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Frost - its nature and control | AG0088 |
W. Thompson, Knoxfield
April, 1995 |  |
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Every spring, fruit growers are faced with the possibility of frost taking part of their hard earned crop.
Nature of frost
On any day of the year the surface soil in an orchard or vineyard goes through a cycle of heat changes. During the day the soil absorbs heat and becomes warmer, (Figure 1); at night the soil, and to a lesser extent the trees, vines and trellis, give off or radiate heat and become colder (Figure 2).
Figure 1. During the day the soil, plants, and vines, absorb and store heat.
Whether the soil surface and surrounding air reaches the frost temperature of 0°C or not depends on:
- the amount of heat stored in the top 300 to 380 mm of soil during the day.
- the amount of heat lost by radiation at night. This depends on the rate of heat loss and length of the night. The rate of loss by radiation is greatest on a clear, still night. Clouds have a blanketing effect.
- the flow of heat from the soil to the radiating soil or plant surface. If this flow of heat cannot keep pace with the heat loss by radiation, then the temperature of the soil surface and the surrounding air drops.
- the moisture content of the air. As the soil and plants cool off at night, the temperature of the air in contact with them may fall below the dew point, causing moisture in the air to condense and form dew. This change from water vapour to water (dew) gives off latent heat which tends to retard the temperature drop. The higher the dew point at night, the less chance of a frost. If the temperature continues to drop, it will reach 0°C and the dew will freeze. This again is a point where the temperature drop is arrested temporarily because latent heat is given out when the dew changes to ice (frost).
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What happens when plant tissue is frosted
Plant tissue contains mainly water held in cells and small conducting vessels. When this water freezes it expands and ruptures the tender cell walls, killing the tissue. When the sap in the damaged tissue melts it leaks away and gives the characteristic " burnt" appearance of a frosted plant.
Figure 2. At night heat is lost from the soil, plants, and vines by radiation and the temperature drops.
The pattern of temperatures on a frosty night
On a calm, clear night, the coldest air is in contact with the soil surface and the temperature increases with height. The height at which the temperature stops increasing and begins to decrease is called the inversion level and this can vary from 8 m to 30 m. During the spring the air temperature at 9 m is often 3°C to 4°C higher than air temperature at 0.3 m. When this is so, we say there is an inversion of 3°C to 4°C in the first 9 m. The greater the inversion of temperature between ground level and 9 m, the better the chance of wind machines minimising frost.
Figure 3. Moist, weed-free compact soil is excellent
Figure 4. Standing cover crop is bad
Principles of frost protection
Although such weather conditions as day temperatures, humidity, and the presence of cloud, over which we have no control play a big part in determining whether there will be a frost or not, there are aspects of management that give a high measure of frost protection.
A moist, compact weed-free soft surface is the key to successful frost control.
Figures 3 and 4 show the effect of different soil conditions on the air temperature at cropping. A standing cover crop in an orchard or vineyard increases the frost risk in three ways:
- By shading the soil it greatly reduces heat stored during the day.
- The cover crop dries out the soil thus lowering its heat capacity.
- The radiating surface, which is the top of the cover crop, is raised. This brings the coldest layer much closer to the fruiting level.
If the cover crop is low and succulent there could be some small compensating influences namely:
- The radiating surface is close to ground level.
- The cover crop being green and succulent would store and subsequently release heat.
- Should heavy dew formation occur within the cover crop then there would be some release of latent heat.
However this practice is nowhere near as good as a clean, compacted soil surface for frost protection.
Straw mulch
A thick cover of straw on the soil increases the frost risk (see Figure 5) for the following reasons:
- It acts as an insulating layer largely preventing heat from being stored in the soil during the day.
- Similarly at night it prevents the heat flow back to the top of the straw which is the radiating surface.
This explains why frost is generally thicker on a layer of straw than on bare ground.
A dry cultivated soil (Figure 6) greatly increases frost risk because:
- A loose, puffy surface greatly reduces heat stored during the day. Firstly, because the air pockets act as an insulating layer, secondly, the heat doesn’t flow well through dry soil and thirdly, dry soil doesn't hold or store as much heat as moist soil.
- At night the heat in the soil finds it hard to get through the air pockets in the loose soil surface to replace the heat lost by radiation.
A cover crop which is mown very close to the soil and bares or exposes a reasonably big area of moist compacted soil offers a fair measure of frost protection (Figure 7). The features of this treatment are:
- The radiating surface is at or near ground level.
- There is reasonably good absorption and storage of heat during the day. At night a fair amount of heat can flow back and reach the radiating surfaces.
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Figure 5. Straw mulch is very bad
Figure 6. Cultivated dry soil is very bad
Figure 7. Slashed cover crop is only fair
Rolling the soil
The effect of rolling on frost protection depends on how well the soil surface can be compacted. Rolling of a dry and/or trashy soil surface compacts the surface very little and has a very limited effect on frost protection. If, however, the soil surface is moist and free of weeds, rolling can give added compaction which increases the frost protection.
The problem of spring weeds
Even following a thorough clean-up of the cover crop, some properties develop a strong growth of spring weeds after the first irrigation or rain. The question often arises "what to do with these weeds". If they are fairly sparse, then leave the soil undisturbed during the frost danger period; but if growth is thick and vigorous and is likely to create a frost hazard or is likely to affect tree of vine growth by competition, then there are three possible solutions:
- Rotary slash as close as possible to ground level.
- Very lightly rotary hoe the mat of weeds when they are only 25 mm or so high and immediately roll. The success of this operation depends on-
- doing it when the weeds are still small and the soil is moist.
- rotary hoeing and rolling must be completed by mid-day so as to permit absorption and storage of heat in the afternoon.
- timing this operation with a weather cycle that is likely to produce rain; this will give added compaction to the soil.
- Spray the regrowth when very small with a "knockdown" weedicide which simply "burns" off the top growth. This effectively bares the soil without the need to disturb the soil surface. However, the method is still fairly costly.
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The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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