Citrus trees are subtropical, evergreen plants, making cold temperatures their most significant environmental threat. Unlike deciduous trees that enter deep dormancy, citrus trees retain their leaves all year, increasing their vulnerability to freezing conditions. They lack the natural winter-hardening mechanisms of plants native to temperate regions. Cold damage can quickly impact the tree’s health, leading to loss of fruit production, limb dieback, or even the death of the entire tree. Understanding specific temperature tolerances and taking appropriate action is important for growers.
Critical Temperature Thresholds for Damage
The precise temperature at which a citrus tree sustains damage depends heavily on the duration of the cold event and the tree’s pre-conditioned state. For the fruit itself, temperatures below 30 degrees Fahrenheit for two hours or more can cause immediate internal damage to the juice vesicles. Mature citrus fruits, such as oranges, may withstand a slightly lower temperature of 26 degrees Fahrenheit for the same duration before freezing occurs.
Wood and leaf tissue are slightly more tolerant, with initial, cosmetic damage to leaves typically starting in the high 20s Fahrenheit. A short-term drop to 22 degrees Fahrenheit may cause minimal leaf damage. However, extending exposure to over eight hours at that temperature can lead to a 50% leaf kill. The total time spent below freezing is often more significant than the minimum temperature reached.
Lethal damage to the structural wood usually occurs when temperatures drop to 20 degrees Fahrenheit or lower for an extended duration. Trees that have successfully acclimated to cooler autumn weather are more cold-hardy because they have reduced cellular water content, which lowers the tissue’s freezing point. Actively growing trees, which are not hardened off, are much more susceptible to damage at higher temperatures than dormant, acclimated trees.
How Tolerance Varies by Citrus Type
Cold tolerance varies significantly across the citrus family, depending on the specific species or cultivar. This variation is linked to differences in cellular structure and the concentration of solutes within the plant’s tissues, affecting the temperature at which ice crystals form. Citrus varieties can be placed on a spectrum from the most cold-hardy to the most cold-sensitive.
The most resilient types are mandarins and their relatives, such as Satsumas and Kumquats, which can survive temperatures near 20 degrees Fahrenheit without major wood damage when fully acclimated. These species are often the preferred choice for growers in regions that experience occasional hard freezes. Sweet oranges and grapefruit fall in the intermediate range, generally suffering severe damage to large branches only when temperatures drop into the mid-20s Fahrenheit.
At the lower end of the spectrum are the acid citrus types, including limes, lemons, and citrons, which are the most susceptible to cold. These varieties can be severely damaged or killed when temperatures remain in the high 20s Fahrenheit for even a few hours. This difference in tolerance means a cold snap causing only cosmetic damage to a Satsuma may be lethal to a lime tree.
Immediate Cold Protection Methods
Preparing a citrus tree for a freeze should begin at least 24 hours before the temperature is expected to drop. Deeply watering the tree is effective, as moist soil absorbs and retains more heat than dry soil, which then radiates upward to the canopy overnight. For maximum heat transfer, any mulch or grass immediately under the tree’s canopy should be pulled back to expose the bare soil.
Covering the canopy is the next step, using insulating materials like frost cloth, blankets, or burlap. The cover must extend all the way to the ground and be secured, creating a tent that traps the heat radiating from the soil. Avoid using plastic sheeting directly on the tree, as this material can conduct cold and damage the leaves where it makes contact.
For young or especially vulnerable trees, additional measures can provide a few degrees of extra warmth. Placing a string of incandescent Christmas lights or a small heat lamp beneath the protective cover can generate enough heat to prevent tissue freezing. Mounding soil or wrapping the trunk of a young tree with insulating material or burlap protects the graft union. This ensures that even if the canopy dies back, the desired variety can regrow from the rootstock.
Post-Freeze Assessment and Recovery
Once freezing temperatures have passed, resist the urge to immediately prune the damaged tree. Premature pruning can slow the tree’s recovery and may remove wood that is still viable. It is best to wait until the danger of subsequent frosts has passed and new spring growth has fully emerged.
The extent of the damage can be assessed by scratching the bark on various branches to check for a living, green layer underneath, which indicates a healthy cambium. If dead leaves cling to the branches for more than a few weeks, it indicates the branch itself has been killed. The only immediate post-freeze action that is beneficial is applying white latex paint or commercial whitewash to any exposed bark. This prevents sunscald damage on defoliated limbs that were previously shaded.
When it is time to prune, only cut back the wood that is definitively dead, making cuts into living tissue to encourage proper healing. During recovery, avoid applying fertilizer until a healthy flush of new growth is evident. Damaged trees cannot efficiently absorb nutrients, and excessive fertilization can lead to toxicity and stress. Irrigation should also be reduced because the defoliated tree needs much less water than a full-canopy tree, which prevents root rot.