Protection By Sprinkler Irrigation
Anthony W. Tyson1, Mr. Ronald E. Sneed2, Dr. Charles M. Mainland3,
Katie B. Perry4, Dr. E. Barclay Poling4, Dr. Douglas C. Sanders3,
and Dr. Richard Unrath3
can cause severe crop losses for man fruits, vegetables and nursery crops.
The most severe damage usually occurs when a freeze or frost takes place
after buds and blossoms have begun to open. A severe freeze can also damage
fruit which is already set, damage foliage, and can even kill limbs or
entire plants. The degree of injury inflicted by low temperatures depends
on a number of factors, including:
- the type of plant
- the stage of development
of the crop. Most crops will not be damaged if a freeze occurs while
they are dormant but can be severely damaged once buds and blossoms
begin to open.
- the amount of
leaf cover over the blossoms and fruit. Leaf cover can provide some
protection particularly against frost damage.
- the severity and
duration of the freeze. The color the temperature and the longer the
freeze period, the more severe the damage will be.
- wind speed. A four
mile per hour wind will prevent frost from forming as long as the temperature
remains above 32°F.
Various means have
been used by producers to minimize the effects of freezing temperatures.
Some of the more common methods are orchard heaters, wind machines, and
overhead sprinkler irrigation.
Extension engineer, University of Georgia
2 Professor and extension Specialist, Biological and agricultural
N. C. State University
3 Professor, Horticultural Science, N. C. State University
4 Assoc. Professor, Horticultural Science, N. C. State University
have been used for centuries to protect orchards. Most heaters are designed
to burn oil and can be placed as freestanding units or supplied by a
pipeline network throughout the orchard. Propane, liquid petroleum and
natural gas have also been used as fuels. The initial cost is generally
lower than for other systems, but the cost of the fuel makes this system
the most expensive in terms of operating cost.
Wind machines can
be effective during a radiation frost. Their purpose is to circulate
warmer air down to orchard level. A single wind machine can protect
up to 10 acres. A typical wind machine is a large fan about 16 feet
in diameter mounted on a 30 foot steel tower. The fan is typically powered
by an industrial engine delivering 85 to 100 horsepower. Helicopters
have been used as wind machines. They hover in one spot until the temperature
increases, then they move to the next area. Repeated visits to the same
area are usually necessary during a typical frost.
is probably the most commonly used means of frost/freeze protection
in the southeastern United States. Heat lost from the plant part to
its environment is replaced by heat released as the applied water changes
to ice. As long as water is supplied at an adequate rate the temperature
of the plant will remain at or near 32° F. Advantages of overhead
irrigation include lower operating cost, convenient to operate and multiple
uses including drought prevention, heat suppression, fertilizer application
and possible limb breakage from heavy ice loads.
systems have also been used successfully for freeze protection especially
in citrus groves in Florida. This system utilizes the sensible heat
of the water to raise air and foliage temperatures. This system does
not provide the same degree of protection as overhead irrigation because
les heat is liberated than by the latent heat of fusion released when
ice is formed on the tree surfaces. A major advantage of the undertree
system is that limbs are not broken under heavy ice loadings. This can
be a serious problem when using overhead irrigation on mature trees
during an extended freeze.
This paper is intended
to present the principles and methodology of frost/freeze protection
using overhead sprinkler irrigation systems.
PRINCIPLES OF FROST
PROTECTION BY IRRIGATION
Advective Freeze vs.
The term frost and
freeze are often used interchangeably; however they are two separate
conditions. An advective or windborn freeze occurs when a cold air mass
moves into an area bringing freezing temperatures. Wind speeds are usually
above five miles per hour and clouds may be present. Due to the high
wind velocities cold protection is very limited during an advective
A radiation frost
occurs on clear nights with calm winds. Air temperatures near the ground
surface drop below freezing and there is normally warmer air higher
in the atmosphere. This phenomenon is known as an inversion. Radiation
frosts are usually shorter in duration than an advective freeze. Frost
protection by irrigation is most effective during radiation frost conditions.
Irrigation for Frost/Freeze
In using overhead
irrigation for frost/freeze protection the heat lost from the plant
part to its environment is replaced by the heat released when water
changes to ice. Specifically, as one pound of water freezes, 144 BTU's
of heat energy are liberated. This is called the latent heat of fusion
of water. As long as liquid water is freezing on the plant at all times,
the surface temperature will remain at or near 32° F. Adequate water
must be applied to compensate for heat losses by radiation, convection
If the irrigation
rate is not adequate, the damage may be more severe than if no protection
had been provided. If wind velocities are high and/or if relative humidities
are low water may evaporate from the plant surfaces. If this occurs
evaporative cooling will actually lower the temperature of the plant.
As one pound of water evaporates 1080 BTU's of heat energy are absorbed
from the surrounding environment. When compared to the 144 BTU's released
by freezing it becomes apparent that 7 ½ times more water must
be freezing than evaporating in order to have a net heating effect.
For this reason frost/freeze protection by irrigation is not usually
recommended if wind velocities exceed five miles per hour. If the relative
humidity is low, the sprinkler system should be stared at a higher than
usual temperature to compensate for the evaporative cooling that will
occur as the first water strikes the plants.
systems should be started when the air temperature in the orchard reaches
34° F. It should operate continuously until the air temperature
increases to above 32° F. and the ice on the plants has begun to
Crops and Critical
The ultimate goal
of frost/freeze protection is to prevent plant parts (particularly the
flowers and fruit) from being damaged by temperatures that drop below
the critical level. This critical temperature varies from crop to crop
and also depends on the stage of flower and fruit development. Table
1 gives the critical temperatures of several tree fruits at various
stages of development. The dates that crops reach these stages depends
on the geographic location and local climate.
Table 1: insert here
DESIGN OF OVERHEAD
IRRIGATION FOR FROST/FREEZE PROTECTION
and Sprinkler Spacing:
The desired precipitation
rate for adequate cold protection will depend on the crop to be protected.
For low growing crops such as strawberries, the precipitation rate should
be between 0.12 and 0.15 inches per hour; medium sized plants such as
blueberries and grapes need 0.14 to 0.16 inches per hour; and large
trees such as apples and peaches need 0.16 to 0.18 inches per hour.
As a general rule precipitation rates should not exceed 0.2 inches per
hour. At these high rates run-off becomes excessive and excessive ice
build-up on trees causes increased damage from broken limbs. Table 2
indicates the degree of protection afforded by various precipitation
rates at several wind velocities.
rate applied by a sprinkler irrigation system will depend on the sprinkler
discharge and the spacing between sprinklers. Generally, the desired
sprinkler spacing will be determined first and then a sprinkler will
be selected with an appropriate discharge rate and wetted diameter.
Sprinklers are typically
spaced in the range of 40x40 feet to 80x80 feet. The precise spacing
will often depend on the spacing of the crop to be watered. For tree
crops, it is desirable to have a row of sprinklers in every other tree
row in order to provide adequate coverage over the whole tree. Also,
the outside row of sprinklers should be located at the edge of the field
to provide adequate coverage for the outside row of trees.
In order to provide
uniform coverage, sprinkler spacing should be from 50 to 60 percent
of the wetted diameter of the sprinkler. Under no circumstances should
the spacing exceed 70 percent of the effective sprinkler diameter. Typically,
every other row of sprinklers is staggered to provide a triangular pattern;
however sprinklers may also be spaced in a square or rectangular pattern.
Once the spacing
is determined, the desired sprinkler nozzle capacity may be determined
using the equation for calculating precipitation rates:
R = 96.3 x Q
S x L
Where: R = application rate (in./hr.)
Q = output of one sprinkler (gal. /min.)
S = spacing between sprinklers (ft.)
L = spacing between rows of sprinklers (ft.)
3 gives precipitation rates for selected nozzle capacities and sprinkler
On orchard crops
the sprinkler systems are typically permanent set with buried PVC pipe.
On annual crops and perennial crops such as strawberries, a solid set
aluminum pipe sprinkler system is often used. This allows the system
to be moved during a land preparation or to other fields as desired.
sprinklers can be used for frost and freeze protection. Sprinklers should
be constructed of brass or some other metal. Plastic sprinklers may
become brittle and break during freezing temperatures.
have a fairly rapid rotational speed - at lest one revolution per minute.
Some manufacturers have special frost protection sprinklers with little
or no counterbalance arm so that they will rotate at a high speed. However,
these sprinklers have a disadvantage in that their diameter of coverage
is somewhat reduced.
Single nozzle sprinklers
are normally used in order to achieve the low application rates desired.
On nights when freezing
temperatures are expected temperatures should be checked at least hourly.
The thermometer(s) should be placed at the level of the plants in a
low spot in the field exposed to the open sky. Accurate thermometers
should be used and they should be checked prior to use by placing them
in a container of well stirred ice water. If accurate they should read
Typically, the sprinkler
system should be started when the temperature reaches 34° F. If
the relative humidity is low the system may need to be started at a
slightly higher temperature.
If a source of power
is available a thermostat may be wired to switch on a warning bell when
the temperature drops to a predetermined setting (usually 36° to
Adequate field drainage
is essential when using sprinkler systems for frost/freeze protection.
During an extended freeze several inches of water may be applied to
a field. This generally occurs during early spring when the soil is
probably already saturated. Consequently, adequate surface and/or subsurface
drainage should be provided.
In designing a sprinkler
system for frost/freeze protection everything must be sized so that
the whole field can be watered at once. Once the system is started,
water must be supplied continuously to the entire field until the freeze
is over. This results in a much higher pumping requirement than is common
for systems designed strictly for irrigation. Irrigation systems are
usually designed with several zones that are operated separately. A
system designed for frost/freeze protection requires that pumps, pipes,
valves and other components must be larger. For this reason, these systems
cost significantly more than those used strictly for irrigation. This
pumping capacity required at various precipitation rates is given in
Because of the large
pumping capacity required, frost/freeze protection systems commonly
pump from a storage reservoir such as a pond or lake. A reservoir may
be spring fed, fed by surface runoff or supplied by pumping should be
sufficient to supply 10 days of frost protection for 14 hours per day.
The actual requirement may be more or less depending on the crop, the
geographic area and the maximum number of days freezing temperatures
are expected once the crop reaches the critical stage.
Pumping units should
be reliable and should be maintained in top working condition. Any equipment
failure during the middle of a freeze could cause catastrophic crop
failure. Most growers prefer to use an internal combustion engine as
a power source. This eliminates the possibility of an electric power
failure shutting down the sprinkler system. In addition, many growers
will have a back-up power unit in case one fails.
irrigation has proven to be one of the most effective means of protecting
a variety of crops against frost/freeze damage. A properly designed system
can protect crops to temperatures as low as 20° F.
In order to be effective
in protecting crops, the sprinkler system must be properly designed. For
this reason, it is always advisable to have the system designed by a competent
irrigation designer. In addition, the system must be operated correctly
to achieve the desired results. It is the grower's responsibility to see
that he is properly informed to operate the system effectively. Timing
is critical in protecting crops from frost/freeze damage, and it makes
little sense to spend the money to install a sprinkler system and then
not know how to operate it properly when a killing freeze occurs.
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