Frost Protection - UTL Repository

PASSIVE PROTECTION METHODS
damage has passed. In a radiation frost, damage typically occurs from the bottom
up in deciduous orchards. Therefore, if a frost event occurs, this practice will
improve the chances for a good crop.
Pruning grapevines to raise the fruit higher above the ground provides some
frost protection because temperature typically increases with height above the
ground during radiation frost nights. In some instances, raising the fruit by 0.3 to
0.5 m can increase the temperature by 1 °C or 2 °C. Canopy density and pruning
can affect the frost sensitivity of deciduous trees. Closed canopies at high density
indirectly increase frost damage sensitivity because of reductions in photosynthesis
and hence sugar accumulation lower in the canopy where it is colder.
COOLING TO DELAY BLOOM
It is well known that operating sprinklers during warm days in the winter can
delay bloom and hence provide a measure of frost protection (Anderson et al.,
1973; Proebsting, 1975). Sprinklers cool the crop because evaporation converts
sensible to latent heat, which causes the temperature to drop. The probability of
subzero temperature falls dramatically in the spring over short periods of time,
so cooling crops to delay bloom decreases the probability of frost damage.
Research on several deciduous tree species has shown that bloom delays of two
weeks or more are possible by sprinkling from breaking of rest to bloom
whenever the air temperature is above 7 °C (Powell and Himelrick, 2000). For
example Anderson et al. (1973) reported budding delays of 15 and 17 days for
cherry and apple trees, respectively when the orchards were sprinkled whenever
the air temperature exceeded 6.2 °C between breaking rest and bud break.
Sprinkling to delay bloom has also been advised as a method to delay bloom of
grapevines (Schultz and Weaver, 1977). However, the benefits of sprinkling
depend on the humidity as well as the temperature. When the sprinklers are
operated, the temperature will drop to near the wet-bulb temperature, so there is
little benefit in attempting to cool by sprinkling in humid environments where
the dew-point temperature is close to the air temperature.
Although research has shown that fruit tree bloom is delayed by sprinkler
operation, Powell and Himelrick (2000) noted that the method was not widely
adopted because of crop production reductions that are not understood (Powell
and Himelrick, 2000). Evans (2000) also reported the use of sprinklers for bloom
delay in apple and peach trees. However, he recommended against the procedure
because, although bloom is delayed, the increased sensitivity of buds to frost
injury counteracts the benefits of bloom delay. Evans noted that the buds regain
hardiness after being wetted if allowed to dry during a cool period. Although
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