A prudent farmer does not grow frost-susceptible crops during winter in areas he knows to be frosty; but he may take a gamble and plant an area that is occasionally hit by frost in winter.

Early frosts can damage autumn crops, and late frosts can damage spring crops. Late frosts can also kill or damage the spring flush of deciduous crops once they have broken dormancy.

In horticultural areas of Queensland where frosts occur, early frosts may be expected as early as April, while late frosts may be expected as late as September. Whenever the frost occurs it may cost the grower his livelihood, and frost management should be part of his overall farm management program. Frost damage can be avoided by careful selection and location of crops, while more sophisticated techniques are available to prevent damage to crops that are exposed to frost. But first you should know the enemy, and then evaluate the management resources available to you.

Frost Damage
When plant tissue freezes, ice crystals form and rupture the cells, causing the typical scorched appearance of frosted plants. But because of the presence of chemicals in the sap, plant tissues freeze at temperatures lower than 0 degrees C - the freezing point of water. This is an important consideration in the prevention of frost damage.

Crop Susceptibility
Some commercial crops are more susceptible than others to cold or frost damage, but most crops will be damaged by a heavy frost. The crops most susceptible to frost are the solanaceous and cucurbit crops, beans, choko, ginger, sweet corn, sweet potato, banana, papaw, pineapple and most flower crops. These crops should never be grown in an area where frosts are likely to occur. Common crops that are tolerant of light frosts are carrot, celery, peas, citrus, custard apple, lychee, macadamia and mango. Although avocado trees and passionfruit vines will tolerate light frosts, their rootstocks may be damaged.

The fruit of strawberries are less tolerant of frosts than the plants. Most other common horticultural crops are relatively resistant to frost damage. This category includes the flower crops carnations, roses and strelitzias. Deciduous crops such as pome and stone fruit, pecans, grapes and kiwi fruit are able to withstand heavy frosts during their dormant period. However once dormancy has broken, late frosts can cause considerable damage to the spring growth flush.

Frosty Situations
While heated air rises, cold air sinks to low spots in the land. The more cold air there is, the greater is its depth on the ground. This is why under Queensland conditions, frosts are relatively common on still winter nights and in the low-lying pockets of land. Only the heavier frosts overflow the low pockets with cold air and reach more elevated land.

Preventing Damage
Belts of trees, buildings or even patches of weed can trap cold air, preventing its flow to lower areas. Removing these barriers to air drainage is very important in preventing frost damage. Frost-sensitive crops are often planted on hillsides to escape frosts by elevation and also by rapid drainage of cold air down the slope. Moist compact soil absorbs the sun's heat readily and warms during the day. At night this stored heat is radiated, warming the air close to the ground by about 1 degree C. This is sufficient to prevent plant damage by a light frost.

On the other hand, dry loose soil is an insulator, and is not warmed by the sun. Similarly, a dry surface mulch insulates the soil from radiation, offering no benefits in frost prevention. Young trees may be damaged by frost. As a tree grows, its foliage gets progressively higher above the frost layer; also the bark hardens and insulates the growing wood from frost damage. The trunks of young trees may be insulated by wrapping them tightly with insulating material such as newspaper or sisal kraft.

Pineapple fruit are often protected by encasing them in paper bags. Growers sometimes light smudge fires to prevent frost damage. The smoke drifts to the low areas where frost is most likely to occur, providing a screen against radiation losses and causing further condensation of dew to wet the ice on the plant surfaces.

While this might be expected to prevent the temperature of the plants from falling below 0 degree C, there is no convincing evidence to confirm that this technique is effective. Fire and heaters are often used to warm the cold air in areas where heavy frosting is common and where costly control measures are warranted. However, the heat generated normally rises and has little effect in preventing frost damage unless used in association with wind machines.

Wind Machines For Frost Protection
On a clear, still night, cold dense air collects close to the ground, while warm air rises and floats on top of the cold air. Long-bladed rotating fans mounted high above the ground are designed to draw down the warm air and mix it turbulently with a lower-lying cold air.

The volume of air moved by the fan is more significant in frost protection than in the velocity of the moving air. Thermometers are usually mounted at various heights on the tower that supports the fan. When frosting is imminent, they may activate an alarm or an automatic switching device. A single wind machine may service up to 4 ha of a mature orchard or a larger area of young planting.

Under very cold conditions, heaters are sometimes used in association with wind machines. Wind machines are very expensive and should be selected, sited and managed with a clear understanding of how frosts affect the particular orchard and how wind machines work.

Sprinkling For Frost Protection
At 0 degrees C the dew on a plant will freeze. As the temperature drops further, the plant sap will eventually freeze, causing considerable damage to the tissues. However, as a mixture of ice and water will not cool below 0 degrees C, further cooling of plant tissue can be prevented by overhead sprinkling when the temperature drops to 0 degrees C.

The sprinkler system should apply water at a rate just sufficient for the leaves to drip. If the application rate is too low, all of the water will freeze and the temperature of the plant tissues will continue to fall. If the application rate is too high, the excess water will turn the soil into a quagmire. However, the heavier the frost, the higher the application rate should be. For most situations, an application rate of 2 to 4 mm per hour is sufficient, with the sprinkler rotating at least once each minute.

In a heavy frost with this system, the weight of ice that builds up on the plants may be sufficient to break branches or stems.

Saving Frosted Plants
Frozen dew may be washed off the plants before they are exposed to the warm sun. If the ice is left to thaw in the sun, surface scorching may develop on the leaves and flower petals. But once the internal plant tissue has frozen, no treatment will prevent the rupture of the cells as the ice forms and again as it thaws.

Monitoring The Temperature
The temperature in a crop may be monitored conveniently with a thermostat mounted at the height of the crop. When the temperature reaches 0 degrees C, the thermostat activates either a system to automatically turn on the sprinkler, or an alarm to alert the grower to turn on the sprinkler manually. Frost damage can be avoided, but planning should begin before the crop is planted.

Extract from Queensland Fruit & Vegetable News

DATE: July 1986

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