Principles of Freeze Protection for Fruit Crops
Wind Displacement Devices
For a number of years, wind machines have increased in importance for freeze protection in fruit crops grown all over the United States, in part because of the increasing cost of petroleum for heating. Wind machines perform best under radiational frost/freeze conditions characterized by calm, clear skies and a moderate to strong inversion. They function by bringing the warmer air aloft near to the surface where it mixes with the colder air, resulting in a rise in temperature in the orchard. Wind machines provide noteworthy advantages in cold protection by minimizing labor requirements and reducing refueling and storage of heating supplies and have low operational cost per acre. They are also more environmental friendly (except for noise) because they don't produce smoke or air pollution.
A typical wind machine consists of one or two 15 foot fans mounted on a steel tower at least 30 feet tall. An estimated 10 brake horsepower per acre delivered to the fan is required for a single machine and about 8 brake horsepower when several machines are used. A single machine can usually protect about 8 to 10 acres. Protection for the fringes of the protected area may not exceed 2 degrees while close to the wind machine 3 to 5 degrees of protection may be provided. When using two engines and fans on the same tower the area protected is increased by about 60%.
Installation costs for wind machines is similar to that of permanent heating systems (pipeline type) but they operate much more cheaply. However, under windy conditions (greater than 5 mph) wind machines will provide no protection. Research has shown that it is hard to regain a higher temperature that is lost when a wind machine stops. Therefore, it is usually recommended to start the machine when air temperature reaches 32øF.
Using heaters or some form of heat with wind machines creates a very efficient system and provides more protection than either system used alone. Further this system can be used to provide protection when forecast minimum temperatures are too low for wind machines to provide sufficient protection. Workers in California report citrus can be protected down to 22øF with wind machines if under ideal conditions when trees are properly hardened.
Very few wind machines are presently used in Alabama for freeze protection. However growers utilize helicopters on a regular basis as required and they provide good protection when moderate to strong inversions ar present. One helicopter can protect about 50 to 100 acres although some growers have tried to use one for 100 to 200 acres. Under rather cold conditions, a helicopter must cover the same area every 30 to 60 minutes to maintain temperatures above critical levels. Once the helicopter locates the strongest inversion, usually at 75 to 150 feet, the copter flies at this level at 15 to 25 mph to thoroughly mix the air. Some have tried flying at 50 mph or greater and little beneficial effect is felt. When hovering slowly over trees a rapid increase of 5 to 8øF is not uncommon.
Heating
Records indicate heating has been used to protect plants from freezes for at least 2000 years. Heating devices are usually of two types, those that heat metal objects that radiate heat (like return stack heater) and those that operate as open fires. Heating a fruit planting on a cold night is simply replacing the heat being lost by radiation or wind. Heaters provide heat primarily in two ways. An estimated 75% of the energy is released as convective heat produced by the mixing of hot gases and heated air. The other 25% of the energy is released as radiation from the flame and objects heated by the flame such as hot metal stacks. This type heat travels in straight lines, is not affected by wind and therefore heaters may provide some protection during windy freezes. Trees close to heaters that are warmed by radiation are usually warmer than air.
The initial costs of equipping fruit orchards with heaters is less than more permanent systems. Permanent heating systems such as those connected by a pipeline network are easy to operate but quite costly to install. These systems may use fuels such as natural gas or propane. Because of the ever increasing price of petroleum products, the use of conventional heaters such as jumbo cone, return stack and short stack have declined dramatically in recent years. Very little use of this type heating is done in Alabama fruit plantings.
Over the years, growers have used other forms of energy for heating including rubber tires, wood and more recently coal. Of these, coal provides the most cost effective heating in orchards. Some growers have felt the smoke created by rubber tires creates a cloud that blocks heat loss from the orchard. However, the smoke particles are too small to block longwave radiation loss like water vapor does in clouds. Growers should keep in mind that when burning of any type is used to protect orchards, local authorities should be consulted on the legality of such practices.
Irrigation - Sprinkler Application
The practice of irrigation for freeze protection is not as old as heating, but it has been in use for over 60 years. Sprinkler irrigation has become an important type of freeze protection, especially with low growing crops such as strawberries and bushes with flexible canes such as blueberries. Tree fruits such as oranges, peaches, cherries and apples can be protected with sprinkler irrigation but some level of limb breakage can be expected. Limb breakage can be excessive on evergreen trees such as oranges or on poorly trained peach trees. The use of sprinkling for freeze protection is the only form of cold protection used whereby improper use can result in more damage than if the planting was left unprotected.
Some of the major limitations on using sprinkler irrigation for freeze protection include limited or no protection (or even increased damage) under windy conditions or when air temperatures are below 22øF and the possibility of increased disease problems or leaching of soil nutrients. When used on strawberries, loss of or damage to green and ripe fruit from excessive moisture or critically low temperatures are problems in addition to the physical problems of working in excessively wet fields.
However, when one considers the advantages of sprinkler irrigation for freeze protection it is easy to understand why its use is rather widespread with certain crops. Costs of operation are significantly lower because energy from water is being used directly for heating rather than indirectly from gas or oil. Sprinkler systems are easy to operate with minimal manpower. However, they still require a watchful eye throughout the night to defrost any sprinkler heads that start freezing and to ensure that the system runs continuously. Design of some systems also permits multiple use of sprinklers for fertilizer application, regular irrigation for drought and evaporative cooling in summer months.
Work in Georgia and other states has shown that a system of micro- sprinklers with one placed above each tree may be successfully used for freeze protection on crops such as peaches. This type of system requires less water because it is only being applied above each tree. However, this system requires substantial installation costs and to date growers have largely chosen to install conventional sprinkler systems when this form of freeze protection is chosen.
Systems commonly used involve sprinkler heads mounted just above crop level on portable aluminum pipe. These are typical installations in low growing crops such as strawberries. Where tree fruit crops are being protected, permanent systems are usually installed. Lateral lines run down the row middles between trees and a short lateral line connects the sprinkler, which is mounted beside a tree, to the system. A galvanized or PVC riser some 10 to 20 feet tall, with the sprinkler mounted on tip, is positioned beside the tree. A 2 to 4-inch diameter post usually several feet high is used to support the riser.
Under the canopy (under the tree) sprinkling has been successfully used in Florida and California for freeze protection, especially with young citrus. Work in Alabama over the past five years as well as in Louisiana with under the tree sprinkling for protection of satsuma mandarin plantings has proven quite successful. The objective of this system is to provide near full protection in moderate freezes and protection mainly to the trunk and lower scaffolds when severe freezes occur. The latter form of protection would allow trees to more rapidly overcome effects of severe freezes and return to production faster. To date a two riser system per tree, one at 2 1/2 feet and a second at 5 feet appear best in Alabama tests. Each sprinkler head applies 24 gal./hour during the freezing period.
The principle on which sprinkler irrigation works for freeze protection involves the latent heat of fusion. When one gram of water freezes, it releases 80 calories of heat. because of the warming effect of freezing water, a plant surface such as a leaf will remain near 32øF as long as a mixture of water, ice and water vapor are present, although the temperature of surrounding air may continue to drop. However, while some water is freezing, additional water is evaporating which cools the air. When one gram of water evaporates it removes nearly 600 calories of heat from the air. When compared to the heating effect of freezing, it becomes obvious that to realize a net effect of heating while these two processes are going on, more than 7 1/2 times more water must be freezing than evaporating. In other words, for every gallon of water that evaporates, more than 7 1/2 gallons of water must be freezing to maintain the plant temperature near 32øF. If this doesn't occur then evaporation will remove heat from the crop causing damage.
Ice is a poor insulator, and the temperature of a plant covered in ice will drop below a dry plant if freezing stops and evaporation starts. Because evaporation is increased by wind, a 5 mph or higher wind speed greatly reduces effectiveness of sprinkler irrigation for freeze protection. A good indicator of the effectiveness of the system is the color of the ice forming. If the system is properly working, the ice will be clear. If the water is freezing before it strikes the plant, the ice will have a milky white appearance because of the presence of air bubbles.
One of the real problems for earlier sprinkler systems and continuing into the present is the fact they are designed for a fixed rate of application. Because of never knowing which rate would be needed on a given night, most systems were designed for the worse possible scenario, which usually meant excess water was being used. However, recently some growers have had systems designed that can be quickly changed by switching riser heads to allow higher rates of water to be applied as needed. Wind poses a real problem for sprinkler irrigation for freeze protection and it has been estimated by a number of workers that an application rate three to four times the normal rate is required for 5 mph wind versus calm. Sprinkler systems provide protection down to near 22øF. When air temperature falls below this level, the degree of protection decreases rapidly until a point is reached where damage becomes worse than if no sprinkling were applied.
When To Start
Sprinkler system for protection varies somewhat with the crop, stage of development and weather conditions, but in general it should be started when air temperature in the coldest part of acreage to be protected drops to 34 to 35øF. However, using an exposed thermometer to provide an estimate of plant temperature (usually several degrees lower than air temperature) is quite commonly practiced by growers. For example when the exposed thermometer on a strawberry bed reaches 31.5 to 32.0øF irrigation systems are turned on. The air temperature at this time is usually 34 to 37øF.
For more information on frost protection, see "Frost and freeze Protection Using Sprinkler Irrigation" in the appendix of the Alabama Micro-Irrigation Handbook.
Irrigation - Soil Application
It is estimated that wetting a soil prior to a freeze can provide 2øF of protection. The water contains heat that is released, and a wet soil allows heat to be continually drawn from lower depths during the night. The water which fills the pore spaces helps conduct this subsurface heat upward. California regularly uses a flood or furrow system to condition soils by wetting them ahead of freezes in fruit plantings. However, this requires relatively flat soil and a large volume of water, two things not prevalent in much of Alabama's fruit growing areas. Water run on the orchard floor supplies considerable heat while freezing of the water provides additional heat. Where growers in the state do not have other methods of freeze protection, it is recommended they pre-wet orchards and fields to the degree possible and consider running water on the surface where systems will allow this to be done.
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