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.

Table 5. Critical temperatures for various tree fruit crops. Numbers are degrees Fahrenheit.

Stage 10% Kill 90% Kill
Silver Tip 15 2
Green Tip 18 10
1/2" Green 23 15
Tight Cluster 27 21
Pink 28 24-25
Bloom 28 25
Post Bloom 28 25
First Swell 18 1
Caylx Green 21 5
Caylx Red 23 9
Pink 25 15
Bloom (early) 26 21
Bloom (late) 27 24
Post Bloom 28 25
Scales Separate 15 0
Buds Exposed 20 6
Tight Cluster 24 15
White Bud 25-26 22-23
Bloom 27-28 23-24
Post Bloom 28 24

Table 6. Critical Temperatures for Peaches in Alabama. Numbers are degrees Fahrenheit1.

Stage 10% Damage 100% Damage
Dormant 5-20 -10
Full Swell/1st Green 15-18 0-5
Calyx Green/Red 21-23 5-10
Pink Bud 25 15-18
Full Bloom 27-28 26
Petal Fall/Wet Shuck 28 26
Dry Shuck 28-29 26
Shuck Split/Off 29 26
1The above temperatures were obtained over a 19-year period in commercial orchards using minimum thermometers.

Table 7. Critical Temperatures for Small Fruit Crops.

Stage 90% Damage
Tight But 22
Tight with White Petals 28
Full Bloom 31
Immature Fruit 28
BLUEBERRY (rabbiteye)
Swelled Flower Buds 21
Individual Flowers Distinguishable 25
Flowers Distinctly Separated, Corollas Expanded but Closed 28
Fully Opened Flowers 31

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