Principles of Freeze Protection for Fruit Crops

Growers should make every effort to utilize 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 establishment of planting. Thus, good management and planning must be a part of effective freeze protection programs from the very beginning. The most widely recognized forms of passive protection are discussed below.

Site Selection

As explained earlier, a given farm may have one or more natural microclimates where minimum temperatures during frost/freeze events may be much warmer or colder than the surrounding area. Thus the producer should make every effort to select favorable sites that tend to have warmer temperatures during freeze events. Monitoring temperatures during freezes on future sites greatly helps identify any microclimates before planting. Where a history of freeze temperatures are known and cold pockets are identified, growers can avoid such areas or plant more hardy, later flowering types and varieties of fruit. On most farms, warmer sites, usually at the highest elevations, will be at least 2 to 4 degrees F warmer during radiational frost/freezes. Removal or thinning of trees that may create cold air dams is desirable.

Soil Type

Grower experience has clearly shown that fruit trees may become active sooner in late winter on dark colored, heavier soils (such as black and red) than on lighter, light colored sands. The latter tend to reflect more heat during the day (trap less heat) and lose it faster during the night. Although a grower can't change the type soil in his orchard, he 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 lower chilling varieties. This effect is somewhat like the differences recognized in the earlier flowering and cropping of plasticulture strawberries because of the retention of warmer soil temperatures.

Variety Selection

Among nearly all fruit types available varieties may 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 established that rootstocks influence 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 some rootstocks as well as certain interstems used in propagating nursery trees can cause bloom delay. However, as of 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. However in the case of citrus, trifoliate orange or its dwarf form, 'Flying Dragon', are very definitely recommended over all other rootstocks because of their ability to impact greater cold hardiness to citrus types budded onto them, such as satsuma.

Nutrition

Deciduous fruit plants, such as peach, 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 which are 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 (may be postharvest) are useful in maintaining proper vigor in peaches. 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.

Orchard Floor Preparation

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 4). Soils which 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 two inches is recommended.

Pruning and Tree Conformation

While the comments made under this topic primarily apply to stone fruit such as peaches, it is recommended that pruning of all tree fruits and grapes be delayed as long as possible in late winter/early spring.

Pruning practices in commercial plantings have vividly 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 most anytime in fall or winter. Research studies over the past 25 years have demonstrated the negative effect of fall-early winter pruning of peaches (early tree death) in the Southeast. Now it is also evident that delaying pruning into February and March (depending upon location) provides the additional benefits of higher live bud count in the spring and delayed flowering both of which contribute to improved annual cropping.

Following radiational freezes, if any fruits remain on trees they 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 past. This may especially be of value where no active protection methods are used.

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. California citrus producers use taller date palms in some citrus plantings to provide the latter 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. The larger trees absorb and reflect long wave radiation to the fruit trees below thereby providing a few degrees of protection.

Chemicals - Cryoprotectants and Antitranspirants

Several commercially available forms of these chemicals have been studied for their ability to provide freeze protection to fruit crops and none have proven effective thus far. 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 cold hardiness of plants, buds and flowers or in delaying flowering or both. Among the compounds tested, only the ethylene-releasing compound ethephon has shown commercial potential. Work in Alabama and other states have clearly shown 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 mid winter 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 underway to gain approval for a label on peaches in Alabama. Preliminary work suggests ethephon will delay flowering in a number of other fruit crops but no significant studies are underway in the Southeast at present time. Studies with apples indicated ethephon did not effectively delay flowering but fall treatment significantly decreases fruit set and vegetative growth.

Plant Covers

Some form of plant covers have 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 which have shown the most promise are woven and spun-bonded polypropylene types of varying thicknesses (weights). The degree of protection provided varies with the weight of the material, but varies from 2 to 3 degrees with light weight covers to 6 to 9 degrees with more heavy weight 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. Growers have quite effectively devised their own system for anchoring the covers down with sandbags and using mechanical rollers to apply and remove covers.

Work on blueberries has shown covers provide outstanding protection. However, some practical system for application and removal of covers for blueberries as well as other lower growing fruit plants must be developed before widespread use will occur. Regardless of which crop covers are used, they must be removed after each freeze event occurs. 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 indicated bloom delays of two weeks or longer could be achieved. This was basically accomplished by sprinkling trees from rest completion until bloom. Intermittent sprinkling was done anytime air temperature in orchard exceeded 45øF. 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 eliminate the problem.

Painting Tree Trunks

Research has clearly shown the benefit of painting trunks of peach and other deciduous fruit trees white in late summer/early fall. This practice is designed to reduce bark splitting and serious damage to tree trunks during freeze events. There is a differential heating effect that 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 warm much greater than the north side resulting in loss of hardiness. Work in Georgia showed that temperatures on the south side of a peach tree could reach 96øF while air temperature was only 55øF. It is quite common to measure differences of 25 to 40 degrees on tree trunks not treated with white paint. Because the south sides of trunks warm more than north sides, freeze damage is nearly always worse on the south side. Although growers have gotten away from painting tree trunks to protect them during winter months, the practice is still quite effective. An interior water based white latex paint is generally preferred and should be diluted at least 50% with water.

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