Freeze Protection Methods
Table of Contents
- Types of Frost
- Effects of Freezing Temperatures on Crops
- Freeze Protection - Passive Methods
- Freeze Protection - Active Methods
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
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
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
Critical temperature for freeze damage
Very tender crops
Strawberries and raspberries (blossom and fruit), tomatoes,
cucumbers, melons, peppers, squash and pumpkins (plants),
0 to -1°C
Potatoes, corn, apples (blossoms), pears (blossoms and fruit),
plums (blossom), cherries (blossom and fruit), beans
-l to -2°C
Half hardy crops
Apples (fruit, buds), blueberries, alfalfa, pears
-2 to -4°C
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.
- 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.
- 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
- 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.
- 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.
- 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
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
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
- 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.
- 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.
- 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.
- 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
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
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
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
- 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.
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.
Figure 1. Temperature inversion under radiative