Principles of Freeze Protection for Fruit Crops

Damage to floral structures may take on many forms. The formation of frost on flowers or fruit may not kill the tissue but can cause scaring of the skin of the fruit and possible flesh damage. The internal freezing of tissue of buds, flowers and fruits is what causes serious damage or death of the floral parts. When small floral structures such as flowers or fruits freeze, they may take on several forms of damage. Severe temperatures usually destroy the entire buds, flower, or ovules (immature seed) and ovaries comprising small fruits resulting in rapid abscission of the structure. Slightly less damage, as in peaches, may cause catfacing (creasing of the skin and flesh) along with partial or complete death of ovules. The damaged fruit may remain until harvest but is poorly shapen, small in size and non-marketable. Even less severe freeze damage may cause only death of ovules (as in peach) and the remaining fruit develops into a very small fruit called a "button" which may persist until harvest. Marginal freeze damage in blueberry may only kill 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. However, 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 can thaw and remain sound. They may however begin slowly drying out (losing juice content) until harvested.

Indices using air temperature have been developed through research and field experience for use in informing producers of the temperature at which serious freeze damage will occur in various floral and vegetative structures. These temperatures may be used in making decisions on when to begin operating active protection systems and when they may be safely turned off. To date, too many unknowns exist regarding tissue temperatures to use this approach to managing freeze protection systems, although considerable research has been done.

Detailed information is provided in Tables 4 and 5 on critical air temperatures to be used in operating freeze protection systems in fruit plantings.

Note that temperatures in Table 6 are similar to those published by Washington State University except the critical temperatures for the most advanced stages are slightly higher in Alabama. It should be noted that the values shown in Tables 5 and 6 are usually for varieties of average hardiness. There are substantial differences among varieties in hardiness from dormant through early, prebloom stages.

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