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Frost Damage, Control and Prevention - Fruit and Vines - Fact

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					                                                                        FACT SHEET

Frost damage, control and prevention - fruit and vines
Introduction
Adequate winter chilling is essential for the normal seasonal cycle of winter dormancy
and spring growth of deciduous trees and vines. Most horticultural crops are sensitive
to frost in early spring as buds and shoots emerge.
A severe frost in spring can cause damage, resulting in partial or total crop loss.
In extreme cases plant death may occur.
Storing heat in the soil is the best form of protection against frost damage.
What is a frost?
The term “frost” is rather a loose one. In its most general sense it refers to the deposit
of crystals of ice on the ground and other exposed surfaces when the air temperature
falls to the freezing point of 0°C. However the temperature at which ice forms varies
with humidity and as a result of this we have two forms of frost – a white frost and a
black frost.
White frost
If the air is moist at the time of the frost, dew will form on the ground before the
temperature falls to O°C. When this temperature is reached, the dew freezes to give a
coating of whitish ice crystals on exposed surfaces and hence the name “white frost”.
Black frost
When the dew point (ie, the temperature at which dew forms) is below 0°C, frost
develops without any visible sign of white ice appearing on exposed surfaces.
Frost danger period
The frost danger period for deciduous crops extends from bud burst to about the 7th of
November. However, records over nearly 40 years show that the most critical periods
in the Riverland occur between September 18th and October 12th. By September 18th
most of the fruit shoots on vines and stonefruit have emerged and frost at this stage
can cause great damage.
After October 12th, the weather gradually warms up and an increasing amount of heat
is stored in the soil. Furthermore the nights are shorter, reducing the time that the soil
loses heat by radiation.


Type of frost
There are two types of frost, radiation and advection frost. The radiation frost is the
most common in South Australia. The two types of frost are not distinct and many
nights have characteristics of both.
Radiation frost
Radiation frosts begin at ground level and gradually rise. Radiation frosts occur when
there is very light or no wind, temperature inversions, clear skies and low humidity.
During the day the soil and plants absorb and store heat. At night the heat is lost by
radiation particularly when there is no barrier (ie clouds) to prevent this heat loss.
Clear skies and light winds allow any stored heat energy in the soil to escape easily.
As a general rule, air temperatures drop by about 1°C per hour when conditions
favour the development of a frost.
Advection frost
This type of frost occurs when a cold air mass moves into the area displacing warm
air. In Australia this usually occurs when a cold Antarctic air mass moves over the
Australian mainland. When this type of frost occurs some characteristics of radiation
frost are also seen as factors causing radiation frosts can still be present. Advection
frosts can occur earlier in the night and while the freezing process is the same as for
radiation frosts the effects are usually seen on the upper part of the plants.
Frost prediction
The Bureau of Meteorology considers a number of factors when estimating the
probability of a frost. These factors include:
General weather situation
Particularly favourable for frost development is the end of a depression, which is
associated with a clearing of cloudy and possibly rainy conditions during the day to
give a clear, still night.
Wind
Conditions of no wind are most favourable for radiation frost development.
Cloud
A cloudy day followed by a cloudless night produces the greatest frost risk. The
cloudy day reduces the sun’s ability to heat the soil surface and when followed by a
clear night what little heat is stored quickly dissipates. However, only a small amount
of cloud at night is required to prevent a frost from developing.
Dew point
The dew point is the air temperature at which dew or condensation begins to form.
Low dew point indicates dry air, which means that very little heat is released as the air
cools and as moisture, condenses. This in turn leads to a rapid fall in temperature
and a greater likelihood of frost.
Experience has been that if the dew point in the evening exceeds about 5.5°C then
there will be sufficient radiation reflected back by the atmosphere (of heat lost from
the ground surface) to retard the rate of cooling and limit the probability of a frost. A
dew point of less than 2.2°C indicates a high probability of frost.
Orchard design
As cold air tends to drain to the lowest point, avoid barriers across the line of airflow
such as buildings and banks of trees, which may pool cold air. Frost sensitive crops
should not be grown in low areas where cold air is trapped by natural topography,
vegetation or by manufactured obstacles that inhibit drainage of cold air.
Site selection is a primary decision to reduce frost damage in sensitive crops. Select
tolerant varieties for frost prone areas. Late pruning in stonefruit and vines delays bud
burst and crop development and reduces exposure of the crop during high frost risk
periods.

Trellis height of grapevines
Temperature increases significantly from ground level to a height of 1.5 m. The
difference in trellis height from 0.75 m to 1.5 m may mean an increase in air
temperature of 1°C. This can reduce the frost risk significantly.
Frost damage
The extent of damage depends on the stage of growth of the crop and the time that
the temperature remains below the danger point for that stage of growth. Damage
occurs when the plant tissue is frozen. Water within the plant cells expands as it turns
to ice rupturing the cell walls.
Young spring shoots and blossoms are more easily damaged than the older limbs
because the cells have thinner walls and higher water contents.
Symptoms
Minor frost damage appears as small whitish – yellow areas on young growth leaves,
which eventually turn brown and fallout leaving tattered and distorted leaves.
Severe frosts cause very distinct damage, shoots and flowers of stonefruit turn a
brown colour and appear limp only hours after freezing. Fruit develops a blackened
heart and may fall over the next few days.
Damage to citrus fruits can be identified a day or two after a frost by the watersoaked
appearance inside the fruit. Rind burn can occur, but often there is damage within the
fruit without visual symptoms.
The tables below illustrate the critical temperatures for stages of development of crop,
at which significant damage can occur.


                                            Stone Fruit

          Growth stage                                           °C
                                              Apple    Apricot   Pear    Peach     Plum
          Buds closed but showing colour      -2.8     -1.1      -2.2    -3.9      -1.1
          Full Blossom                        -1.7     -0.6      -1.7    -2.2      -0.6
          Small, green fruit                  -1.1     0         -1.1    -1.1      -0.6

                                              Citrus
                        The temperatures at which tissues begin to freeze are:
          Variety                                                            °C
          Oranges, Grapefruit, Mandarins
          Green Oranges                                                 –1.4 to –1.9
          Half ripe oranges, Grapefruit and Mandarins                   -1.7 to –2.8
          Ripe Oranges, Grapefruit and Mandarins                        –1.7 to – 3.9

          Lemons
          Buds and Blossoms                                                  -2.8
          Button Lemons ( up to 12mm diameter)                           –0.8 to – 1.4
          Green Lemons (greater than 12mm diameter)                      –1.4 to –2.8
          Tree Ripe Lemons                                               –0.8 to –3.3
                      Reference: (Opits,K.W., Brewer, R.F., and Platt,R.G., 1979)

                                             Grapevines

                   For grape vines the temperatures at which damage occurs are:

                          Growth Stage                             °C
                          Woolly bud stage                        -3.5
                          Early Bud Stage                         -1.1
                          Shoots up to 150 mm                     -0.5
                          Shoots over 150 mm                        0

Predisposing factors
Frost damage predominantly occurs in low-lying areas, where there is dense ground
cover, or where sunlight cannot warm the ground during the day.
Control
The key principle to preventing frost is good soil management which maximises heat
storage in the soil during the day, for release at night. Control measures should be
aimed at increasing the temperature around the buds and the shoots.
Cultural control measures
A weed free, compact, moist, soil maximises the amount of heat that can be
stored during the day.
Clean up the soil surface
Most orchards have an inter-row sward, consisting of either a cover crop or
combination of weeds. The use of this system to benefit soil structure is preferred to
frequent cultivating. However, if vegetative cover is present this insulates the soil
during the day, not allowing any substantial bank of heat to accumulate. Slashing
close to the ground or spraying with a knockdown herbicide before bud burst is
essential in known frost prone areas. This is a sensible compromise to cultivation
which can harm soil structure.
Compact the soil surface
If cultivation is carried out, the soil must then be compacted and irrigated. Air pockets,
as a result of cultivation insulate the soil and reduce its ability to store heat.
Cultivation should not be carried out during the frost prone period as it also removes
heat from the soil because water brought to the surface evaporates and removes heat
in the process.
Irrigate lightly
Moisture in the soil stores a considerable amount of heat, and moist soil conducts
heat more readily than dry soil. Heavy irrigation gives no more frost protection than
light irrigation, with the added danger of damage to feeder roots. Aim to wet up only
the top 30 cm to 40 cm of soil. This can be achieved by applying about 10 to 15 mm
of irrigation on a weekly basis.
Full cover irrigation will give up to 2°C more soil temperature than drip irrigation
due to the greater area of moist soil.
Mechanical frost control
Mechanical methods include the use of overhead sprinklers, burning oil in heaters or
frost pots and wind machines. Mechanical systems of frost control are generally
expensive to operate.

Before undertaking mechanical measures for frost protection every possible
effort should be made to reduce the frost hazard through the cultural means as
outlined earlier.
Overhead irrigation systems
The most common method of active frost protection is the use of overhead sprinklers
while the frost risk is present.
When water is applied to crops under zero or sub zero temperature conditions, it
freezes. On freezing the water releases heat, which offsets the heat lost by the crop
to its cooler surroundings. To achieve successful frost control a film of continuously
freezing water must be maintained on the surface of the formed ice.
When the ice begins to melt, usually after dawn when the air temperature begins to
rise, heat must be applied to the ice or this heat will be drawn from the crop. This
heat is supplied by water applied to the crop via the irrigation system.
Generally an application rate of 3.5 mm/hr is adequate for frosts down to –4.0°C.
Undertree sprinklers used during frost will give some protection but not at the same
level as overhead sprinkler systems.
Although overhead sprinklers are an effective means of frost control, they can have
two major disadvantages.
Firstly, during severe or prolonged frosts, ice can build up on the tree limbs to the
extent that limbs break.
Secondly during a long series of frosts, and when frosts begin early in the night,
sprinklers must be operated for many hours; soils can become waterlogged promoting
the development of root rots such as phytophthora.
Orchard heaters
Heaters or frost pots are only effective in radiation frost conditions. The burning of oil
in frost pots heats up air creating localised convection currents which cause a mixing
of the layer of air from the ground up to the inversion layer thus keeping the air
warmer at crop height. Many pots are required to create this convection,
approximately 120 to 170 per hectare.
Frost pots are expensive to operate and buy. They are also not readily obtainable.
However frost pots maybe worthy of consideration for crops of high monetary value
located in a small-localised high frost prone areas such as nurseries.
Wind machines
The effectiveness of wind machines needs to be assessed for individual properties.
Wind machines operate by drawing warm air down from above the inversion layer that
exists at times of a radiation frost condition. They force cold air from the treatment
area and replace it with warm air from above.
Before investing in a wind machine, a grower should research several factors
including orchard topography, height of the inversion layer and the temperature
gradient to the inversion layer. Depending on conditions and topography wind
machines can provide approximate temperature increase of around 2°C and protect
an area of around 4 to 5 hectares.
It is important to check with neighbours on the noise factor of these machines.
Frost alarms
In recent years electronic frost alarms have become available. Electronic sensors are
placed in the orchard and record the temperature changes. The sensors can be pre-
set to a certain temperature. When that temperature is reached, an alarm sounds via
a radio link, alerting the grower to a possible frost, enabling him to take preventative
steps such as activating the irrigation system.
The placement of these sensors in known frost prone areas minimises the risk of
substantial crop losses.



Last update: August 1998
Agdex: 200/10
Author:
David Pocock and Ashley Lipman, Industry Consultants, Renmark and Waikerie.

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