Key limes (Citrus aurantifolia) are a tropical species prized for their aromatic fruit and distinct flavor. They are dramatically less cold-tolerant than most other cultivated citrus varieties, such as lemons or mandarins. Because the Key lime evolved in consistently warm climates, it is acutely sensitive to temperature fluctuations, limiting its successful outdoor cultivation to the warmest regions.
The Ideal Temperature Range for Growth
The Key lime tree thrives best within a relatively narrow and warm temperature spectrum. Optimal daytime temperatures range from approximately 77°F to 95°F, which allows for maximum photosynthetic activity and nutrient absorption. Maintaining temperatures consistently within this range promotes vigorous vegetative growth, peak health, and reliable fruit set.
Nighttime temperatures are also a major factor, with the tree preferring lows that do not drop below 60°F. Keeping the environment above 68°F is particularly beneficial for active growth, as cooler temperatures slow down the tree’s metabolism. When temperatures fall below 50°F, even briefly, the tree experiences significant stress, diverting energy from growth and fruit production toward survival.
Critical Cold Damage Thresholds
The Key lime’s sensitivity means that temperatures uncomfortable for other citrus can be damaging or lethal to this tropical species. A drop below 40°F signals the beginning of danger, causing the tree to cease growth and leading to stress in new foliage. The tree reacts by shutting down its systems, which often manifests as leaf discoloration or drop.
Severe damage to the canopy begins when temperatures dip to 30°F to 32°F. At this range, water within leaf cells and small twigs can freeze and rupture the cell walls, resulting in significant dieback. Even brief exposure can destroy a large percentage of the leaves and smaller branches. The lethal threshold is reached when temperatures fall below 29°F, especially if sustained for several hours. The cold penetrates deeper wood tissues, including the trunk and the graft union, which can lead to the death of the entire tree above the soil line. The duration of the cold is a major variable; a short dip is far less damaging than a prolonged freeze.
Protecting Key Limes During Cold Weather
Proactive protection is necessary when cold weather is forecast, especially when temperatures are expected to drop below the 40°F threshold.
Container Trees
For Key limes grown in containers, the simplest strategy is to relocate them indoors. Place them near a sunny window, away from cold drafts or direct heating vents. Container trees can also be temporarily moved into an unheated garage or shed, provided the temperature remains above 40°F to prevent freezing.
In-Ground Trees
In-ground trees require external intervention to survive a freeze. Begin by thoroughly watering the soil a day or two before the cold snap, as moist soil retains more heat than dry soil. Protection should focus on insulating the tree and trapping heat. Drape a specialized frost cloth or thick blanket over the entire canopy, ensuring the edges are secured to the ground. For younger trees, wrapping the trunk with insulating material, such as burlap, is important to protect the sensitive graft union.