Drier La Niña Winter Conditions Can Contribute to Sudden Freezes
Although the California avocado growing regions are expected to experience a moderate La Niña phase with warmer average temperatures through spring 2021, the lower rainfall amounts associated with this climate phase can lead to sudden cold spells or freezes. As Dr. Ben Faber noted in a recent blog post, some of California’s most severe freezes have occurred during weak La Niña phases.
Advection and radiation freezes pose the most threat to California avocado groves. Advection freezes are caused by the movement of arctic air into the region. Radiation freezes occur at night when clear skies and calm conditions are present that allow cold pockets of air to settle in low areas of the grove.
To prepare for potential frosts or freezes, it’s important to remember that different prevention measures may be used for a frost versus a freeze. A frost is caused when objects cool at night and radiate their heat loss, thus chilling the surrounding air. In Southern California, warm air is typically close to the ground due to a low ceiling, thus causing a temperature inversion that protects orchards. However, windy conditions can disrupt this inversion and press cold air to the ground. In comparison, a freeze occurs when cold air moves in and the air temperature decreases at both high and low levels.
- To prevent damage when cold weather events are in the forecast, consider the following.
- To protect against frost or freeze, orchard heaters can be used to distribute heat. The downside of heaters is the cost of running them and possible fire hazards.
- Wind machines should only be used in frost, not freeze, conditions and should not be used when it is windy. This economical option can be paired with orchard heaters to improve effectiveness.
- If frost threatens and no temperature inversion is present, the best practice is to run microsprinklers during the day and turn off the water prior to sunset. If the temperature drops below freezing, restart the water and run it until sunrise. If ice forms on the fruit or leaves, heat will be released as the ice melts and protect the plants.
- If watering the entire grove prior to a cold weather event is cost prohibitive, it is recommended that growers opt to water only those portions of their groves that tend to be coldest.
If your grove is affected by a frost/freeze event, please view Post-freeze Avocado Grove Management on the California Avocado Commission’s website. For more complete information, visit the Commission’s online library of frost/freeze protection articles.