*Both citrus fruit and foliage can be damaged if temperature falls below freezing for a prolonged period. However, weather conditions prior to cold temperature, duration of cold, position of the tree in the grove or yard, maturity of the fruit, and health and age of the tree can affect tree and fruit hardiness.
*Trees are more cold hardy when exposed to cooler temperature over several weeks prior to freezes. Sudden cold snaps can be particularly damaging to citrus. Cold tolerance develops most readily when trees are not flushing. Warm temperatures at any time during the winter may cause citrus trees to resume growth and reduce their cold tolerance.
*Ice formation in citrus tissues – not low temperature – kills or damages citrus trees and fruit.
*Fruit damage occurs when the temperature falls below 28°F for at least four hours.
*Frozen fruit can be salvaged for juice.
*Four hours at 20°F can kill 3/8 inch or smaller wood and temperatures below 28°F for 12 continuous
hours may kill larger limbs and possibly the entire tree.
*The most popular method of freeze protection employed by growers is irrigation. Small micro-sprinklers at the base of the tree emit a mist that turns to ice and engulfs the trunk and lower canopy. As the water changes to ice it give off heat (called latent heat of fusion) which protects the tree. As long as water is constantly changing to ice the temperature of the ice-water mixture will remain at 32°F.
*The major problems in the use of irrigation for cold protection occur when inadequate amounts of water are applied or under windy (advective) conditions.
*Depending on the type of freeze (advective/windy freeze or radiation frost) water can provide protection in one situation and cause damage in another, it is important to know what principles are involved and understand that dew point and what can happen when using water during a freeze.
Information provided by Mongi Zekri, multi-county citrus agent, University of Florida, IFAS, SW Florida Extension Service.