Orange trees become frost damaged when fruit freezes at 27 to 28 degrees Fahrenheit. The duration of cold weather can be more significant than a brief drop in temperature. Orange trees are damaged when the water inside the fruit, leaves, twigs and wood freezes, and the cell membranes rupture. The fruit becomes hollow and frozen, and then dries out. It is possible for an orange tree to recover from frost damage.
Severe frost damage can cause the loss of an entire commercial crop or ruin a home garden harvest. Frost damage causes the juice vesicles inside the orange to rupture. Movement of the liquid is disrupted and fruit may be severely damaged. Moderately damaged fruit may remain on the tree, but becomes hollow and dried up inside. Freezing can also split the tree trunk, allowing bacteria and fungi to enter.
Badly damaged oranges fall from the tree. Some damaged fruit can remain on the tree. Frost damaged leaves appear water-soaked and dark green. Damage to fruit and leaves appears 2 days to 2 weeks after exposure to frost. Orange fruit skin may appear mottled, blemished or pitted. As the oranges unfreeze they remain a mottled color and have a weak structure.
Growers sometimes wrap the orange tree trunk with insulating material to protect it from frost damage. Soak trees well when a severe frost is expected. Trees which are well irrigated withstand frost more easily. Plant orange trees in a protected area of the garden. Citrus trees do well when planted near a wall because the heat reflects and warms the tree. Keep the area around the tree trunk free from debris and plant material so heat radiates from the ground.
Do not prune a frost-damaged orange tree until late spring or summer. Warm weather increases the ability of the tree to recover from frost. A frost damaged tree branch may appear dead, but still has the ability to recover. In spring, new leaves begin to grow and it is easier to assess damage. Prune all dead wood and branches. Make the pruning cut directly below the damaged area. Healthy areas are green, damaged areas are brown. The tree's energy is available for new growth after the dead wood has been removed.
Citrus trees thrive in a protected area of the garden. Do not plant citrus in a windy area or a cold spot in the garden. Cold air moves downward. Citrus trees do best when planted on a high spot. Consult your garden center for the varieties that do well in your local area.