Citrus trees are evergreen, never become fully dormant and cannot withstand temperatures as low as those tolerated by deciduous trees. However, citrus trees can become preconditioned or acclimated to cool air temperatures that occur in late fall and winter. This preconditioning, called acclimation of citrus trees, induces a degree of dormancy known as quiescence. Trees in active growth are more severely injured by cold than those that are acclimated. Recovery from freeze damage depends largely on tree vigor and health.
Symptoms of Freeze Damage
Factors responsible for freeze damage include minimum temperature, duration of freezing temperatures, and stage of tree acclimation. Susceptibility of trees to freezing temperatures can also be related to tree vigor, scion and rootstock, crop load, and grove and soil conditions.
Leaf and Wood Injury
Citrus freeze injury is caused by the formation of ice crystals in the intercellular spaces of plant tissues. Ice formation in leaves and the subsequent damage are diagnosed by dark water-soaked areas on the leaf surface. Such areas may or may not turn brown after thawing. New growth, when frozen, will turn a blackish color. Leaf drop within a few days is a sign that the wood is likely not damaged or killed. Leaf retention on the twigs usually indicated wood kill. Wood damage can be checked by scraping the outer layer of bark. Green tissue implies live wood, brown tissue implies freeze-damaged dead wood. Extensive bark splits may cause limbs to die and later break off many years after the freeze event. Trees should not be pruned after the freeze event until the extent of the damage is determined.
Fruit severely injured during a freeze may drop quickly or may drop more slowly over time. However, the external appearance of the fruit is not significantly changed. The first evidence of freeze injury in citrus fruit is characterized by the presence of water-soaked areas on the segment membranes. Shortly after the freeze, cutting the fruit progressively from the outside to the inside, starting at the stem end, will show the amount of ice formed and its location and distribution within the fruit. The deeper the ice crystals form, the greater is the injury to the fruit. The amount of overall juice loss is dependent on damage severity and weather conditions following the freeze.
Recovering from Freeze Damage
The type of care needed for citrus trees that have been freeze injured will be determined by a number of factors, including the time of year at which the freeze occurs, the overall condition of trees at the time of injury, and the weather conditions immediately following injury. Twigs and branches may continue to die for a period of several months to a couple of years following a severe breeze. Therefore, making it impossible to determine the full extent of the injury. Citrus trees on which twigs and branches have been killed by cold should receive extra care during the following growing season.
- Pruning – No attempt should be made to prune or even assess freeze damage until the new spring flush gets fully expanded and mature. In early spring, freeze-damaged trees often produce new growth that soon dies back. Early pruning does not promote recovery and delaying pruning to the proper time will save money. It is advisable to remove heavy brush from the grove immediately after the pruning operation.
- Fertilization– Should be reduced until the trees are back to their original canopy size and foliage density. Fertilizer should be applied more frequently, but rates should be reduced in proportion to the amount of tree damage and to the expected crop load. Foliar sprays of micronutrients will be beneficial, due to the nutrient deficiency symptoms that are intensified i freeze-damaged trees. Leaves require nutrients in order to regenerate large amounts of growth necessary to replace lost foliage.
- Irrigation– When leaves are lost, transpiration from the tree canopy is greatly reduced. Therefore, the tree will require less water, and thus the amount of water supplied to the tree should be reduced. Over-irrigation in the winter following a cold injury should be avoided, because it may induce new vegetative growth which could be damaged by subsequent freezes.
- Weed and Disease Control– To ensure rapid recovery from freeze damage, an effective weed control program should be implemented. Weeds will compete heavily with the trees for available water, nutrients, and light. Fungicide applications for greasy spot control after a freeze in May and July are essential. Even on non-fruiting trees, one or two fungicide applications should be made to help prevent infection of new flushes and of next year’s crop.
One of the most devastating environmental stresses to citrus trees is a freeze event. However, citrus trees are vigorous and can recover quickly from cold damage, if given proper and timely care.
If you would like to know more about citrus freeze damage symptoms and recovery, visit edis.ifas.ufl.edu or contact your local extension office.