Principles of Freeze Protection for Fruit Crops

Alabama is presently divided into ten forecast zones. Each zone is an area comprising several counties that possess similar features so relative weather conditions do not differ greatly across the entire zone. However, with each zone there will be microclimates resulting in considerable differences in minimum temperatures that may be attained during a freeze event. Because of the effects of microclimates, growers must carefully monitor their individual farm's characteristics regarding the impact of major freeze occurrences. This should involve measurements of temperatures at numerous locations across the farming area throughout the duration of the freeze. Records on occurrence of frosts, cloud cover, wind speed, dew point and wet bulb temperatures are also helpful. These records should be placed in a farm weather file along with copies of the forecast conditions for the same freeze events. Over time, these records will allow the grower to carefully compare his temperatures and other weather measurements with those from the nearest forecast sites. Notes should also be kept on the condition of the orchard floor, when and if freeze protection methods were used, stage of crop development, variety and block identity and extent of freeze damage. Once orchards or plantings are identified as being in warmer or colder sites on the farm, better management decisions can be made regarding optimum deployment of weather protection systems.

Obtaining temperature records from a site prior to establishment is a sensible approach to planning for future fruit plantings. Under ideal conditions a grower would maintain a winter/spring temperature record (recording the minimum temperatures) of potential new sites for future plantings. These records should be maintained for a minimum of one to three years in order to compare these locations to existing orchards and thereby determine if one or more of these should be avoided because of extremely cold minimum temperatures. If a producer wanted very specific information on one or more sites, he could establish an inexpensive temperature monitoring station. Temperature data obtained from such a field station could be used to quite accurately determine when a variety of a particular chilling requirement (such as an 850-hour selection) would have its rest requirement satisfied. By collecting heat units (using data from the same field station), commonly referred to as growing degree hours (GDH), once the variety's rest is satisfied, a fairly accurate prediction of 50% bloom date is possible. This information would help the grower determine if a variety of a given chilling requirement would tend to flower too early to be used in a particular location. Obviously most producers don't go to this much trouble in examining a future site but the potential for doing so still remains.

To properly monitor the temperature conditions in a fruit planting several minimum/maximum thermometers should be placed in inexpensive wood shelters (for measurements at 5-foot height) and arranged across an area. These thermometers indicate the highest and lowest temperatures reached in any period. Normally warm, cold and intermediate sites should be included. One thermometer every five acres may be adequate but more or less could be used based on terrain differences and numbers of different cold pockets. Even small changes in elevation such as 2 to 3 feet in a 5 to 10-acre planting can create cold pockets. Some growers prefer to hang thermometers from branches of plants, but this can be misleading if air temperature measurements are being used in managing freeze control systems because exposed thermometers can read several degrees lower than air on clear, calm nights. Helicopters may be effectively used during freeze events to monitor the temperatures of inversion layers and their height above the surface. By steadily measuring temperatures on orchard sites at least every 30 minutes, decisions can be made on where protection is needed and when. Radios can be used to instruct helicopters as to which blocks require protection at any given time.

Today, systems are available for electronic monitoring plant tissue (leaves and fruit) using thermocouples during a freeze. Thermocouples are devices for measuring temperature that may be inserted into buds, flowers and fruit. They are rather inexpensive and easy to use. This type system will give a very accurate picture of the severity of the freeze as the night progresses.

Most growers have not yet chosen to use measurements of crop temperatures to manage their freeze protection system. One of the primary problems with this system is that an index of threshold temperatures for various plant parts is not currently available although considerable research has been done. For example, citrus has been carefully studied and it is well established that 21.5øF is the average freezing temperature for leaves. Therefore, leaf temperature should never be allowed to drop below 22øF to prevent damage. In the case of stone fruit and small fruit similar threshold temperatures aren't readily available for all stages of bud, flower and fruit development stages. Adding to the difficulty of using tissue temperatures is the fact that floral parts as well as ripe fruits have the ability to supercool (drop below their normal freezing points before freezing and not freeze in some situations. And whether fruits will supercool during a given freeze is not easy to determine. Further, the temperature at which plant tissue freezes is also affected by the presence of moisture on the plant surface. Dry plant tissue freezes at lower temperatures than tissue that is wet. Work in California has shown citrus fruit that are covered with ice are cooled much more rapidly than dry fruit (snow does not provide as much cooling effect) during radiational freezes. Studies have also shown that cold temperatures in the winter cause fruit plants including fruit buds to gain cold hardiness (freeze at lower temperatures) while deacclimation (loss of hardiness) occurs with warmer temperatures. It has also been shown that fruit buds regain hardiness in response to colder temperatures several times slower than they lose hardiness as a result of warmer temperatures. Therefore the threshold freezing point for fruit buds and more advanced floral stages fluctuates as these tissues acclimate and deacclimate in response to changing temperatures. Fruit buds and flowers which develop slowly during colder temperatures are usually able to withstand lower temperatures during freeze than similar plant tissues that are growing rapidly during warm conditions.

A system has been developed in Washington for taking shoot samples from orchards during the winter and examining the buds to determine the threshold temperatures for freeze damage. This information may then be used to decide on use of freeze protection. This method is not currently being used in the Southeast.

Because of all the uncertainties of when different plant tissues freeze under varying freeze conditions, growers will most likely continue to use air temperature measurements to decide if protection is needed.

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