The hard freeze of December 1990 and 2007 and similar episodes earlier in the century vividly demonstrated that no citrus tree is immune from freeze damage. A given citrus tree will be more tolerant of cold temperatures once it has become semi-dormant (for example as in January) as compared to its level of tolerance in the fall or in late winter as temperatures increase again in March. It may take a temperature of 12°F to kill a mature navel tree in February, but only 20°F prior to the tree going into dormancy in mid-November. Young trees are at much greater risk of being killed from a frost than are older trees. Lemons and limes are much more sensitive to frost than are oranges. Grapefruit are intermediate. To maximize the ability of a tree to survive a frost event, the soil surrounding the tree should be bare and firm. Mulches should not be present during the cold months. The soil should be wet to a depth of
six to eight inches, prior to the freeze as the water can retain more heat per unit of weight than can the soil. A hose may be left on through the freeze event, and this running water will supply additional heat
to the tree. Bare soil and wet soil can absorb more heat from the sun during the day, and reradiate this heat back to the tree at night.
The same insulative materials that are used to wrap pipes to prevent their freezing can be used to wrap trunks of young citrus. Straw and cardboard may be used to wrap the trunks of mature citrus trees, although this usually unnecessary unless very cold weather is expected (i.e. less than 20ºF for lemon and colder for orange). The canopies of younger trees can be covered with blankets, tarps, plastic or whatever else looks like it might work. These covers will absorb heat from the surroundings and
radiate this energy back to the tree. It is not necessary to totally enclose the tree unless extremely cold weather is forecasted. Covers should be removed during the day to allow maximum heat absorption by
the trees surroundings. If the covering is plastic, especially clear plastic, it should be removed during the day to avoid cooking the tree. Avoid wetting the covers themselves, as often the water on the
covers will freeze, and freeze leaf tissue in contact with the cover.
Propane stoves, incandescent lights, electric heater and the like have been used effectively in conjunction with covers to keep backyard citrus from freezing. (Information from Kern County California) - Millet
