Principles of Freeze Protection for Fruit Crops

METHODS OF FREEZE PROTECTION

Frost/freeze protection may be divided into passive and active forms. Passive protection involves methods such as site selection, variety and rootstock selection and cultural practices such as pruning and orchard floor preparation that do not require expenditure of outside energy sources during the frost/freeze event. Active protection systems replace or prevent radiant heat loss by using methods such as heaters, irrigation or wind machines that require outside energy to operate during the freeze. The form of active protection chosen depends upon the crop, site, economics and other variables. A discussion of the advantages and disadvantages of passive and active methods of protection is given below. Table 3 provides an overview.

Table 3. Overview of Freeze Protection Alternatives1.

1. PASSIVE PROTECTION SYSTEMS

Description of Protection Method

Crops Protected
Site Selection - selecting sites with 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 in winter/early spring resulting in earlier plant activity. All fruit crops
Variety Selection - selecting varieties which historically have best cropping during severe freeze events. All fruit crops, but especially stone fruit and blueberries.
Rootstock Selection - some rootstocks impart greater hardiness to scion varieties and delay flowering. Citrus, stone fruit; except for citrus, differences in hardiness induced by recommended rootstocks for other fruit crops is inadequate to recommend one over another.
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/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; assists to some degree with blueberries.
Nutrition - using mid summer or postharvest application of nitrogen can induce sufficient vigor for strong fruit bud development and some delay in flowering, thereby increasing cropping. Mainly on stone fruits and blueberries.
Planting Canopy Trees With Orchard Crop - use of large canopy trees (such as date palms used in citrus plantings in California) can afford some freeze protection. Pine trees are being used in citrus plantings in south Alabama. Mainly on citrus.
Chemicals - cryoprotectants and antitranspirants; commercially available forms of these chemicals have not provided freeze protection to flower buds, flowers and young fruits through foliar sprays. Are not 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 GA3 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 upon the material used several degrees of protection are afforded. Strawberries and possibly blueberries, and small citrus.
Evaporative Cooling - special dormant season use of overhead misting of stone fruits from the period rest is satisfied until bud swell delays flowering but damages buds. More studies 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.

2. ACTIVE PROTECTION SYSTEMS (Requires supplemental energy)
Wind Displacement Devices - using wind machines and helicopters to raise temperatures in plantings. All fruit crop plants, but mainly used in tree fruits.
Heating - using heating devices such as orchard heaters to raise temperatures in plantings. All fruit crop plants but mainly used in tree fruits.
Irrigation - sprinkler application; using 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 may be pumped onto orchard floor ahead of freezes as done in California. Requires relatively flat soils. All fruit crop plants, but terrain dictates use; most common on tree fruits.

1 Freeze protection in this table focuses attention primarily on preventing or minimizing crop damage. However, most of the methods listed also provide some level of plant protection. Painting of tree trunks is the only method used strictly for plant protection.


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