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CHAPTER 2 - RECOMMENDED METHODS OF FROST PROTECTION


Introduction

This chapter presents information on important aspects of frost protection methods without complicated equations or concepts. More detailed information is given in following chapters. References are not included in this chapter to reduce its size and to simplify reading.

Crop sensitivity and critical temperatures

Frost damage to crops results not from cold temperature but mainly from extracellular (i.e. not inside the cells) ice formation inside plant tissue, which draws water out and dehydrates the cells and causes injury to the cells. Following cold periods, plants tend to harden against freeze injury, and they lose the hardening after a warm spell. A combination of these and other factors determine the temperature at which ice forms inside the plant tissue and when damage occurs. The amount of frost injury increases as the temperature falls and the temperature corresponding to a specific level of damage is called a "critical temperature" or "critical damage temperature", and it is given the symbol Tc. Generally, most critical temperatures are determined in growth chamber studies by cooling at a fixed rate down to a predetermined temperature that is maintained for 30 minutes. Then the percentage damage is recorded.

Categories for frost hardiness of vegetable and other horticultural plants are given in Tables 4.1 and 4.2. For agronomic and other field crops, ranges for critical damage temperature are given in Table 4.5. Critical temperature values are given for almonds (Table 4.6), other deciduous tree crops and grapevines (Table 4.7 and 4.8), small-fruit vines, kiwifruit and strawberries (Table 4.9), and citrus (Table 4.10). In most of these tables, T10 and T90 values are provided, where T10 and T90 are the temperatures where 10 percent and 90 percent of the marketable crop production is likely to be damaged. Generally, both the T10 and T90 temperatures increase with time after the buds start developing until the small-nut or -fruit stage, when the crops are most sensitive to freezing. The T90 value is quite low at the onset of growth but it increases more rapidly than the T10 and there is little difference between T10 and T90 when the crop is most sensitive. The Tc values for deciduous orchards and vineyards vary with the phenological stage (Tables 4.6-4.8). Photographs showing the common phenological stages of many of these crops can be found on the Internet, including sites such as fruit.prosser.wsu.edu/frsttables.htm or www.msue.msu.edu/vanburen/crittemp.htm. Although the Tc values provide some information on when to start and stop active frost protection methods, they should be used with caution. Generally, Tc values represent bud, flower or small-fruit temperature where a known level of damage was observed. However, it is difficult to measure sensitive plant tissues, and these temperatures are likely to differ from air temperature, which is what growers typically measure. Except for large fruits (e.g. oranges), bud, flower and small-fruit temperature tends to be colder than air temperature, so active protection methods should be started and stopped at higher air temperatures than indicated in the tables in Chapter 4. For large fruits, like citrus, the evening air temperature will often drop faster than the fruit temperature, so heaters or wind machines can be started when the air temperature is at or slightly below the Tc temperature. The Tc values in Chapter 4 provide guidelines for timing active protection methods, but the values should be used with caution because of other factors such as the difference between plant and air temperature; degree of hardening; and the concentration of ice-nucleation active (INA) bacteria.

Passive protection

Passive protection includes methods that are implemented before a frost night to help avoid the need for active protection. The main passive methods are:

Passive methods are usually less costly than active methods and often the benefits are sufficient to eliminate the need for active protection.

Site selection and management

Growers are usually aware that some spots are more prone to frost damage than others. The first step in selecting a site for a new planting is to talk with local people about what crops and varieties are appropriate for the area. Local growers and extension advisors often have a good feeling for which locations might be problematic. Typically, low spots in the local topography have colder temperatures and hence more damage. However, damage can sometimes occur in one section of a cropped area and not in another, without apparent topographical differences. In some cases, this might be due to differences in soil type, which can affect the conduction and storage of heat in the soil.

Dry sandy soils transfer heat better than dry heavy clay soils, and both transfer and store heat better than organic (peat) soils. When the water content is near field capacity (i.e. a day or two after thoroughly wetting the soil), soils have conditions that are most favourable for heat transfer and storage. However, organic soils have poor heat transfer and storage regardless of the water content. When selecting a site in a region prone to frost, avoid planting on organic soils.

Cold air is denser than warm air, so it flows downhill and accumulates in low spots much like water in a flood (Figure 6.4). Therefore, one should avoid planting in low-lying, cold spots unless adequate cost-effective active protection methods are included in the long-term management strategy. This is important on both a regional and farm scale. For example, on a regional scale, valley bottoms near rivers are usually colder than the slopes above. These spots can also be identified from topographical maps, by collecting temperature data, and by locating spots where low-level ground fogs form first. Low spots consistently have colder nights, when the sky is clear and the wind is weak, during the entire year. Accordingly, temperature measurements to identify cold spots can be made at any time during the year.

Planting deciduous crops on slopes facing away from the sun delays springtime bloom and often provides protection. Subtropical trees are best planted on slopes facing the sun where the soil and crop can receive and store more direct energy from sunlight.

Cold air drainage

Trees, bushes, mounds of soil, stacks of hay, and fences are sometimes used to control air flow around agricultural areas and the proper placement can affect the potential for frost damage. A careful study of topographical maps can often prevent major frost damage problems. Also, the use of smoke bombs or other smoke generating devices to study the down slope flow of cold air at night can be informative. These studies need to be done on nights with radiation frost characteristics, but not necessarily when the temperature is subzero. Once the cold air drainage flow pattern is known, then proper placement of diversion obstacles can provide a high degree of protection.

If a crop already exists in a cold spot, there are several management practices that might help reduce the chances of frost damage. Any obstacles that inhibit down-slope drainage of cold air from a crop should be removed. These obstacles might be hedgerows, fences, bales of hay, or dense vegetation located on the downslope side of the field. Land levelling can sometimes improve cold air drainage through a crop so that incoming cold air continues to pass through the crop. Row lines in orchards and vineyards should be oriented to favour natural cold air drainage out of the crop. However, the advantages from orienting crop rows to enhance cold air drainage must be balanced against the disadvantages due to more erosion and other inconveniences. Grass and plant stubble in areas upslope from a crop can make air colder and will enhance cold air drainage into a crop. Air temperature measured within grape vineyards and citrus orchards with plant residue or grass cover typically varies between 0 °C and 0.5 °C colder than grape vineyards and citrus orchards with bare soil, depending on soil conditions and weather. Without the crop present, the differences would probably be greater. Therefore, having bare soil upslope from a crop will generally lead to higher air temperatures over the upslope soil and less likelihood of cold air drainage into the crop.

Plant selection

It is important to choose plants that bloom late to reduce the probability of damage due to freezing, and to select plants that are more tolerant of freezing. For example, deciduous fruit trees and vines typically do not suffer frost damage to the trunk, branches or dormant buds, but they do experience damage as the flowers and small fruits or nuts develop. Selecting deciduous plants that have a later bud break and flowering provides good protection because the probability and risk of frost damage decreases rapidly in the spring. In citrus, select more resistant varieties. For example, lemons are least tolerant to frost damage, followed by limes, grapefruit, tangelos and oranges, which are most tolerant. Also, trifoliate orange rootstock is known to improve frost tolerance of citrus compared with other rootstocks.

For annual field and row crops, determining the planting date that minimizes potential for subzero temperature is important. In some instances, field and row crops are not planted directly to the outdoors, but are planted in protected environments and transplanted to the field after the danger of freezing has passed. Several Excel application programs on probability and risk are included with this book and their use is discussed in the probability and risk chapter. If freezing temperatures cannot be avoided, then select crops to plant based on their tolerance of subzero temperatures.

Canopy trees

In Southern California, growers intercrop plantings of citrus and date palms, partly because the date palms give some frost protection to the citrus trees. Because the dates also have a marketable product, this is an efficient method to provide frost protection without experiencing relevant economic losses. In Alabama, some growers interplant pine trees with small Satsuma mandarin plantings and the pine trees enhance long-wave downward radiation and provide protection to the mandarins. Shade trees are used to protect coffee plants from frost damage in Brazil.

Plant nutrition management

Unhealthy trees are more susceptible to frost damage and fertilization improves plant health. Also, trees that are not properly fertilized tend to lose their leaves earlier in the autumn and bloom earlier in the spring, which increases susceptibility to frost damage. However, the relationship between specific nutrients and increased resistance is obscure, and the literature contains many contradictions and partial interpretations. In general, nitrogen and phosphorus fertilization before a frost encourages growth and increases susceptibility to frost damage. To enhance hardening of plants, avoid applications of nitrogen fertilizer in late summer or early autumn. However, phosphorus is also important for cell division and therefore is important for recovery of tissue after freezing. Potassium has a favourable effect on water regulation and photosynthesis in plants. However, researchers are divided about the benefits of potassium for frost protection.

Pest management

The application of pesticide oils to citrus is known to increase frost damage and application should be avoided shortly before the frost season.

Proper pruning

Late pruning is recommended for grapevines to delay growth and blooming. Double pruning is often beneficial because resource wood is still available for production following a damaging frost. Pruning lower branches of vines first and then returning to prune higher branches is a good practice because lower branches are more prone to damage. Pruning grapevines to raise the fruit higher above the ground provides protection because temperature during frost nights typically increases with height. Late-autumn pruning of citrus leads to more physiological activity during the winter frost season. Citrus pruning should be completed well before frost season. For example, serious damage has been observed in citrus that were topped in October when a freeze occurred in December. If deciduous trees are grown in a climate sufficiently cold to cause damage to dormant buds, then the trees should not be pruned. Otherwise, deciduous tree pruning can be done during dormancy with few problems.

Plant covers

Plant row covers are warmer than the clear sky and hence increase downward long-wave radiation at night, in addition to reducing convectional heat losses to the air. Removable straw coverings and synthetic materials are commonly used. Because of the labour costs, this method is mainly used on small plantings of short plants that do not require a solid frame. Sometimes, disease problems occur due to deficient ventilation. Woven and spun-bonded polypropylene plastics are sometimes used to protect high value crops. The degree of protection varies from about 1 °C to 5 °C, depending on plastic thickness. White plastic is sometimes used for nursery stock but not for fruit and vegetable crops. Partially covering grapevines with black polyethylene has been observed to increase air temperature next to the foliage by as much as 1.5 °C. However, clear plastic is generally more effective.

Avoiding soil cultivation

Soil cultivation creates air spaces in the soil and it should be avoided during frost-prone periods. Air is a poor heat conductor and has a low specific heat, so soils with more and larger air spaces will tend to transfer and store less heat. If a soil is cultivated, compacting and irrigating the soil will improve heat transfer and storage.

Irrigation

When soils are dry, there are more air spaces, which inhibit heat transfer and storage. Therefore, in dry years, frost protection is improved by wetting dry soils. The goal is to maintain the soil water content near field capacity, which is typically the water content 1 to 3 days following thorough wetting. It is unnecessary to wet the soil deeply because most of the daily heat-transfer and storage occurs in the top 30 cm. Wetting the soil will often make it darker, and increases absorption of solar radiation. However, when the surface is wet, then evaporation is also increased and the energy losses to evaporation tend to counterbalance the benefits from better radiation absorption. It is best to wet dry soils well in advance of the frost event, so that the sun can warm the soil.

Removing cover crops

For passive frost protection, it is better to remove all vegetation (cover crops) from orchards and vineyards. Removal of cover crops will enhance radiation absorption by the soil, which improves energy transfer and storage. Cover crops are also known to harbour higher concentrations of ice-nucleation active (INA) bacteria than many orchard and vine crops, so the presence of vegetation on orchard and vineyard floors increases the INA bacteria concentrations on the crop and hence the potential for frost damage.

Generally, mowing, cultivation and spraying with herbicides are methods to remove floor vegetation. If possible, the cover crop should be mowed sufficiently early to allow the residue to decompose or the cut vegetation should be removed. For grass taller than about 5 cm, there is little difference in orchard floor surface temperature, but the surface temperature increases as the canopy gets shorter, to the highest minimum surface temperature for bare soil. Orchard floor minimum surface temperature differences as high as 2 °C have been reported between bare soil and 5-cm high grass. However, the air temperature difference is likely to be less than 2 °C. Cultivation should be done well before the frost season and the soil should be compacted and irrigated following the cultivation to improve heat transfer and storage. The most effective method is to use herbicides to kill the floor vegetation or keep down the growth. Again, this should be done well in advance of the frost-prone period.

Soil covers

Plastic covers are often used to warm the soil and increase protection. Clear plastic warms the soil more than black plastic, and wetting the soil before applying the plastic further improves effectiveness. Sometimes vegetative mulches are used during dormancy of tree crops to help prevent damage to roots due to freezing and soil heaving; however, vegetative mulches reduce the transfer of heat into the soil and hence make orchard crops more frost prone after bud break. In general, vegetative mulches are only recommended for locations where soil freezing and heaving are a problem. For non-deciduous orchards, pruning up the skirts of the trees allows better radiation transfer to the soil under the trees and can improve protection.

Trunk painting and wraps

The bark of deciduous trees sometimes splits when there are large fluctuations in temperature from a warm day into a frost night. Painting the trunks with an interior water-based latex white paint diluted with 50 percent water in the late autumn when the air temperature is above 10 °C will reduce this problem. White paint, insulation and other wraps are known to improve hardiness against frost damage in peach trees. The paint or wraps decrease the late winter high cambial temperatures due to daytime radiation, which improves hardiness. Wrapping tree trunks with insulation (i.e. materials containing air spaces that resist heat transfer) will protect young trees from frost damage and possible death. Critical factors are to use insulation that does not absorb water and the trunks should be wrapped from the ground surface to as high as possible. Fibreglass and polyurethane insulation wraps with higher resistance to heat transfer provide the best protection of commercially available wraps. Typically, the trunk wraps are removed after 3 to 4 years. Wrapping young citrus tree trunks with water bags was reported to give even better protection than fibreglass or polyurethane foam. The main drawback to trunk wraps is increased potential for disease problems, so the bud unions should be at least 15 cm above the ground. Applying fungicide sprays prior to wrapping helps to reduce disease problems.

Bacteria control

For freezing to occur, the ice formation process is mostly initiated by presence of INA bacteria. The higher the concentration of the INA bacteria, the more likely that ice will form. After forming, it then propagates inside the plants through openings on the surface into the plant tissues. Commonly, pesticides (copper compounds) are used to kill the bacteria or competitive non-ice-nucleation active (NINA) bacteria are applied to compete with and reduce concentrations of INA bacteria. However, this frost protection method has not been widely used; for further information refer to Chapter 6.

Active protection

Active protection methods include

All methods and combinations are done during a frost night to mitigate the effects of subzero temperatures. The cost of each method varies depending on local availability and prices, but some sample costs based on prices in the USA are given in Table 7.1. In some cases, a frost protection method has multiple uses (e.g. sprinklers can also be used for irrigation) and the benefits from other uses need to be subtracted from the total cost to evaluate fairly the benefits in terms of frost protection.

Heaters

Heaters provide supplemental heat to help replace energy losses. Generally, heaters either raise the temperature of metal objects (e.g. stack heaters) or operate as open fires. If sufficient heat is added to the crop volume so that all of the energy losses are replaced, the temperature will not fall to damaging levels. However, the systems are generally inefficient (i.e. a large portion of the energy output is lost to the sky), so proper design and management is necessary. By designing a system to use more and smaller heaters that are properly managed, one can improve efficiency to the level where the crop is protected under most radiation frost conditions. However, when there is little or no inversion and there is a wind blowing, the heaters may not provide adequate protection.

The energy requirement to match losses on a radiation frost night is in the range 10 to 50 W m-2, whereas the energy output from heaters is in the range of 140 to 280 W m-2, depending on the fuel, burning rate, and number of heaters. One hundred stack heaters per hectare burning 2.85 litre h-1 of fuel with an energy output of 37.9 MJ litre-1 would produce approximately 360 W m-2. The net benefit depends on weather conditions, but one can expect about 1 °C increase in the mean air temperature from the ground up to about 3 m, with somewhat higher temperatures measured at 1.5 m height. However, direct radiation from the heaters supplies additional benefit to plants within sight of the heaters. Because the energy output is much greater than the energy losses from an unprotected crop, much of the energy output from heaters is lost and does not contribute to warming the air or plants. If the heating system were perfectly designed and managed to replace the energy lost from the volume of air under the inversion layer with little or no loss of convective heat to the sky, then the energy output requirement would be close to the energy requirement needed to prevent frost damage and the heating would be efficient. To achieve the best efficiency, increase the number of heaters and decrease the temperature of the heaters. However, this is often difficult to accomplish because of equipment costs, labour, etc. If the temperature inversion is weak or if the fires are too big and hot, the heated air rises too high and energy is lost to the air above the crop, thus decreasing efficiency. Modern heaters have more control over the temperature of emitted gases to reduce buoyancy losses and improve efficiency. The most efficient systems have little flame above the stack and no smoke. Operating the heaters at too high a temperature will also reduce the lifetime of the heaters. Liquid-fuel and gas fuel heaters typically output energy at close to twice the rate of solid-fuel heaters. When there is a strong inversion (i.e. a low ceiling), the heated volume is smaller, and the heaters are more effective at raising the temperature, if the fires are not too big (i.e. the temperature of gases leaving a stack heater should be near 635 °C) so that the heated air rises slowly. Heater operation is less efficient in weak inversion (i.e. high ceiling) conditions because there is a bigger volume to heat. More frost damage occurs on the edges and more heaters are needed on the edges to avoid this damage. In the past, it was widely believed that smoke was beneficial for frost protection. However, smoke does not help and it does pollute the environment, and should be avoided.

Heater distribution should be relatively uniform with more heaters in the borders, especially upwind, and in low cold spots. Borders should have a minimum of one heater per two trees on the outside edge and inside the first row.

On the upwind border, one heater per two trees is recommended inside the second row as well. Heaters on the borders, especially upwind, should be lit first and then light every fourth row through the orchard (or every second row if needed). Then monitor the temperature and light more rows of heaters as the need increases. Heaters are expensive to operate, so they are commonly used in combination with wind machines or as border heat in combination with sprinklers. See Chapter 7 for more information on heater management.

Use of liquid-fuel heaters decreased as oil prices and concerns about air pollution increased. Liquid-fuel heaters require considerable labour for placement, fuelling and cleaning in addition to the capital costs for the heaters and the fuel. Note that isolated small orchards require more heaters than large orchards or those surrounded by other protected orchards.

Fuel recommendations for lighting heaters varies from ratios of 1:1 oil to gasoline [petrol] to 8:5 oil to gasoline [petrol]. Buckets or tanks towed by a tractor, which allow two lines of burners to be filled simultaneously, are used to refill the heaters after a frost. When direct heating is used, to minimize fuel consumption the protection is started just before reaching critical damage temperatures. The temperature should be measured in a Stevenson screen, fruit-frost shelter or Gill shield that prevents thermometer exposure to the clear sky.

Labour requirements to refill liquid-fuel heaters are high, so centralized distribution systems using natural gas, liquid propane or pressurised fuel oil have become more popular. In more elaborate systems, ignition, the combustion rate and closure are also automated, in addition to fuel distribution. The capital cost to install centralized systems is high, but the operational costs are low. Propane-fuel heaters require less cleaning and the burning rates are easier to control than oil-fired heaters. Because the burning rate is less, more heaters are needed (e.g. typically about 100 per hectare of stack heaters and about 153 per hectare of propane-fuel heaters), but the protection is better because more heaters at a lower burning rate are more efficient. Under severe conditions, the propane supply tank can sometimes freeze up, so a vaporizer should be installed to prevent the gas line from freezing.

The ratio of radiation to total energy released is 40 percent for burning solid fuels in comparison with 25 percent for burning liquid fuels, so solid fuels are more efficient at heating the plants, especially under windy conditions. The main disadvantage of solid fuels is that energy release diminishes as the fuel is used up, so the energy release becomes limiting when it is needed most. Another drawback is that solid fuels are difficult to ignite, so they must be started early. They are also difficult to extinguish, so fuel is often wasted.

Wind machines

Wind machines alone generally use only 5 percent to 10 percent of the fuel consumed by a fuel-oil heater protection system. However, the initial investment is high (e.g. about $ 20 000 per machine). Wind machines generally have lower labour requirements and operational costs than other methods; especially electric wind machines.

Most wind machines (or fans) blow air almost horizontally to mix warmer air aloft in a temperature inversion with cooler air near the surface. They also break up microscale boundary layers over plant surfaces, which improves sensible heat transfer from the air to the plants. However, before investing in wind machines, be sure to investigate if inversions between 2.0 and 10 m height are at least 1.5 °C or greater on most frost nights.

When electric wind machines are installed, the grower is commonly required to pay the power company "standby" charges, which cover the cost of line installation and maintenance. The standby charges are paid whether the wind machines are used or not. Internal combustion wind machines are more costly effective, but they require more labour. Wind machine noise is a big problem for growers with crops near cities and towns, and this should be considered when selecting a frost protection method. Generally, one large wind machine with a 65 to 75 kW power source is needed for each 4.0 to 4.5 ha. The effect on temperature decreases approximately as the inverse square of the distance from the tower, so some overlap of protection areas will enhance protection.

Wind machines generally consist of a steel tower with a large rotating two-blade fan (3 to 6 m diameter) near the top, mounted on an axis tilted about 7° downward from the horizontal in the tower direction. Typically, the height for fans is about 10-11 m, and they rotate at about 590-600 rpm. There are also wind machines with four-blade fans. When a fan operates, it draws air from aloft and pushes it at a slightly downward angle towards the tower and the ground. The fan also blows cold air near the surface upwards and the warm air above and cold air below are mixed. At the same time that the fan is operating, it rotates around the tower with about one revolution every three to five minutes. The amount of protection afforded depends on the unprotected inversion strength. In general, the temperature increase at 2.0 m height resulting from the fans is about 30 percent of the inversion strength between 2 m and 10 m height in an Q unprotected crop. Wind machines are typically started when the air temperature reaches about 0 °C. Wind machines are not recommended when there is a wind of more than about 2.5 m s-1 (8 km h-1) or when there is supercooled fog, which can cause severe fan damage if the blades ice up.

Fans that vertically pull down warm air from aloft have generally been ineffective and they can damage plants near the tower. Wind machines that blow vertically upwards are commercially available and there has been some testing of the machines. However, there were no published research reports found when preparing this book.

Helicopters

Helicopters move warm air from aloft in a temperature inversion to the colder surface. The area covered by a single helicopter depends on the helicopter size and weight and on the weather conditions. Estimated coverage area by a single helicopter varies between 22 and 44 ha. Recommendations on pass frequency vary between 30 to 60 minutes, depending on weather conditions. Waiting too long between passes allows the plants to supercool and the agitation from a passing helicopter can cause heterogeneous ice nucleation and lead to severe damage. Heterogeneous ice nucleation occurs when water is supercooled (i.e. at temperature below 0 °C) and some foreign matter or agitation initiates ice formation. In the case of helicopters, agitation can cause ice formation if the passes are too infrequent and the plant tissue temperature becomes too low.

The optimal flying height is commonly between 20 and 30 m and the flight speeds are 8 to 40 km h-1. Pilots often load helicopter spray tanks with water to increase the weight and increase thrust. Under severe frosts with a high inversion, one helicopter can fly above another to enhance the downward heat transfer. Thermostat-controlled lights at the top of the canopy are used to help pilots see where passes are needed. On the sides of hills, heat transfer propagates down-slope after reaching the surface, so flying over the upslope side of a crop usually provides more protection. Flights are stopped when the air temperature upwind from the crop has risen above the critical damage temperature.

Sprinklers

The energy consumption of sprinklers is considerably less than that used in frost protection with heaters, so the operational costs are low compared to heaters. Also, the labour requirement is less than for other methods, and it is relatively non-polluting. The main disadvantages with using sprinklers are the high installation cost and the large amounts of water needed. In many instances, limited water availability restricts the use of sprinklers. In other cases, excessive use can lead to soil waterlogging, which could cause root problems as well as inhibit cultivation and other management. Nutrient leaching (mainly of nitrogen) is a problem where sprinkler use is frequent.

The secret to protection with conventional over-plant sprinklers is to re-apply water frequently at a sufficient application rate to prevent the plant tissue temperature from falling too low between pulses of water. For non-rotating, targeted over-plant sprinklers, the idea is to continuously apply water at a lower application rate but targeted to a smaller surface area. For conventional under-plant sprinklers, the idea is to apply water at a frequency and application rate that maintains the ground surface temperature near 0 °C. This increases long-wave radiation and sensible heat transfer to the plants relative to an unprotected crop. For under-plant microsprinklers, which apply less water than conventional sprinklers, the goal is to keep only the ground under the plants near 0 °C in order to concentrate and enhance radiation and sensible heat transfer upwards into the plants.

Over-plant conventional sprinklers

Over-plant sprinkler irrigation is used to protect low-growing crops and deciduous fruit trees with strong scaffold branches that do not break under the weight of ice loading. It is rarely used on subtropical trees (e.g. citrus) except for young lemons, which are more flexible. Even during advection frosts, over-plant sprinkling provides excellent frost protection down to near -7 °C if the application rates are sufficient and the application is uniform. Under windy conditions or when the air temperature falls so low that the application rate is inadequate to supply more heat than is lost to evaporation, the method can cause more damage than experienced by an unprotected crop. Drawbacks of this method are that severe damage can occur if the sprinkler system fails, the method has large water requirements, ice loading can cause branch damage, and root disease can be a problem in poorly drained soils.

Application rate requirements for over-plant sprinklers differ for conventional rotating, variable rate, or low-volume targeted sprinklers. As long as there is a liquid-ice mixture on the plants, with water dripping off the icicles, the coated plant parts will be protected. However, if an inadequate precipitation rate is used or if the rotation rate of the sprinklers is too slow, all of the water can freeze and the temperature of the ice-coated plants can fall to lower temperatures than unprotected plants.

Conventional over-plant sprinkler systems use standard impact sprinklers to completely wet the plants and soil of a crop. Larger plants have more surface area, so a higher application rate is needed for tall plants than for short plants. For over-plant sprinklers to be effective, the plant parts must be coated with water and re-wetted every 30 to 60 seconds. Longer rotation rates require higher application rates. Also, bigger plants require more water to coat the plants. See Table 2.1 for guidelines on application rates for various plants.

Sprinkler distribution uniformity is important to avoid inadequate coverage, which might result in damage. If cold air is known to drift in from a specific direction, increasing sprinkler density on the upwind edge of the crop or even in an open field upwind from the crop can improve protection. In most cases, the sprinkler heads should be mounted at 30 cm or higher above the top of the plant canopy to avoid the plants blocking the spray. For frost protection, specially designed springs are often used, which are protected by an enclosure to prevent icing of the heads. Clean filters are needed to be sure that the system operates properly, especially when river or lagoon water is used.

TABLE 2.1
Application rates for overhead sprinkler protection of tall (orchard and vine) and short (field and row) crops depending on the minimum temperature and rotation rate, for wind speeds between 0 and 2.5 m s-1

MINIMUM TEMPERATURE

TALL CROPS

SHORT CROPS

°C

30 s rotation mm h-1

60 s rotation mm h-1

30 s rotation mm h-1

60 s rotation mm h-1

-2.0

2.5

3.2

1.8

2.3

-4.0

3.8

4.5

3.0

3.5

-6.0

5.1

5.8

4.2

4.7

NOTE: Application rates are about 0.5 mm h-1 lower for no wind and about 0.5 mm h-1 higher for wind speeds near 2.5 m s-1. The "short crop" rates cover field and row crops with canopies similar in size to strawberries. Taller field and row crops (e.g. potatoes and tomatoes) require intermediate application rates.

Starting and stopping the sprinklers

Over-plant sprinklers should be started when the wet-bulb temperature is higher than the critical (Tc) temperature. Starting when the wet-bulb temperature reaches 0 °C is less risky and it may be prudent if there are no problems with water shortage, waterlogging or ice loading. Even if the sun is shining on the plants and the air temperature is above 0 °C, sprinklers should not be turned off unless the wet-bulb temperature measured upwind from the crop is above the critical damage temperature. If soil waterlogging or water shortages are not problems, permitting the wet-bulb temperature to slightly exceed 0 °C before turning off the sprinklers adds an extra measure of safety.

The wet-bulb temperature can be measured directly with a psychrometer (Figure 3.9) or it can be estimated from the dew-point and air temperatures. Wet-bulb temperature measurements are explained in Chapter 3. A simple, inexpensive dew-point measurement is accomplished with a thermometer, a shiny can, water, salt and ice (Figure 7.11). First pour some salted water into the shiny can. Then start adding ice cubes to the can while stirring the mixture with the thermometer. Watch the outside of the can to see when water condenses or ice deposits on the surface. Immediately read the thermometer temperature when the water or ice forms. Shining a flashlight (pocket torch) onto the can surface will help you to see water or ice form and to read the thermometer. Under very cold, dry conditions, more salt and ice might be needed to reach the ice or dew-point temperature. There is a small difference between the ice point and dew-point temperature (explained in Chapter 3), but for estimating sprinkler start and stop air temperatures there is negligible error by assuming they are equal.

TABLE 2.2
A range of minimum starting and stopping air temperatures (°C) for frost protection with sprinklers as a function of wet-bulb and dew-point temperature (°C)

DEW-POINT TEMPERATURE

WET-BULB TEMPERATURE (°C)

°C

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.0













0.0

0.0

-1.0









-1.0

-0.9

-0.2

-0.1

0.6

0.7

-2.0





-2.0

-1.8

-1.2

-0.8

-0.4

-0.2

0.4

0.6

1.2

1.4

-3.0

-3.0

-2.7

-2.2

-1.9

-1.4

-1.1

-0.6

-0.3

0.2

0.5

1.0

1.3

1.8

2.1

-4.0

-2.5

-2.1

-1.7

-1.4

-0.9

-0.6

-0.1

0.2

0.7

1.0

1.5

1.8

2.3

2.6

-5.0

-2.0

-1.6

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

-6.0

-1.5

-1.1

-0.7

-0.3

0.1

0.5

0.9

1.4

1.7

2.1

2.5

2.9

3.3

3.7

-7.0

-1.1

-0.6

-0.3

0.2

0.5

1.0

1.3

1.8

2.1

2.6

2.9

3.4

3.7

4.2

-8.0

-0.7

-0.2

0.1

0.6

0.9

1.4

1.7

2.2

2.5

3.0

3.3

3.8

4.1

4.8

-9.0

-0.3

0.3

0.5

1.1

1.3

1.9

2.1

2.7

2.9

3.5

3.7

4.3

4.5

5.1

-10.0

0.1

0.7

0.8

1.5

1.6

2.3

2.4

3.1

3.2

3.9

4.0

4.7

4.9

5.6

NOTE: Select a wet-bulb temperature that is above (warmer than) the critical damage temperature for your crop and locate the appropriate column. Then choose the row with the correct dew-point temperature and read the corresponding air temperature from the table to turn your sprinklers on or off. Use the lower air temperatures at low elevations (0-500 m) and increase to the higher temperatures at higher elevations (1500-2000 m).

After measuring the dew-point temperature, the start and stop air temperatures are found using the critical (Tc) temperature for your crop, the dew-point temperature, and Table 2.2. For more exact information, see Tables 7.5 and 7.6 and the related discussion in Chapter 7.

Sprinkler application rates

The application rate requirement for over-plant sprinkling with conventional sprinklers depends on the rotation rate, wind speed and unprotected minimum temperature. Table 2.1 provides commonly used application rates for tall and short crops. For both tall and short crops, the application rates increase with wind speed and they are higher for slower rotation rates.

If there is a clear liquid-ice mixture coating the plants and water is dripping off the ice, then the application rate is sufficient to prevent damage. If all of the water freezes and it has a milky white appearance like rime ice, then the application rate is too low for the weather conditions. If the application rate is insufficient to adequately cover all of the foliage, then damage can occur on plant parts that are not adequately wetted. Under windy, high evaporation conditions, inadequate application rates can cause more damage than if the sprinklers are not used.

Targeted over-plant sprinklers

Use of targeted over-plant microsprinklers has been studied as a method to reduce application rates for over-plant sprinklers, but installation costs are high and the method has not been widely accepted by growers except those with water deficiency problems. Targeted sprinklers spray the water directly on to the plants, with minimal amounts of water falling between plant rows. A big advantage of using targeted sprinklers is that conventional sprinklers often have application rates of 3.8 to 4.6 mm h-1, whereas targeted sprinklers commonly have application rates of 2.8 to 3.1 mm h-1. Under windy conditions, because of non-uniform application, targeted sprinkler application rates higher than 3.1 mm h-1 might be needed to protect crops. In one study on the use of targeted sprinklers over grapevines, there was an 80 percent water saving over conventional over-plant sprinklers.

In grower trials, a low-volume system applied approximately 140 litre min-1 ha-1, compared with the grower's conventional system application of 515 to 560 litre min-1 ha-1 to grapevines during two radiation frost events. In the first year, the unprotected minimum temperature was -3.9 °C, but no difference in crop loads or pruning weights were observed between the targeted and conventional systems. In the second year, -5.8 °C was observed on one night and some of the impact sprinkler heads froze up and stopped turning. The frost damage losses were similar in both the conventional and low-volume sprinkler blocks. The grower pointed out that it was important to orient the non-rotating sprinkler heads to obtain a uniform coverage of the vine rows. Consequently, the labour requirement is high. It was also important to start and stop the sprinklers when the wet-bulb temperature was above 0 °C.

Sprinklers over covered crops

Sprinkling over covered crops in greenhouses and frames provides considerable protection. Protection levels of 2.4 °C to 4.5 °C have been observed using an application rate of 7.3 mm h-1 over glass-covered plants. Sprinkling at 10 mm h-1 onto plastic greenhouses during a frost event was observed to maintain temperatures inside up to 7.1 °C higher than outside. The energy use was about 20 percent of the energy used in an identical plastic greenhouse that was heated to the same temperature difference.

Under-tree conventional sprinklers

Under-tree sprinklers are commonly used for frost protection of deciduous tree crops in regions where the minimum temperatures are not too low and only a few degrees of protection are needed. In addition to the lower installation and operational cost, one can also use the system for irrigation, with fewer disease problems and lower cost, so it has several advantages relative to over-plant sprinklers. Limb breakage due to ice loading, soil oxygen deficiency and sprinkler system failure are less of a problem with under-plant sprinkler systems, having lower application rate (2.0 to 3.0 mm h-1) requirements.

Once started, the sprinklers should be operated continuously without sequencing. If water supply is limited, irrigate the most frost-prone areas or areas upwind from unprotected orchards. Good application uniformity improves protection. Hand-moved sprinkler systems should not be stopped and moved during a frost night. However, under mild frost conditions (Tn > -2.0 °C), to cover a larger area the sprinkler lines can be placed in every second row rather than every row. For moderate to severe frosts, closer spacing of the sprinkler lines may be necessary.

Several researchers found that cover crops are beneficial for protection when under-tree sprinklers are used for frost protection. This recommendation is based partially on the idea that the presence of a cover crop provides more surface area for water to freeze upon and hence more heat will be released. The recommendation is also partly based on the idea that the height of the liquid ice mixture and hence the height where the surface temperature is maintained at 0 °C is elevated closer to the tree buds, flowers, fruits or nuts that are being protected. The difficulty in having a cover crop is that although there might be additional protection, if and when the system is used, it is also more likely that active protection will be needed if a cover crop is present. Where water and energy resources are limited and frosts are infrequent, it might be wiser to remove the cover crop and reduce the need for active protection. In climates where frosts are common and there are adequate resources to operate the under-plant sprinklers, then maintaining a cover crop may improve protection. However, energy and water usage will increase.

Under-plant microsprinklers

In recent years, under-plant microsprinklers have become increasingly popular with growers for irrigation and interest in their use for frost protection has followed. More protection is afforded by covering a larger area with a full coverage sprinkler system; however, with microsprinklers, water is placed under the plants where radiation and convection are more beneficial than water placed between crop rows. However, if you spread the same amount of water over a larger area, the ice is likely to cool more than if the water is concentrated in a smaller area. Again, the best practice is to supply sufficient water to cover as large of an area as possible and be sure that there is a liquid-ice mixture over the surface under the worst conditions that are likely to occur.

Trickle-drip irrigation

Low-volume (trickle-drip) irrigation systems are sometimes used for frost protection with varied results. Any benefit from applying water comes mainly from freezing water on the surface, which releases latent heat. However, if evaporation rates are high, it is possible that more energy can be lost to vaporize water than is gained by the freezing process. Because of the wide variety of system components and application rates, it is difficult to generalize about the effectiveness of low-volume systems. One should be aware that operating a low-volume system under frost conditions might damage the irrigation system if freezing is severe. Heating the water would reduce the chances of damage and provide more protection. However, heating may not be cost-effective.

Under-plant sprinklers with heated water

Some researchers have hypothesized that freezing water on the surface to release the latent heat of fusion provides little sensible heat to air. Because of the low trajectory of the under-plant spray, evaporation is reduced relative to over-plant systems, and preheating water might provide some benefit for the under-plant sprinklers. Applying water heated to 70 °C with under tree sprinklers in a citrus orchard was reported to increase temperature by 1 °C to 2 °C on average. Where inexpensive energy is available or water is limited, or both, using an economical heating system to warm water to about 50 °C has been recommended to lower the required application rates. However, the same benefit might be realized by increasing the application rate from say 2.0 mm h-1 to 2.6 mm h-1, so increasing the application rate might be more cost-effective if water is not limiting.

Surface irrigation

Flood irrigation

In this method, water is applied to a field and heat from the water is released to the air as it cools. However, effectiveness decreases as the water cools over time. Partial or total submersion of tolerant plants is possible; however, disease and root asphyxiation are sometimes a problem. The method works best for low-growing tree and vine crops during radiation frosts.

Because of the relatively low cost of flood irrigation, the economic benefits resulting from its use are high and the method is commonly used in many countries. As much as 3-4 °C of protection can be achieved with this method if irrigation is done prior to the frost event. The depth of water to apply depends on the night-time energy balance and the water temperature. Table 2.3 provides an estimate of the depth to apply as a function of the maximum water temperature on the day preceding the frost event.

TABLE 2.3
Depth (d) in millimetres of flood irrigation water to apply for frost protection corresponding to the maximum water temperature (Twx) in °C on the day prior to a frost night

Twx (°C)

35

30

25

20

15

10

d (mm)

42

50

60

74

100

150

Furrow irrigation

Furrow irrigation is commonly used for frost protection and the basic concepts are similar to flood irrigation. Furrows work best when formed along the drip-line of citrus tree rows where air warmed by the furrow water transfers upwards into the foliage that needs protection, rather than under the trees where the air is typically warmer, or in the middle between rows, where the air rises without intercepting the trees. The furrows should be on the order of 0.5 m wide with about half the width exposed to the sky and half under the tree skirts. For deciduous trees, the water should run under the trees where the warmed air will transfer upwards to warm buds, flowers, fruit or nuts. The furrows should be under the trees and 1.0 to 1.5 m wide but should not extend past the drip line.

Furrow irrigation should be started early enough to ensure that the water reaches the end of the field before air temperature falls below the critical damage temperature. The flow rate depends on several factors, but it should be sufficiently high to minimize ice formation on the furrows. Cold runoff water should not be re-circulated. Heating the water is beneficial, but it may or may not be cost-effective, depending on capital, energy and labour costs.

Foam insulation

Application of foam insulation has been shown to increase the minimum temperature on the leaf surfaces of low growing crops by as much as 10 °C over unprotected crops. However, the method has not been widely adopted by growers because of the cost of materials and labour as well as problems with covering large areas in short times due to inaccuracy of frost forecasts. When applied, the foam prevents radiation losses from the plants and traps energy conducted upwards from the soil. Protection is best on the first night and it decreases with time because the foam also blocks energy from warming the plants and soil during the day and it breaks down over time. Mixing air and liquid materials in the right proportion to create many small bubbles is the secret to generating foam with low thermal conductivity. More detailed information on the use of foam insulation is presented in the chapter on active protection methods.

Combination methods

Under-plant sprinklers and wind machines

Under-plant sprinklers with low trajectory angles can be used in conjunction with wind machines for frost protection. The addition of wind machines could potentially increase protection by up to 2 °C over the under-plant sprinklers alone, depending on system design and weather conditions. In addition to heat supplied by the water droplets as they fly from the sprinkler heads to the ground, freezing water on the ground releases latent heat and warms air near the surface. While this warmed air will naturally transfer throughout the crop, operating wind machines with the sprinklers will enhance heat and water vapour transfer within the mixed layer to the air and plants. Typically, growers start the lower cost sprinklers first and then turn on the wind machines if more protection is needed. Unlike using heaters with wind machines, the sprinkler heads near the wind machine can be left operating. Because operating wind machines artificially increases the wind speed, evaporation rates are higher and wind machines should not be used if sprinklers wet the plants.

Surface irrigation and wind machines

The combination of wind machines and surface irrigation is widely practiced in California and other locations in the USA, especially in citrus orchards. Growers typically start with the surface water and turn on the wind machines later to supplement protection when needed. As with under-plant sprinklers, the wind machines facilitate the transfer to the air and trees of heat and water vapour released from the water within the mixed layer.

Combination of heaters and wind machines

The combination of wind machines and heaters improves frost protection over either of the methods alone (e.g. a wind machine with 50 heaters per hectare is roughly equal to 133 heaters per hectare alone). A typical combination system has a 74.5 kW wind machine with about 37 evenly spaced stack heaters per hectare, with no heaters within 30 m of the wind machine. Because the fan and heater operation tends to draw in cold air near the ground on the outside edge of the protected area, placing more heaters on the outside edge warms the influx of cold air. One heater for every two trees on the outside edge and inside the first plant row is recommended. Heaters can be widely spaced within the area affected by each wind machine. There should also be one heater for every two trees inside the second row on the upwind side of the crop. The wind machines should be started first, and the heaters are lit if the temperature continues to fall.

Sprinklers and heaters

Although no research literature was found on the use of sprinklers and heaters in combination, the method has been used. It has been reported that a grower used a round metal snow sled mounted horizontally on a pole at about 1.5 m above each heater to prevent water from extinguishing the heater. The heaters were started first and the sprinklers were started if the air temperature fell too low. This combination reduced ice accumulation on the plants and, on some nights, the sprinklers were not needed.

Forecasting and monitoring

Forecasting the minimum temperature and how the temperature might change during the night is useful for frost protection because it helps growers to decide if protection is needed and when to start their systems. First consult local weather services to determine if forecasts are available. Weather services have access to considerably more information and they use synoptic and/or mesoscale models to provide regional forecasts. Local (microscale) forecasts are typically unavailable unless provided by private forecast services. Therefore, an empirical forecast model "FFST.xls", which can be easily calibrated for local conditions, is included with this book. The model uses historical records of air and dew-point temperature at two hours past sunset and observed minimum temperatures to develop site-specific regression coefficients needed to accurately predict the minimum temperature during a particular period of the year. This model will only work during radiation-type frost events in areas with limited cold air drainage. The procedure to develop the regression coefficients and how to use the FFST.xls program are described in Chapter 5.

Another application program - FTrend.xls - is included with this book to estimate the temperature trend starting at two hours past sunset until reaching the predicted minimum temperature at sunrise the next morning. If the dew-point temperature at two hours past sunset is input, FTrend.xls also computes the wet-bulb temperature trend during the night. The wet-bulb temperature trend is useful to determine when to start and stop sprinklers. FTrend.xls is explained in Chapter 5.

Probability and risk

Probability and risk of damage is an important factor in making frost protection decisions. Several aspects of probability and risk and computer applications are presented in Chapter 1 of Volume II.

Economic evaluation of protection methods

Chapter 2 of Volume II discusses the economics of various frost protection methods and presents an application program to help evaluate the cost-effectiveness of all major protection methods.

Appropriate technologies

Although this book presents information about most known methods of frost protection, whether or not a method is appropriate depends on many factors. Chapter 8 discusses what methods are currently used and discusses what technologies are appropriate in countries with limited resources.


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