Preparing for the Potential Frosts and Freezes of January
For California avocado growers, January is the month that groves tend to be susceptible to frost and freeze damage. As the threat of colder weather arrives, it’s important for growers to understand the mechanics of frosts and freezes, and what they can do to protect their groves from unseasonably cold conditions.
Frost vs. Freeze
To begin with, it’s important to differentiate between a frost and a freeze. A frost is caused when objects cool during the night and radiate their heat loss, chilling the surrounding air. Cold air tends to settle near the ground while warm air rises. In Southern California, the “ceiling” is often low, which means that warm air is closer to the ground. This phenomenon is called a temperature inversion and it tends to protect orchards from the cold.
That said, when forecasts indicate windy conditions this disrupts the inversion and thus the colder air is pressed to ground level. Knowing whether a temperature inversion is present or not is a key factor in determining whether to use wind machines as a preventative measure (this is discussed in further detail below).
A freeze occurs when a mass of cold air moves in and the temperature of the air decreases at both ground level and in the higher air levels. Protecting groves in these conditions is difficult and usually involves adding heat to the grove with orchard heaters.
To prevent frost damage in a mature grove, the two most-relied-upon methods are orchard heaters and wind machines.
Orchard heaters tend to distribute heat to all parts of the grove when well distributed, but can be expensive to run and can present a fire hazard.
Wind machines are economical and less labor intensive and, when used with orchard heaters, can increase the effectiveness of the heaters. Wind machines should only be used in frost conditions — not during freeze conditions in which temperatures fall 4 or more degrees below damage points. In addition, wind machines should not be used in conditions (usually windy) in which no temperature inversion is present. If wind machines are used in these conditions, they can cause more damage because they simply stir up the cold air at tree level.
In situations in which there is no temperature inversion, the best practice is to run microsprinklers during the day and then turn off the water prior to sunset. If temperatures drop below 32˚ during the night, the water should be started again and run until sunrise. According to Dr. Ben Faber, even if the running water freezes this is the chosen method. If ice forms on the fruit and leaves, it will release heat as it melts with the sunrise and protect the plants.
Because it can be expensive to run water, growers can opt to choose those portions of their grove that tend to be the coldest in order to save money. When the temperature is below freezing, Dr. Faber recommends against running water in a block during the night and then turning it off to water another block as this will lower the temperature in both blocks and lead to damage.
It should be noted that these methods are less effective for protecting young trees. To protect young trees, insulative wrap or frames with a burlap cover may be needed.
Caring for Frost Damaged Trees
If trees lose leaves to frost/freeze, it’s important to protect exposed branches from sunburn with whitewash. Damaged trees will require less water due to loss of leaves, so refrain from irrigation until the root zone soil begins to dry. This also helps protect the damaged tree from root rot.
Refrain from pruning until the extent of damage is clearly visible. This may take months. Remove dead wood once new shoots are 2 – 3’ long and be certain to thin out suckers.
If a tree is severely frozen, it’s best to contact a Farm Advisor to determine the best next steps.