Effects of freezing temperatures on 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.

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