several general suggestions apply to all systems. If frost protection is to be practiced successfully, it must be handled with the same care and attention as spraying, fertilizing, pruning, and other cultural practices. Success depends on proper equipment used correctly, sound judgment, attention to detail, and commitment. Growers should not delegate protection of the crop to someone who has no direct interest in the result. It is important to prepare and test the system well before the frost season begins, to double-check the system shortly before an expected frost, and not to take down the system before the threat of frost has definitely passed. Problems that are handled easily during the warm daylight can become monumental and even disastrous during a cold, frosty night when every second counts.
Related Topics: DORMANCY AND ACCLIMATION; FLOWER BUD FORMATION, POLLINATION, AND FRUIT SET; TEMPERATURE RELATIONS
Barfield, B. J. and J. F. Gerber, eds. (1979). Modification of the aerial environment of plants. St. Joseph, MI: ASAE Monograph.
Hoffman, G. J., T. A. Howell, andK. H. Solomon, eds. (1990). Management of farm irrigation systems. St. Joseph, MI: ASAE Monograph. Perry, K. B. (1998). Basics of frost and freeze protection for horticultural crops.
HortTechnol. 8:10-15. Rieger, M. (1989). Freeze protection for horticultural crops. Hort. Rev. 11:45-109. Rosenberg, N. J., B. L. Blad, and S. B. Verma (1983). Microclimate: The biological environment. New York: John Wiley and Sons.
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