Freeze protection methods for crops

Damage to crops by freezing temperatures causes crop yield losses somewhere in Ontario every year. Such damages range from cold set-back of alfalfa in spring to loss of tomatoes in a home garden in the fall. Some of these losses can be prevented. A number of different methods are available for preventing freeze damage to crops. It is important for growers to be aware of these so that they can evaluate which procedures are feasible and economical for combating freeze damage. This Factsheet provides some information on various freeze prevention methods that are available.

The methods are described in terms of active and passive techniques. Active methods are those which are used when the danger of a freeze is present and include such techniques as adding heat and covering crops. Passive methods are those which are used well in advance of the freeze and include proper scheduling of planting and harvesting within the safe freeze-free period, proper crop and field selection, among others. Specific examples of both methods will be discussed in more detail.

The terms frost and freeze are often used interchangeably. In this Factsheet the word freeze will be used for the subfreezing temperature conditions that cause crop damage, and has the same meaning as 'killing frost'. The word frost will refer to the condition that exists when air temperatures drop to the freezing point of water (0°C), or lower, but which may or may not result in freeze damage to crops.

Types of Frost

Frosts are frequently classified as either advective or radiative, depending on the atmospheric conditions under which they occur. An advective frost occurs when cold air from another region moves into an area and winds remain relatively strong. Radiative frosts are produced locally and occur only during clear, calm nights (see OMAFRA Factsheet The Behavior of Frost in Ontario, Agdex 079, Order No. 85-055).

Effects of Freezing Temperatures on Crops

To properly evaluate the benefit of freeze prevention methods it is necessary to understand the effect of below freezing temperatures on the crop(s) concerned. Some effects are well known while others are less clear and require more research. The minimum temperature (known as the "critical" temperature) which must be reached before damage occurs may be influenced by many factors. These include plant species, variety, growth or development stage, plant vigor, soil conditions, surface cover; freeze intensity and duration; thawing conditions, cloud and wind conditions during the freeze; and others.

Many plants have less freeze-resistance when they become mature than during earlier stages of growth. A healthy, growing plant can often withstand a frost better than a weak plant.

The critical temperatures needed for damage to occur may vary depending on the duration that temperatures remain below freezing. For example, buds of fruit trees may be damaged if exposed to -2°C for more than 24 hours, but may survive if exposed to -6°C for less than 2 hours. Thus the critical temperature for a radiative frost lasting for only a few hours in the early morning may be lower than for an advective frost which may continue even during daytime hours.

Thawing conditions often affect the extent of damage after a frost. For example, tobacco leaves which are thawed out gradually after freezing have been known to suffer less damage than if thawing was rapid.

The effect that freezing temperatures have on crops will vary. In some cases it results in a total loss of the plant parts affected. For example, frozen apple blossoms will not produce fruit. In other instances it will only result in a decline in yield or quality. If potato tops are frozen prematurely, the result will be only a partial loss in yield and/or quality of tubers. A premature frost can affect both yield and quality of silage and grain corn as well as other cereal crops. Sometimes a frost can cause a decline in the ability to store a crop. For example, partly frozen potatoes may break down sooner in storage and also cause other healthy tubers to deteriorate. Whether or not freeze prevention methods are economical will depend a lot on the amount of loss in crop yield or quality that results from a frost. Therefore growers should be well aware of the effects of below freezing temperatures on their crops.

Table 1 indicates approximate critical temperatures for some crops grown in Ontario. These are temperatures of the crop, not in a weather screen 1.5 metres above ground. Keep in mind that the crop temperature may be below freezing even though the temperature in the weather screen is several degrees (C) above freezing. (The opposite is also common for crops such as potato tubers that are beneath the soil surface.)

Table 1. Critical temperatures that result in freeze damage to crops

Freeze Protection - Passive Methods

Passive methods used well in advance of the actual freeze danger are probably the most economical and effective. Some are really only common sense and already widely used, but it is nevertheless useful to list them. Following are some examples of passive methods which can be used.

1. Site selection The land selected to grow a frost-sensitive crop should have an adequate freeze-free period. For high risk crops, avoid low-lying fields where cold air tends to drain to and be trapped. Dense windbreaks, forested areas, road embankments or other obstructions can result in "pools" of cold air by preventing the cold air from moving down the slopes. Thus the freeze risk above such obstructions may be increased. However, freeze risk of land areas below windbreaks situated along a slope may be reduced since cold air movement from higher ground is partly prevented. Protective shelterbelts properly located can create a more favourable climate which will promote earlier maturity in heat-loving crops and thus reduce the risk of freeze damage in the fall. Locations near large water bodies are usually less prone to frost as air masses over water cool less rapidly at night than over land. Coastal areas frequently experience land breezes at night which help prevent frost. Planting orchards on north-facing slopes has helped to delay blooming until the danger of frost is past in some areas.
   
   Growers should know the risk of spring and autumn frost in their area and be aware of the variations that can be expected on their farms. Typical temperature patterns that exist in both the horizontal direction as well as vertically above crops under different types of frost situations have been described in another OMAFRA Factsheet, The Behavior of Frost in Ontario, Agdex 079. Ideally it is desirable to know when susceptible plant parts are likely to reach critical temperatures needed for damage to occur. While much information can be gained through experience, it may sometimes be helpful to make measurements of minimum temperature at various locations on the farm, particularly if the terrain is hilly or if the farm is near a lake. Temperature records can also be useful for adjusting minimum temperature forecasts for on-farm conditions. Information from nearby climate stations may often be helpful in determining frost risk, even though the station may not always represent on-farm conditions accurately. In general, it is best to seek assistance from an experienced agricultural meteorologist or climatologist if temperature data are to be collected and interpreted.
2. Land clearing Thinning hedgerows or clearing forested areas can sometimes reduce the risk of frost in sloping terrain by allowing cold air to drain to lower areas. It is preferable to seek professional advice before attempting this method since sometimes it can increase frost risk below the windbreak. In small clearings in forests, the risk of frost increases with size of clearing up to about one hectare. However, as clearings become larger than a few hectares the risk of frost is usually lowered by allowing for more air movement.
3. Crop management Select crop species and varieties which will mature within the available freeze-free period. For example, when growing grain corn, hybrids that reach maturity before killing frost should be selected. If spring frost is a threat to strawberry blossoms, growing late-flowering varieties or delay removing the straw mulch may help. Dwarf apple trees may be more likely affected by frost than taller varieties since air layers near the ground tend to be colder than at higher levels during nights with frost.
4. Plant and harvest frost sensitive crops within the available freeze-free period on your farm. Plant early enough to ensure crops are mature before killing frost in the fall. It may be advantageous to take a slightly bigger risk in spring than in fall with some crops which can be replanted if freeze damage does occur. Plant around a relatively low risk date (e.g. one which would result in freeze damage in less than 1 yr in 10, or 10% risk) rather than the average date which would result in damage in 5 yrs out of 10 (50% risk). If planting before the risk of freezing temperatures is over in spring is desirable to capture early, higher priced markets, then only plant out acreage which can be protected by one of the active freeze prevention methods. Know the risk of experiencing frost in spring and fall in the field in which the crop is being grown.
   
   Some plants can be hardened to withstand frost by exposing the seeds or young seedling plants to varying temperature conditions, although much of this work is still in the experimental stage. Greenhouse plants are often hardened by exposure to outside conditions prior to transplanting in the field. Treatment of seeds with certain chemicals has been shown to increase hardiness in some plants. Application of proper amounts of required nutrients can also help to maintain plant hardiness.
5. Soil management The condition of the soil will affect the risk of freeze damage to both above and below ground plant parts. Loose soil surfaces reduce conduction of heat to the surface at night and therefore tend to have lower surface temperatures than compacted soils. Thus it is advisable not to cultivate the soil just before a killing frost is expected if plant parts near the ground need to be protected.
   
   Moisture in the soil has some counteracting effects. Excessively wet soils gain less heat energy during the day as more of the sun's energy goes into evaporating moisture. This can reduce the heat available to the crop at night. On the other hand excessively dry soils are poorer heat conductors and are able to store less heat, and therefore result in a higher risk of frost. A dry peat soil is a particularly poor heat conductor and has a very low heat storage capacity, so that nighttime minimum temperatures over such surfaces may be considerably lower than over mineral soils. It may be possible to improve the heat characteristics of peat soil by the addition of mineral soil.
   
   Mulches on the soil surface increase risk of frost by behaving as insulators. Less heat is absorbed by the soil during the day and less is released at night. Mulches can help to avoid freeze damage, however, if they completely cover the sensitive plant parts. Delaying the removal of straw mulches in strawberries in spring can sometimes help to delay the bloom date past the time when there is danger of frost. However, the straw will also delay the warming of the soil, and if it remains underneath the blossoms at the time of frost, it will increase the risk of damage.
   
   Cover crops under orchards act similarly to mulches and thus can increase the risk of frost. They may have other beneficial effects, however, such as reducing soil erosion, which outweigh the freeze risk factor.
   
   Covering of plant parts beneath the soil surface with a layer of soil is a way of protecting against frost. Well-hilled potatoes are less prone to experience frost damage to tubers than if the hills are poorly formed. Dry soils cool more rapidly near the surface and therefore adding moisture to the soil may sometimes help to reduce risk of frost damage to tubers.
   
   While soil management practices may only provide a few degrees (C) of protection, even this small amount could affect freeze dates by 1 to 2 weeks or more, and in some cases could mean the difference between a total loss of the crop and relatively little damage.
   
   The above passive methods of freeze protection are worth taking so that active methods are not necessary in most seasons because the latter are expensive and can only be afforded when the crop has a high value per unit area.

Freeze Protection - Active Methods

Active protection takes place just before and during the occurrence of the frost after a warning has been issued in the weather forecast. They are usually only effective under radiative frost conditions when winds are light or calm, and are most suitable in low-lying, frost prone areas. Advective freezes usually cannot be prevented by active means.

It is very important to have good forecasts of on-farm minimum temperature and wind conditions for active freeze protection. Moreover, knowledge of the critical temperatures that cause crop damage is needed. Farmers should know the nighttime temperature variations as they occur over their land and which fields are most prone to frost, so that action can be taken in these fields first. The basic concept of these methods is very simple. They either depend on the reduction of heat loss from the surface, stirring the air to break up the temperature inversion, or adding heat to maintain the temperature above the danger point.

In order to determine if it is economical to invest in the equipment, materials and labor for active freeze protection many factors must be considered. These include the degree of risk, the likely duration and severity of frosts, value of the crop, and effectiveness of the method to be used. Some of the active methods are described below.

1. Covering This method reduces heat loss from the surface. Home gardeners and growers of small acreages of low-growing commercial crops often use materials such as straw mulch, boxes, tar paper, plastic, etc. to reduce the heat loss from the surface. The cost of the materials, their storage and the time and labour needed to place the covers are the main drawbacks to this method for large areas of crops. Foams have also been used experimentally to protect plants but materials and applicators are not readily available on a commercial basis.
   
   Some materials are more effective in reducing radiative heat loss than others. Clear plastic may transmit some long-wave radiation whereas dark, opague covers do not. Any cover is effective in reducing heat loss by convection. When covers are placed, particularly thin materials such as plastics, care must be taken to prevent contact with the plant to reduce heat loss by conduction, as the temperature of the exposed surface is usually lower than the air below it. Straw mulches should cover all plant parts as any protruding leaves are more susceptible to freeze damage. Mulches underneath plants prevent heat coming out of the soil at night from reaching these plants and thereby result in lower plant temperatures.
   
   Covers should be removed during the day as air humidity would be higher under the cover and this would increase the danger of certain plant diseases.
2. Fog or smoke Clouds and fog are well-known for their ability to reduce radiative heat loss from the surface. Smoke from smudge pots or burning tires or refuse and mist from fine water nozzles have been used in attempts to reduce this heat loss. Since it is difficult to maintain the smoke over the sensitive crop area and to produce droplets the optimum size to intercept the long-wave radiation, this method is not very effective. In addition, our environmental laws now prohibit the use of this method, where smoke is involved.
3. Wind Machines During freezes which occur on calm, clear nights, the air layer near the ground is colder than the air aloft. This is known as a temperature inversion. Wind machines or helicopters are sometimes used to bring the warmer air down to the crop level to replace the cold air layer at the surface. This method can be effective when there are large temperature differences between air layers near the surface and those up higher. Equipment and operating costs are high. Effectiveness varies in the range of 1 to 4 degrees C.
4. Sprinkling A very low rate of application of water through irrigation can be effective in preventing freeze damage through the release of heat during cooling and freezing. Protection from freezing temperatures as low as -6°C have been reported for low growing berry and vine crops, when 1.5 to 2.5 mm per hour of water was applied.
   
   Sprinkling of the crop should begin with the onset of freezing conditions and a film of water continuously maintained until temperatures have risen above the freezing level (0°C). If sprinkling is discontinued prematurely, heat will be drawn from leaves to melt the ice and freeze damage may result. This method creates another problem if the frost lasts too long, because the plants must be able to support the added weight of ice that builds up on the leaves and branches. A forecast of the duration that temperatures are expected to remain below freezing is very useful when using this method.
   
   In spite of the problems, this method has proven effective for low growing crops such as strawberries, tomatoes, beans, cucumbers, peppers and squash as well as vine crops and tree fruits. It is important to recognize that this method only prevents the temperature of the protected plant from falling below the freezing point. It does not warm the plant parts nor does it raise the air temperature appreciably. Moreover, sprinklers need to provide constant, uniform coverage.
   
   There is controversy over the use of irrigation as a protection method prior to frost occurrence. The added moisture has the beneficial effects of increasing the capacity of the soil to store heat and improving conduction of heat to the surface. Nevertheless, heating of the soil during day time is reduced because increased evaporation uses up heat energy. Moisture may also change the critical temperature which is needed to cause freeze damage to a crop. Since there are counteracting factors, a general recommendation cannot be made.
5. Heating This method is intended to add enough heat to the layer of air surrounding the crop and through radiant heat to the crop to maintain the temperature above the freezing point. Many small heaters uniformly spaced throughout the crop are the most effective in doing this. Large fires or heaters create a "chimney effect" and draw cold air in at the surface, which may create colder conditions in parts of the crop area (Figure 1).

   Fuel costs are high whether solid fuel bricks, oil or propane gas heaters are used. Capital and labour costs add to the expense and therefore only crops which have a very high value per unit area can be protected from frosts using this method.

   Taller crops such as grapes and tree fruits are protected most effectively. The best results occur when the air is calm, so that a steep temperature inversion exists. This method can provide protection from frost as low as -4° C.

Summary

Crops can be protected from freeze damage by proper site selection and certain crop and soil management practices (passive methods) or by taking action when frost warnings are issued. The various passive and active methods of freeze prevention are described herein. The terms frost and freeze are defined and the atmospheric conditions in which active freeze prevention can be taken. No discussion of the economics of freeze prevention is included.

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Figure 1. Temperature inversion under radiative frost conditions.