ANR-1057B, New Oct 2000. Arlie A. Powell, Extension Horticulturist, Professor, and David G. Himelrick, Extension Horticulturist, Professor, both in Horticulture at Auburn University

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Methods of Freeze Protection for Fruit Crops

Frost/freeze protection methods are either passive or active. Passive protection methods do not require outside energy sources during the frost/freeze event. Passive protection methods include site selection, variety and rootstock selection, and cultural practices such as pruning and orchard floor preparation. Active protection methods replace heat or prevent radiant heat loss by using outside energy during the freeze. Active methods include wind machines, heaters, and water application via sprinklers, microsprinklers/ drippers, flooding, or furrowing. Table 1 provides an overview of the advantages and disadvantages of passive and active protection methods. The freeze protection methods listed in this table are primarily used to prevent or minimize crop damage. However, most of these methods also provide some level of plant protection. Painting of tree trunks is the only method used strictly for plant protection.

Table 1. Advantages and Disadvantages of Freeze Protection Methods

Passive Protection Systems
Protection Method	Description	Crops Protected
Site Selection	Select sites with the warmest minimum temperatures during freezes.	All fruit crops, but especially tree fruit and blueberries
Soil Type	Heavier, darker-colored soils tend to hold more heat than sandy, lighter-colored soils do in winter/early spring, resulting in earlier plant activity.	All fruit crops
Variety Selection	Select varieties that historically have the best cropping during severe freeze events.	All fruit crops, but especially stone fruit and blueberries
Rootstock Selection	Select rootstocks that impart greater hardiness to scion varieties and delay flowering.	Citrus, stone fruit, and pome fruit, except for citrus. Differences in hardiness induced by recommended rootstocks for other fruit crops are inadequate to recommend one over another.
Nutrition	Midsummer or postharvest application of nitrogen can induce sufficient vigor for strong fruit bud development and some delay in flowering, thereby increasing cropping.	Mainly stone fruits and blueberries
Orchard Floor Management	A weed-free, firm soil with good soil moisture beneath plants creates a warmer microclimate and reduces freeze damage.	All fruit crops
Pruning/Tree Conformation	Reducing the severity of pruning, delaying pruning until late winter or early spring, and leaving taller trees until final pruning reduces freeze damage and increases cropping. Summer pruning can delay flowering in blueberries.	All tree fruits but especially stone fruits; blueberries to some degree
Canopy Trees	Planting large canopy trees with the orchard crop can afford some freeze protection. Date palms are used in citrus plantings in California. Pine trees are being used in citrus plantings in south Alabama.	Mainly citrus
Chemicals - Cryoprotectants and Antitranspirants	Commercially available forms of these chemicals have not provided freeze protection to flower buds, flowers, or young fruits through foliar sprays.	Not consistently effective on any fruit crops
Chemicals - Growth Regulators	Among chemicals tested, only the ethylene-releasing compound ethephon has shown commercial promise. Ethephon increases winter fruit bud hardiness and delays flowering 4 to 7 days. Studies with gibberellic acid (GA(3)) have shown promise for bloom delay, but the need for multiple applications and cost are limitations.	Peaches, possibly other stone fruits and blueberries
Plant Covers	Woven and spun-bonded polypropylene covers of varying thicknesses (weights) are among the latest form of freeze protection to be tested on fruit crops. Depending on the material used, several degrees of protection are afforded.	Strawberries, possibly blueberries and small citrus
Evaporative Cooling	Overhead misting of stone fruits delays flowering but damages buds. Misting is done in the dormant season from when rest is satisfied until bloom swell. More studies are needed before application can be recommended.	Potential use on peaches and nectarines
Painting Tree Trunks	Using white paint on trunks of peaches and other tree fruits reduces winter trunk damage.	All tree fruits
Active Protection Systems
Wind Displacement Devices	Use wind machines and helicopters to raise temperatures in plantings.	All fruit crop plants, but mainly tree fruits
Heating	Use heating devices such as orchard heaters to raise temperatures in plantings.	All fruit crop plants but mainly tree fruits
Irrigation - Sprinkler Application	Use a conventional overhead system, overhead microsprinkler system, or low-riser trunk application system.	All fruit crop plants, but different systems are used on different fruit types.
Irrigation - Soil Application	Water can be pumped onto the orchard floor before a freeze, as is done in California. Requires relatively flat soils.	All fruit crop plants, but terrain dictates use; most common on tree fruits.

Passive Protection

Growers should use all forms of passive protection that are available for the crop being grown. A number of these forms of protection, such as site selection, must be used before the planting is established; therefore, good management and planning must be a part of freeze protection programs from the very beginning. The most widely used forms of passive protection are discussed below.

Site Selection

A given farm may have one or more natural microclimates where minimum temperatures during frost/freeze events may be much warmer or colder than those in the surrounding area. For this reason, growers should try to select sites that will be warmer during freeze events.

Monitoring temperatures of future sites during freezes can help a grower identify any microclimates before planting. Once a history of freeze temperatures is known and cold pockets are identified, growers can avoid such areas or they can plant more-hardy, later-flowering types and varieties of fruit.

Warmer sites are usually at the highest elevations and are at least 2 to 4 degrees warmer during radiational frosts/freezes. Removing or thinning trees that may create cold air dams is helpful.

Soil Type

Growers have found that fruit trees may become active sooner in late winter on heavier, dark-colored soils (such as black and red) than on less dense, light-colored sands. Light-colored soils tend to reflect more heat during the day (trap less heat) and lose heat faster during the night.

Although growers cannot change the type of soil in their orchards, they can make decisions that can lessen freeze problems. For example, it may be more advantageous to plant higher-chilling varieties that are slower to break bud on heavier, darker soils than it is to plant lower-chilling varieties on them.

Variety Selection

Among almost all fruit types, available varieties can be divided into hardiness groups based on their ability to survive and/or produce crops under adverse freeze conditions. Large differences in varietal hardiness are evident among crops such as peaches, plums, nectarines, grapes, and blueberries, while more subtle differences are evident among others. Growers should refer to available Extension publications for recommendations on varieties well before establishing plantings.

Rootstock Selection

Studies have shown that rootstocks influence the performance of scion varieties in many ways, including yields, fruit quality, longevity, cold hardiness, and time of flowering. Some rootstocks have been eliminated because of their lack of hardiness. For example, Siberian C peach rootstock is not recommended in the Southeast because trees die prematurely from lack of hardiness.

Recent studies, particularly in peaches, have shown that some rootstocks as well as certain interstems used in propagating nursery trees can cause bloom delay. However, at the present time, the differences among rootstocks recommended for peaches and other deciduous fruit types do not justify recommending one over another because of greater bloom delay or hardiness induced.

In the case of citrus, trifoliate orange or its dwarf form, Flying Dragon, are highly recommended over all other rootstocks because of their ability to provide greater cold hardiness to citrus types budded onto them, such as satsuma.

Nutrition

Deciduous fruit plants that are not nutritionally sound, especially in regard to nitrogen, are more subject to winter damage. Fruit buds of such trees are also less healthy and more easily damaged by freezes. Nitrogen-deficient trees also tend to flower earlier in the spring and are more subject to crop loss. Trees of good vigor retain their foliage longer in the fall and flower later in the spring.

Maintaining adequate vigor throughout the summer is very critical to stone fruit such as peaches. Therefore, summer pruning programs and/or summer fertilization are useful in maintaining proper vigor in peaches (fertilization may be postharvest). However, tree fruit such as apples and pears with a low fertility requirement do not normally require mid- to late summer fertilization, whereas such applications do benefit blueberries. Stimulation of late fall growth may prevent plants from properly hardening off and may predispose them to winter injury.

Orchard Floor Management

Most fruit plantings are maintained with weed-free strips along plant rows and sod middles for travel and erosion control. A weed-free, firm, moist soil can add 1 to 4 degrees of protection during a radiational frost/freeze event (Table 2). Soils that are dry, freshly cultivated, or covered with live or dead grass give the opposite effect. Growers should make every effort to properly prepare the orchard floor in early fall for maximum release of radiant heat during freeze events. Mowing the orchard floor grass to a height not exceeding 2 inches is recommended.

Table 2. Air Temperature at 5 Feet above Soil Surface as Influenced by Orchard Floor Conditions

Orchard Floor	Temperature Ranges
Bare, firm, moist ground	Warmest
Shredded cover crop, moist ground	0.5 degrees F colder
Low-growing cover crop, moist ground	1 to 3 degrees F colder
Dry, firm ground	2 degrees F colder
Freshly disced, fluffy ground	4 degrees F colder
Higher cover crop	2 to 4 degrees F (or more) colder

Pruning and Tree Architecture

While delayed pruning is very important for stone fruit such as peaches, the pruning of all tree fruits and grapes should be delayed as long as possible in late winter or early spring. Pruning practices in commercial plantings have demonstrated the benefits of delayed pruning of peaches. Delaying pruning until budbreak (pink bud or later if possible) results in less winter kill of fruit buds and a 2- to 4-day delay in flowering. Producers in the Northeast have practiced delayed pruning for years, while southern producers always felt they could prune at any time in fall or winter. Research studies over the past 25 years have demonstrated that fall or early winter pruning of peaches can cause early tree death in the Southeast. Now it is also evident that delaying pruning into February and March (depending on the location) provides the additional benefits of higher live bud count in the spring and delayed flowering, both of which contribute to improved annual cropping.

After a radiational freeze, any fruit remaining on a tree will be on the higher branches. For this reason, growers could hedge their risk of crop loss by not pruning the upper terminals of trees until the risk of late freezes has passed. Delayed pruning of upper branches may especially be of value where no active protection methods are used.

Tree Canopy Protection

Most growers choose not to use larger canopy trees to afford freeze protection to small trees growing beneath them, but the technique does work. The larger trees absorb and reflect long-wave radiation to the fruit trees below, thereby providing a few degrees of protection.

California citrus producers use taller date palms in some citrus plantings to provide the citrus with a measure of freeze protection. In Alabama, this is being done only by planting pines in small satsuma mandarin plantings. Competition between the two plant types must be carefully controlled; otherwise, the yields of the fruit crops become limited.

Chemicals--Cryoprotectants and Antitranspirants

Several commercially available forms of cryoprotectants and antitranspirants have been studied for their ability to provide freeze protection to fruit crops, and none has proven effective thus far in providing consistent protection. Some of the latest chemicals to be tested include formulations of non-ice-nucleating bacteria used in foliar sprays to nullify the effects of ice-nucleating bacteria. These latter chemicals have shown promise, but no recommendations are available yet.

Chemicals--Growth Regulators

Over the past 40 years, numerous growth regulators have been tested for their effectiveness in increasing the cold hardiness of plants, buds, and flowers and in delaying flowering. Among the compounds tested, only the ethylene-releasing compound ethephon has shown commercial potential. Work in Alabama and other states has clearly shown that ethephon applied in early fall at the onset of chilling enhances cold hardiness of buds and delays flowering of peaches by 4 to 7 days. The net effect of this ethephon spray is to greatly reduce damage from midwinter and late-winter freezes on peaches. Ethephon provides the same effect on cherries and has a federal label for use. Some northeastern states have a state label for use on peaches, and efforts are currently under way to gain approval for a label on peaches in Alabama.

Preliminary work suggests that ethephon will delay flowering in a number of other fruit crops, but no significant studies are currently under way in the Southeast. Studies with apples indicated that ethephon did not effectively delay flowering but showed that fall treatment significantly decreases fruit set and vegetative growth.

Plant Covers

Some form of plant cover has been used for frost/freeze protection for many years. However, only in the last 10 to 20 years have specially manufactured covers gained appreciable commercial acceptability. Among the covers that have shown the most promise are woven and spun-bonded polypropylene types of varying thicknesses, or weights. The degree of protection provided varies with the weight of the material: from 2 to 3 degrees with lightweight covers and 6 to 9 degrees with heavier covers. Co-polymer white plastic has provided protection to nursery stock but is generally not used on fruit and vegetable crops.

Light- and medium-weight covers provide outstanding protection for low-growing crops such as strawberries and blueberries. Some growers have devised a system for anchoring the covers with sandbags and using mechanical rollers to apply and remove the covers, although it is not very practical; therefore, some practical system for application and removal of covers for low-growing fruit plants must be developed before widespread use of covers will occur. Regardless of which crop covers are used, they must be removed after each freeze event. If left on, they deacclimate the plants and can cause pollination problems.

Evaporative Cooling

The use of evaporative cooling with sprinkler irrigation has been successfully used on a number of fruit crops. Work with apples, cherries, peaches, and other crops has indicated that bloom delays of 2 weeks or longer can be achieved. This was basically accomplished by sprinkling trees from rest completion until bloom. Intermittent sprinkling was done any time the air temperature in the orchard exceeded 45 degrees F. This approach was especially promising on peaches for a number of years but never gained grower acceptance because cropping on sprinkled trees was greatly reduced, and no remedy was found to solve the problem.

Painting Tree Trunks

Research has clearly shown the benefit of applying white paint to the trunks of peach and other deciduous fruit trees in late summer/early fall. This practice is designed to reduce bark splitting and serious damage to tree trunks during freeze events.

A differential heating effect occurs on tree trunks in the winter months on clear days. Because of the angle of the sun, the south and west sides of tree trunks become much warmer than the north side, resulting in loss of hardiness. Work in Georgia has shown that temperatures on the south side of a peach tree can reach 96 degrees F while air temperature is only 55 degrees F. It is quite common to measure differences of 25 to 40 degrees on tree trunks not treated with white paint. Because the south side of the trunk warms more than the north side does, freeze damage is nearly always worse on the south side.

Although growers do not commonly paint tree trunks to protect them during winter months, the practice is quite effective. An interior, water-based, white latex paint is generally preferred and should be diluted with water at least 50 percent.

Active Protection

The form of active protection a grower chooses depends on the crop, the site, economics, and other variables.

Wind Displacement Devices

For a number of years, wind machines have become increasingly important for freeze protection in fruit crops throughout the United States. This increase in importance has occurred in part because of the rising 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 closer to the surface where it mixes with the colder air, causing the temperature in the orchard to rise. Wind machines have several advantages over heating: they minimize labor requirements, reduce refueling and storage of heating supplies, have low operational costs per acre, and are more environmentally friendly (except for noise) because they do not produce smoke or significant air pollution. Costs for installing wind machines are similar to the costs for installing permanent heating systems (pipeline type), but wind machines are cheaper to operate.

One drawback to using wind machines is that they do not provide protection if winds are greater than 5 mph. 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 that the machine be started when the air temperature reaches 32 degrees F.

A typical wind machine consists of one or two 15-foot fans mounted on a steel tower that is 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 is required for two or more machines. A single machine can usually protect about 8 to 10 acres. The temperature in the fringes of the protected area may increase only 2 degrees, while temperatures of the air closer to the wind machine may increase 3 to 5 degrees. When two engines and fans are placed on the same tower, the area protected is increased by about 60 percent.

Using heaters or some form of heat with wind machines creates a very efficient system and provides more protection than either system can provide alone. The use of both wind machines and heat is recommended when forecast minimum temperatures are too low for wind machines to provide sufficient protection alone. In California, under ideal conditions, citrus has been protected down to 22 degrees F with wind machines when trees were properly hardened.

Very few wind machines are currently used for freeze protection in Alabama. However, growers use helicopters on a regular basis, and helicopters provide good protection when moderate to strong inversions are 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, it flies at this level at 15 to 25 mph to thoroughly mix the air. Some have tried flying at 50 mph or greater, but little beneficial effect has been felt. When helicopters hover slowly over trees, temperatures can rapidly increase 5 to 8 degrees.

Heating

Records indicate that heating has been used to protect plants from freezes for at least 2,000 years. Heating a fruit planting on a cold night is simply replacing the heat being lost by radiation or wind. There are basically two types of heating devices: those that heat metal objects that radiate heat (like a return stack heater) and those that operate as open fires.

Heaters provide heat primarily in two ways. An estimated 75 percent of the energy is released as convective heat produced by the mixing of hot gases and heated air. The other 25 percent of the energy is released as radiation from the flame and objects heated by the flame, such as hot metal stacks. This type of heat travels in straight lines and is not affected by wind; therefore, heaters may provide some protection during windy freezes. Trees close to heaters that release heat through radiation are usually warmer than the surrounding air.

The initial cost of equipping fruit orchards with heaters is less than that of 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 the jumbo cone, return stack, and short stack has declined dramatically in recent years. Very little of this type heating is used 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 thought that the smoke created by rubber tires creates a cloud that blocks heat loss from the orchard. However, the smoke particles do not block long-wave radiation loss like water vapor does in clouds. Growers should remember to consult local authorities about the legality of any burning practices used to protect orchards.

Irrigation--Sprinkler Application

The practice of irrigation for freeze protection is not as old as the practice of 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 with bushes with flexible canes, such as blueberries. Tree fruits such as oranges, peaches, cherries, and apples can be protected with sprinkler irrigation, but some limb breakage can be expected. Limb breakage can be excessive on evergreen trees such as oranges or on poorly trained peach trees. Improper use of sprinkling for freeze protection can cause more damage than will occur if the planting is left unprotected.

There are several major problems with using sprinkler irrigation for freeze protection. Windy conditions (5 mph or greater) require an application rate three to four times the normal rate for calm weather. When air temperatures fall below 22 degrees F, the degree of protection decreases rapidly to a point at which damage becomes worse than it would have been if no sprinkling were applied. Using sprinkler irrigation can also increase problems with diseases and leaching of soil nutrients. When used on strawberries, sprinkling can cause loss of or damage to green and ripe fruit from excessive moisture or critically low temperatures. These problems are in addition to the physical problems of working in excessively wet fields.

Another problem with sprinkler systems is the fact that they are designed for a fixed rate of application. Because the specific rate needed on a given night cannot be determined ahead of time, most systems are designed for the worst possible scenario, which usually means excess water is used. Recently, some growers have had systems designed so that the riser heads can be quickly changed to allow higher rates of water to be applied as needed.

In spite of these problems, the advantages of using sprinkler irrigation for freeze protection make its use rather widespread with certain crops. Costs of operating sprinkler systems irrigation are significantly lower than costs of heating systems. In sprinkler irrigation systems, energy for heating comes directly from water rather than indirectly from gas or oil as with heating systems. In addition, sprinkler systems are easy to operate with minimal man power. Sprinklers do need to be checked throughout the night to ensure that the system runs continuously and so that any sprinkler heads that start freezing can be defrosted. Finally, some systems are designed to be used for multiple purposes including fertilizer application, regular irrigation for drought, and evaporative cooling in summer months.

Work in Georgia and other states has shown that a system of microsprinklers, with one placed above each tree, can be successfully used for freeze protection on crops such as peaches. This type of system requires less water because water is only being applied above each tree. However, this type system is expensive to install, and growers usually choose to install conventional sprinkler systems when they choose this form of freeze protection.

Typical installations in low-growing crops such as strawberries have sprinkler heads mounted just above crop level on portable aluminum pipe. Permanent systems are usually installed where tree fruit crops are being protected. 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, 10 to 20 feet tall with the sprinkler mounted on top, is positioned beside the tree. A 2- to 4-inch diameter post that is usually several feet high supports the riser.

Under-the-canopy, or under-the-tree, sprinkling has been successfully used in Florida and California for freeze protection, especially with young citrus. Work in Alabama and Louisiana during the past 5 years 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 in severe freezes. The latter form of protection allows trees to more rapidly overcome effects of severe freezes and return to production. To date, a two-riser system per tree, with one riser at 2-1/2 feet and a second at 5 feet, appears best in Alabama tests. Each sprinkler head applies 24 gallons per hour during the freezing period.

How sprinkling works. The principle on which sprinkler irrigation works for freeze protection involves the latent heat of fusion. When 1 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 32 degrees F as long as a mixture of water, ice, and water vapor are present, although the temperature of the surrounding air may continue to drop. However, while some water is freezing, additional water is evaporating, which cools the air. When 1 gram of water evaporates, it removes nearly 600 calories of heat from the air. When this is 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 32 degrees F. If this does not occur, evaporation will remove heat from the crop and cause damage.

Ice is a poor insulator, and the temperature of a plant covered in ice will drop below the temperature of a dry plant if freezing stops and evaporation starts. Because evaporation is increased by wind, a 5 mph or higher wind speed greatly reduces the 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 working properly, 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.

When to start sprinkling. Sprinkler system use for freeze protection varies somewhat with the crop, stage of development, and weather conditions, but in general, it should be started when the air temperature drops to 34 to 35 degrees F in the coldest part of the area to be protected. However, using an exposed thermometer to provide an estimate of plant temperature (which is usually several degrees lower than air temperature) is a common practice among growers. For example, when the exposed thermometer on a strawberry bed reaches 31.5 to 32 degrees F, irrigation systems are turned on. The air temperature at this time is usually 34 to 37 degrees F.

For more information about frost protection with sprinkler irrigation, 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 the soil before a freeze can provide 2 degrees of protection. Water contains heat that is released, and a wet soil allows heat to be continually drawn from lower depths during the night. The water that fills the pore spaces helps conduct this subsurface heat upward. California regularly uses a flood or furrow system to condition soils in fruit plantings by wetting them before freezes. 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 that 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.

Critical Temperatures for Fruit Plants

Frosts and freezes can damage floral structures in many ways. The formation of frost on flowers or fruit can cause scarring of the skin of the fruit and possible flesh damage but will probably not kill tissue. The internal freezing of tissue of buds, flowers, and fruits causes serious damage or death of the floral parts.

When small floral structures such as flowers or fruits freeze, they may be damaged in several ways. Severe temperatures usually destroy the entire buds, flower, ovules (immature seed), and ovaries of small fruits, resulting in rapid abscission of the reproductive structure. Slightly less freeze damage may cause catfacing (creasing of the skin and flesh, as in peaches), along with partial or complete death of ovules. The damaged fruit may remain until harvest but is poorly shaped, small in size, and unmarketable. Even less-severe freeze damage may cause death of ovules only (as in peach); the remaining fruit may develop into a very small fruit called a "button" which may persist until harvest. Marginal freeze damage in blueberry may kill only the upper portion of the pistil (stigma and style).

When small fruit of deciduous fruit plants (such as apple, peach, and blueberry) freeze solid, they are usually totally killed or will be badly malformed. Leaves of these same fruits commonly freeze solid during freeze events but will thaw as temperatures rise and remain undamaged (unless extremely low temperatures occurred). Ripe citrus fruits, however, can freeze solid, and if the damage is not too severe, they can thaw and remain sound. They may, however, begin slowly drying out (losing juice content) until harvested.

The air temperatures at which serious freeze damage will occur in various floral and vegetative structures have been determined through research and field experience. Producers can use these temperatures to make decisions about when to start and stop active protection systems. Although considerable research has been done, too many unknowns exist to use tissue temperatures to manage freeze protection systems.

Tables 3, 4, and 5 provide detailed information on critical air temperatures to be used in operating freeze protection systems in fruit plantings. The temperatures shown in Tables 3 and 4 are for fruit varieties of average hardiness. The hardness of varieties differs substantially from dormant through early, prebloom stages. Temperatures in Table 4 are similar to those published by Washington State University except that the critical temperatures for the most advanced stages are slightly higher in Alabama.

Table 3. Critical Temperatures for Various Tree Fruit Crops(*)

Floral/Fruit Stage	Temperature at Which 10 Percent Kill Occurs (degrees F)	Temperature at Which 90 Percent Kill Occurs (degrees F)
Apples
Silver tip Green tip 1/2"green Tight cluster Pink Bloom Postbloom	15 18 23 27 28 28 28	2 10 15 21 24 to 25 25 25
Peaches
First swell Calyx green Calyx red Pink Bloom (early) Bloom (late) Postbloom	18 21 23 25 26 27 28	1 5 9 15 21 24 25
Pears
Scales separate Buds exposed Tight cluster White bud Bloom Postbloom	15 20 24 25 to 26 27 to 28 28	0 6 15 22 to 23 23 to 24 24
(*)Temperatures as published in Washington State University Extension Bulletins 0914, 0915, and 0916

Table 4. Critical Temperatures for Peaches in Alabama(*)

Floral/Fruit Stage	Temperature at Which 10 Percent Damage Occurs (degrees F)	Temperature at Which 100 Percent Damage Occurs (degrees F)
Dormant	5 to 20	10
Full swell/first green	15 to 18	0 to 5
Calyx green/red	21 to 23	5 to 10
Pink bud	25	15 to 18
Full bloom	27 to 28	26
Petal fall/wet shuck	28	26
Dry shuck	28 to 29	26
Shuck split/off	29	26
(*)The above temperatures were obtained over a 19-year period in commercial orchards, using minimum thermometers.

Table 5. Critical Temperatures for Small Fruit Crops

Floral/Fruit Stage		Temperature at Which 90 Percent Damage Occurs (degrees F)
Strawberries
Tight bud		22
Tight with white petals		28
Full bloom		31
Immature fruit		28
Blueberries (rabbiteye)
Swelled flower buds		21
Individual flowers distinguishable		25
Flowers distinctly separated,		28
Corollas expanded but closed
Fully opened flowers		31

Summary

The cost of fruit production continues to increase, and the reliability of annual cropping has become a necessity for farming operations to be economical. Because freezes represent the number-one problem in maintaining annual cropping of fruits in Alabama, using one or more forms of freeze protection has become a necessity most years. But the decision regarding which form of freeze protection to use rests entirely with the grower. The particular method of protection growers choose will depend on the crop, the particular site, cost versus benefit, and a number of other variables. Growers should do a great deal of planning before deciding on a particular form of protection.

The first thing growers should do is select and implement the various forms of passive protection applicable to their farms. Except when row covers are used, growers must next decide on one or more forms of active protection such as using wind machines, heating, or sprinkler irrigation. With experience, growers will begin to master the "art" as well as the "science" of knowing just how to manage a helicopter/heating program, sprinkler irrigation, or another form of active protection.

And the most critical aspect of freeze protection management is always having a system ready for each freeze event and placing its operation in reliable hands. Playing catch-up or having to make too many last-minute decisions can be frustrating and disastrous.

For more information about the principles of freeze protection, see Extension publication ANR-1057A, "Principles of Freeze Protection for Fruit Crops."

Acknowledgments

The authors gratefully acknowledge the use of materials from the following references in the preparation of this publication.

Anderson, J.L. and S.D. Seeley, "Bloom Delay in Deciduous Fruits" In Horticultural Reviews, vol. 15. NY, NY: John Wiley & Sons, Inc., 1991.

Puffer, R.E. et al., "Frost Protection in Citrus." University of California Agricultural Extension publication AXT-108,1987.

Sneed, R.E. et al., "Frost and Freeze Protection Using Sprinkler Irrigation." North Carolina State University Extension Circular 101, 1988.

Lyrene, P.,"Humidity as a Factor in Freeze Protection with Sprinkler Irrigation." Proc. Fla. State Hort. Soc. 50:(45-59), 1996.

Perry, K., "Frost/Freeze Protection for Horticultural Crops." North Carolina State University Extension Leaflet 705-A, 1994.

Perry, K., "Frost/Freeze Protection for Apple Orchards." North Carolina State University Extension publication AG-303, 1983.

Ballard, J.K., "Critical Temperatures for Blossom Buds - Peaches." Washington State University Extension Bulletin 0914, 1981.

Ballard, J.K., "Critical Temperatures for Blossom Buds - Apples." Washington State University Extension Bulletin 0915, 1981.

Ballard, J.K., "Critical Temperatures for Blossom Buds - Pears." Washington State University Extension Bulletin 0916, 1981.

Perry, K., "Basics of Frost and Freeze Protection for Horticultural Crops." HortTechnology 8(1):10-15, 1988.

Yoshikawa, R.T. et al., "Frost Protection" In Peach Management Handbook. University of California-Davis, CA, 1990.

Gerber, J.F. et al., "Protecting Citrus from Cold Damage." University of Florida Extension Circular 287, 1966.