Know the Signs of Freeze Damage

By Amir Rezazadeh

It is important to know how cold temperatures impact citrus trees. Citrus trees are native to subtropical and tropical regions. Although some mandarins and tangerines are quite cold-hardy, other citrus trees are not particularly cold-hardy and temperatures below 20 degrees for more than four hours may kill most citrus trees.

Chilling damage in citrus depends on several factors. These include minimum temperatures, duration of cold temperatures, stage of tree acclimation, tree age, tree health, scion and rootstock, crop load and soil condition.

Freeze damage in citrus is due to ice formation in the cell membranes of fruit, leaves, twigs and wood of a tree. Disruption of the cell membranes following ice formation damages cell walls. Dark, water-soaked areas on leaf surfaces are indications of freeze damage. Severely frozen leaves are bleached or tan to brown in color. New succulent leaves will turn black in color upon freezing. Wood damage may appear as the scraping of the outer layer of bark. Generally, green tissue refers to live wood, while brown tissue implies freeze-damaged deadwood.

In young trees, ice formation may result in bark splitting. Bark splitting in larger trees may cause serious injury.

Fruit damaged by freeze may drop, but this will not happen in all cases. Fruits may not show a significant change on the exterior. The exterior of the fruit on certain cultivars, such as grapefruit, may appear blemished or pitted due to low but not freezing temperatures.

Among the citrus cultivars, citrons, lemons and limes are most easily killed or damaged by freezing weather. Sweet oranges and grapefruits are more cold-hardy, but temperatures in the mid-20s will damage even large branches. Tangerines and mandarins are even more cold-hardy cultivars that can withstand temperatures at 20 degrees without significant wood damage. Keep in mind that the damages due to the temperatures stated refer to wood and leaves, but fruit easily freezes if exposed to a higher temperature (26 to 28 degrees).

Amir Rezazadeh is a multi-county fruit and field crops Extension agent at the University of Florida Institute of Food and Agricultural Sciences St. Lucie County Extension office in Fort Pierce.