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Freeze damage on citrus trees occurs when water inside the fruit, leaves, twigs and wood of a tree freezes rupturing the cell membranes. Unlike deciduous trees which protect themselves from cold by shedding their leaves in the fall and entering a dormant state, citrus trees continue growing year-round. Extended periods of cool weather prior to a freeze may allow a citrus tree to prepare somewhat. This is why sharp freezes following warm weather are more damaging than gradual temperature changes. However, virtually all freezes will cause damage of some kind.
Regardless of what steps you take, there are times when nothing you have done helps and your citrus is damaged by freezing. However, as long as the damage is not too severe, your tree can recover with a little encouragement from you. One of the keys to dealing with freeze damage is not to do something right away to but to wait awhile until the extent of the damage becomes apparent. In some instances, twig and branch death from a severe freeze can continue for as long as two years after a severe freeze. Act too soon and you run the risk of either pruning away parts of your tree that can recover on their own or missing parts that look healthy enough at first glance but are really fatally damaged.
In the case of citrus fruit, the interior of the fruit may suffer extreme damage while the peel appears normal. Occasionally, the exterior of the fruit may appear blemished or pitted. Badly damaged fruit may fall from the tree; however, this may not happen in all cases or in situations where the fruit suffers moderate damage. In any case, over time, the frozen interior of the fruit will dry out and the fruit will hollow.
The appearance of citrus leaves damaged by freezing can be a little deceptive in that they can appear firm and green at the outset. It is only later, as they thaw, that they soften and droop. In instances where the damage is not severe, freeze-damaged leaves can recover. However, if the damage is fatal, the leaves will loose their structure completely, dry out and fall. While alarming, leaf fall alone does not indicate tree death. If the wood remains healthy, the tree will recover and put out new growth in the spring. As for twigs, damage to the twig will almost invariably result in leaf death. In the case of serious damage, the leaves will dry out but may stay attached for a time, several weeks in some cases. However, if the twig is not badly damaged, the leaves will fall more rapidly.
Signs of freezing damage in branches and trunks include the loosening and splitting of bark. Patches of damage may appear oozing canker-like areas (cold cankers) occasionally mistaken for the disease gummosis.
The first step in the pruning process is to wait until late spring or the summer following the winter the damage occurred. This will give you time to assess the damage. In addition, freeze-damaged trees occasionally put out a false start of new growth in the early spring which soon dies back. Delaying pruning until after this occurs will save you time and energy.
When pruning always remember to prune living wood, ideally at crotches, to ensure that you cut away all of the damage. In the case of young trees that have been banked, the tree may survive and put out a new top even if you have to cut away all the wood above the bank.
In very severe cases, a citrus tree may be damaged all the way to the ground. In such cases, the root area may still put out new growth and the tree may, in time, recover. However, if the original tree was a graft and the tree is killed off below the bud, any resulting new growth will be of the variety of the rootstock and not of the graft or scion. It will be up to you to decide whether to re-graft, allow the rootstock to continue growing or start again.
The above images have been reproduced from Fact Sheet HS-120, a series of the Department of Horticultural Sciences, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Publication date: June 1992. Reviewed: June 1994.