The Problem With Citrus Freezing

Extent of Cold

• Much of the anxiety experienced by citrus growers and winter freezes centers around the actual air temperature that occurs around the trees. Weather forecasts provide only approximate low temperatures expected. Cold air sinks across the landscape, so upper parts of the tree may escape the coldest temperatures, while lowest branches will experience the deadliest temperatures. Upper branches and leaves within the tree can protect lower and inner buds and fruits from cold as well. The duration of the freeze also affects what damage, if any, occurs. A nighttime temperature of 25 degrees Fahrenheit that lasts for several hours will cause more severe damage than if that temperature was reached for only 30 minutes before conditions warmed.

Prevention

• Planting citrus trees on a slope oriented to capture the warming morning sun's rays, or in an area sheltered from cold, drying winds helps diminish the extent of cold. Proper site placement may be effective in preventing citrus freezing even if temperatures briefly drop into the upper 20s. Substantial resources, as measured in time, labor and money, are required to prevent damage on citrus trees. Running irrigation systems, covering trees with frost cloth, burning heaters or warming lights or hiring a helicopter to hover over orchards to mix the air are all methods you can try. However, these tactics do not guarantee success in preventing any damage to the trees.

Damage

• The sap in citrus leaves and branches, as well as juices in fruits, does not freeze at precisely 32 degrees Fahrenheit because impurities lower the freezing point in the liquid solution. Citrus trees may not sustain any or minimal leaf scalding by limited encounters with temperatures in the 26- to 32-degree range. Once the temperature drops into the 15- to 25-degree range, substantial tissue destruction begins, and a tree can be killed, depending on the citrus tree type. Dead leaves may brown and wilt or fully drop from the tree. Small twigs completely dry out from the freeze, rupturing the sap cells in the vascular tissues. Fruits may persist or drop away.

Aftermath

• Citrus trees affected by subfreezing temperatures do not manifest the damage immediately. It can take weeks for the full extent of tissue death to be understood. Fruits may not initially look harmed, but a freeze causes water to build up under the rind and the flesh to rot, even though the fruit's outer skin seems normal. A couple of weeks later, the fruit finally becomes soft and its interior quality pithy and dry; it is no longer usable even for salvaged juice. This is the reason why citrus farmers choose to pick fruits after a freeze. Rather than risk no fruit crop, frozen fruits are quickly picked and made into juice before the fruits degrade. Not until midspring, once the new leafy growth emerges, will you truly know how much damage occurred. You will need to prune to remove all the dead branches. Living branches display new leaves and flower buds in spring, and the tree must rejuvenate more branches so that in the following year, there is an ample number of flowers to later develop into a good fruit crop.