Citrus trees do not go dormant to protect themselves from cold winter temperatures. Freezing ruptures the cells in fruit, leaves, twigs and roots. Long periods of cool weather can help a citrus tree prepare itself for a freeze, but a sharp freeze after warm weather will almost certainly damage the tree.
Damage to Fruit
The peel can appear normal while the interior fruit is badly damaged. Sometimes citrus peels develop pits and blemishes. Badly damaged fruit may fall, but moderately damaged fruit can stay in place. Over time, the frozen interior of the fruit will dry out and become hollow.
Damage to Leaves
Leaves of citrus trees may droop or wilt when the temperature drops, but this is not freeze damage. Freezing turns leaves hard and brittle; severe freezing causes them to dry out and drop from the tree.
Damage to twigs usually kills leaves. The leaves can appear green after a freeze, but they will soften and droop after they thaw. Seriously damaged leaves will dry out but can stay on the tree for weeks.
Damage to Bark, Branches and Trunk
Freezing can cause loose bark and can split the larger branches and trunk. Branches and twigs may keep dying for two years after a severe freeze.
Pruning Damaged Trees
After a severe frost, branches and twigs can continue dying for as long as two years. If you prune too soon, you risk removing parts that will recover on their own, or you can miss fatally damaged parts that look healthy.
Since it's difficult to determine the damage of a tree until it begins growing in the spring, it's best to delay pruning damaged limbs until late spring or summer.
Prune living wood at the crotches to make sure you have cut away all the damaged wood. If you banked young trees with soil to protect them, cut away all the wood above the bank, and they will still survive.
Trees Frozen to the Ground
A severe freeze can damage a citrus tree all the way to the ground, but the root area may still put out new growth so the tree will recover in time.
If you lose a lot of leaves, but not branches or twigs, increase slightly the amount of fertilizer you ordinarily use. Don't fertilize until after the tree begins new growth. Frequent light applications are better than one heavy application.