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Source: Adapted from Ventskevich, 1961; Rogers, 1970.

Source: Adapted from Ventskevich, 1961; Rogers, 1970.

Site Selection

Many factors are involved to create pockets of very low temperatures. Before planning a crop or an orchard, the best method of frost protection is careful selection of the site. The site should be selected taking into account the climatic conditions prevailing in that location, its slope, and the soil characteristics. There is a possibility of cold air buildup in low paddocks or behind barriers such as fences, hedges, and wooded areas (Hutton, 1998).

Such paddocks are not the best locations for planting orchards and frostsensitive crops. Removal or thinning of trees that create cold air dams is desirable. If a site has good cold air drainage, then it is likely to be a good production site as far as frost damage is concerned. Frost-sensitive fruit trees are usually planted on hillside slopes from which the cold air drains rapidly to the bottom of the valley. Such sites are usually 2 to 4°C warmer during radiational frost.

Frost-Resistant Cultivars

Planting frost-resistant cultivars and crop varieties is one approach to avoid frost damage to fruit trees and field crops. Oats are more tolerant to frost damage than barley, and barley is slightly more tolerant than wheat. The varieties could be those in which genetic resistance to freezing stress has been incorporated. Growers should refer to available extension publications on varieties that could withstand the low temperatures.

Optimizing Sowing Dates

The best and most cost-effective strategy to save field crops from frost is the choice of the optimum dates for crop plantings. As crops enter the flowering and grain-forming stage, their tolerance to frost is drastically reduced. If the sowing dates of crops are adjusted in such a way that these stages do not fall in the period of heavy frost, then its damaging action is avoided. In the case of wheat, it is necessary for anthesis to occur after the high-risk frost period is over. Results of experiments in NSW have shown that a wheat crop can be saved from frost damage to a great extent if the crop flowers in mid-September in areas around Trangie, in late September around Narrabri, and in early October around Tamworth. A late date of anthesis, however, needs to be balanced against the damage that can occur if grain filling takes place during the period of high temperatures or moisture stress. Each week's delay after these dates in anthesis can reduce yields dramatically (Boer, Campbell, and Fletcher, 1993).

Storing Heat in the Soil

Frost frequency and intensity is greater in orchards in which the soil is cultivated, dry, and covered with weeds or mulch as compared to orchards in which the soil is moist, compact, and weed free (Johns, 1986). This is because soil that is bare or weed free, compact, and moist stores more heat during the daytime than soil that is covered with shade and is dry. At night this heat is released to the lower layers of the air surrounding the crop plants and fruit trees, minimizing the damage from frost.

Standing weeds increase the incidence of frost in three ways: by shading the soil, which hampers the heat flow to the soil; by drying the soil; and by raising the cold radiating surface which comes close to the fruit level. Thick mulches also increase the incidence of frost through hampering the heat flow to the soil during the day and retarding the heat flow to the top of the straw during the night. A dry cultivated soil increases the incidence of frost, because cultivation creates more air pockets in the soil which act as insulating layers and hamper the flow of heat to the soil, lowering its heat storage during the day.

Therefore, keeping the soil moist with frequent light irrigation, maintaining it weed free, and making it compact with rollers is the best technique to minimize frost damage in orchards, vineyards, and wide-row crops.

Plant Cover

Planting large canopy trees with orchard plants provides some freeze protection. Date palms in California and pine trees in southern Alabama are used as canopy cover for citrus plantings (Perry, 1994)

"Brushing" is commonly used for protecting vegetable crops from frost damage. Shields of coarse brown paper are attached to arrowhead stems on the poleward side of the east-west rows of plants. The fields present a brushy look. During the day the shields act as windbreaks against cold wind, while at night they reduce radiation loss to the sky. Woven or spun-bonded polypropylene covers of varying thickness are among the latest forms of protection used on fruit crops. Depending on the material used, several degrees of protection are achieved. Copolymer white plastic has provided protection to nursery stock but is not used on fruit and vegetable crops. Light-and medium-weight covers provide excellent protection for low-growing crops such as strawberries.

Nutrition

Deciduous fruit plants, such as peach, that are not nutritionally sound, especially in regard to nitrogen, are more subject to frost damage. Fruit buds of such trees are less healthy and more easily damaged by frost. Using midsummer or postharvest application of nitrogen can induce vigor for strong fruit bud development and some delay in flowering in stone fruits such as peaches. However, tree fruits with low fertility requirements, such as apples and pears, do not normally require mid- to late-summer fertilization, whereas such applications do benefit blueberries (Perry, 1994).

Chemicals

Some inexpensive materials which could be stored easily until needed and are portable and easily applied to provide frost protection have been tested. The possibilities of using cryoprotectants, antitranspirants, and growth regulators are encouraging.

A number of materials that could change the freezing point of plant tissue, reduce the ice nucleating bacteria on the crop and thereby inhibit frost formation, or affect growth, i.e., delay dehardening, have been examined. Several products are advertised as frost protection materials; however, none of the commercially available materials has successfully withstood the scrutiny of scientific testing. Growers should be very careful about accepting the promotional claims of these materials (Ullio, 1986; Powell and Himelrick, 1998).

Growth regulator applications that could increase the cold hardiness of the buds and flowers, delay flowering, or both seem to hold the most promise at this time. Among the growth regulators tested, only the ethylene-re-leasing compound ethephon has shown promise (Gallasch, 1992; Powell and Himelrick, 1998). Ethephon increases winter fruit bud hardiness and delays flowering of peaches by four to seven days. It provides the same effects on cherries. In the United States, ethepon has been federally labeled for use on cherries, and it is on several state labels for use on peaches.

Irrigation

Irrigation is the oldest, most popular, and most effective method of protection from frost. Irrigation is done with sprinklers mounted above or below the crop canopy. Sprinkling the canopy with water releases the latent heat of fusion when water turns from liquid to ice. As long as ice is being formed, latent heat released by water efficiently compensates for the heat lost from the crops to the environment.

For most situations, sprinkler rotating once each minute and an application rate of 2 to 4 mm of water per hour is sufficient. A backup power source is essential, as power failure can be devastating. Once started, irrigation must continue until the morning sun hits the trees (Wickson, 1990). During the other seasons the sprinklers can be used for evaporative cooling, artificial chilling, delayed flowering, fruit drop prevention, sunburn injury, and color improvement of fruits (Spieler, 1994).

Heaters

Heating of orchards for protection against frost has been relied upon for centuries. The high cost of fuel has now provided an incentive to look at other methods. There are several advantages to using heaters. Most heaters are designed to burn oil and can be placed as freestanding units or connected by a pipeline network throughout the crop area. The advantage of connected heaters is the ability to control the rate of burning and shut all heaters down from a central pumping station simply by adjusting the pump pressure. A pipeline system can also be designed to use natural gas. Propane, liquid petroleum, and natural gas systems have been used for citrus.

Heaters provide protection by three mechanisms. The hot gases emitted from the top of the stack initiate convective mixing in the crop area and break the inversion. The bulk of a heater's energy is released in this form. The remaining energy is released by radiation from the hot metal stack. A relatively insignificant amount of heat is also conducted from the heater to the soil. Around the periphery, more heaters are required, because the ascending plumes of hot air allow an inflow of cold air.

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 operating costs. There is no added risk to the crop. Whatever heat is provided will be beneficial.

Wind Machines

The purpose behind using wind machines is to circulate warmer air down to the crop level. Wind machines are effective only under radiation frost conditions. They should be installed and operated after a thorough understanding of how frost affects a particular area or orchard (Lipman and Duddy, 1999). A typical wind machine with fans about 5 m in diameter and mounted on a 10 m steel tower can protect approximately four hectares of area, if the area is relatively flat and round. The fan is powered by an engine delivering 85 to 100 HP. Wind machines used in conjunction with heaters provide the best protection. When these two methods are combined, the required number of heaters per hectare is reduced by about half.

Wind machines provide noteworthy advantages in frost protection by minimizing labor requirements, reducing refuelling and storage of heating supplies, and requiring a low operational cost per hectare. Wind machines use only 5 to 10 percent of the energy per hour when compared to heaters. The original installation cost is quite similar to that for a pipeline heater system, making wind machines an attractive alternative to heaters for frost protection. They are also more environmentally friendly (except for noise) because they do not produce smoke or air pollution.

An overview of the advantages and limitations of the methods mentioned in this section is given in Table 3.4. Each grower must choose the proper method of frost protection for the particular site considered. Once the decision has been made, and if frost protection is to be practiced successfully, three guidelines apply to all systems:

1. Operation for protection against frost must be handled with the same care and attention as spraying, fertilizing, pruning, and other cultural practices.

2. Frost protection equipment must be used correctly with sound judgment and attention to detail and commitment.

3. Operation should not be delegated to someone with no direct interest in the result.

Frost Forecast

Season Ahead

The Southern Oscillation Index, particularly the SOI phase during autumn, provides a modest tool to determine the dates of the last frost and the number of frost days in eastern Australia with a lead time of three to four months (Stone, Nicholls, and Hammer, 1996).

The SOI phases representing either a consistently negative or a rapidly falling phase during late autumn indicate a greater chance of frosts with the last frost occurring late in the season. The reason for more frosts under these patterns is because these are associated with El Niño, resulting in less rainfall, more clear skies, and more radiational cooling of the earth's surface during winter and early spring.

TABLE 3.4. Frost protection alternatives—advantages and disadvantages

Protection method Crops protected

Advantage

Disadvantage

Site selection

Resistant varieties

All fruit crops especially tree fruit

Grain and fruit crops

A location with good Good site may not be cold air drainage is available a good production location

Resist frost action Many varieties of crops already at the upper limit of frost resistance

Very difficult to create genetic resistance to frost

Optimize sowing dates Grain and fruit crops Very effective

Improving soil heat

All fruit crops

Easy and inexpen-

storage

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