The question of whether to cover a lemon tree in winter is best answered with a qualified “yes,” depending entirely on the temperature forecast and the duration of the cold snap. Lemon trees are subtropical evergreens that never fully go dormant, making them highly susceptible to cold damage. They flourish in mild climates, and their vulnerability increases when temperatures drop toward the freezing point. Protection is necessary to ensure the survival of the tree’s wood and overall health, which can be compromised by extended exposure to cold. While a brief, light frost might cause minimal damage, a hard, prolonged freeze requires immediate action to shield the plant from lasting injury.
Identifying Critical Temperature Thresholds
Lemon trees are among the least cold-tolerant citrus varieties, making them highly vulnerable to freezing temperatures. Damage depends on which part of the tree is exposed and the duration of the cold. Generally, temperatures falling below 32°F (0°C) signal the need for vigilance, though fruit and leaves can be damaged at slightly higher temperatures.
The fruit is the first to suffer, with damage possible when temperatures reach 30°F or below for just a couple of hours. Lemon fruit is mostly water, which freezes easily, causing the juice sacs inside to rupture and dry out, rendering the fruit unusable. Wood and foliage can withstand slightly lower temperatures, but the threshold for serious injury is typically around 26°F to 27°F, especially if the cold is sustained for several hours.
A light frost, a brief dip just below freezing, typically only damages the outer layer of leaves and small twigs. A hard freeze involves temperatures dropping well into the 20s or lower for extended periods, risking severe damage to the main branches and trunk. The duration of the cold is often more damaging than the minimum temperature reached, as prolonged exposure allows ice crystals to form within the plant tissues, causing cell membrane rupture.
Practical Steps for Covering the Tree
When temperatures are predicted to drop into the mid-to-high 20s, covering the lemon tree is the most direct protective measure. The goal is to trap heat radiating from the soil and create a small, insulated microclimate around the foliage. Effective covers include specialized frost cloth, burlap, or heavy blankets, which provide insulation while allowing air circulation. Plastic sheeting should not be placed directly against the foliage, as it can trap moisture and cause damage from temperature fluctuation.
Before applying the cover, construct a simple frame or cage around the tree using stakes or PVC pipes. This support structure prevents the insulating material from resting directly on the branches, which can cause damage from weight, especially if the cover becomes wet. The cover must extend all the way to the ground and be securely anchored to seal in the ground heat effectively. Leaving a gap at the bottom will allow the trapped warmth to escape.
The cover must be removed or vented once the temperature rises above freezing during the day, especially if the sun is out. Removing the cover prevents excessive heat buildup, which can cook the foliage and lead to mold or moisture issues. Monitoring the forecast allows the gardener to apply the cover in the early evening before the coldest temperatures hit and remove it the following morning. This temporary procedure should be repeated for the duration of the threat.
Alternative Winter Protection Strategies
Protection strategies that do not involve full wrapping can supplement covering or serve as the primary defense for smaller plants. For lemon trees grown in containers, the simplest strategy is to move them indoors or into a sheltered location. A garage, shed, or unheated sunroom provides protection from wind and direct frost while keeping the plant in a state of semi-dormancy. If moved indoors, placing the container near a south- or west-facing wall can help, as the masonry absorbs heat during the day and radiates it at night.
For in-ground trees, passive heating methods can raise the ambient temperature within the canopy by a few degrees. Hanging large-bulbed incandescent holiday lights throughout the branches provides a small but beneficial amount of warmth. It is also important to ensure the soil beneath the tree is well-watered before a freeze, as moist soil holds and radiates more heat than dry soil, insulating the roots.
Applying a thick layer of organic mulch, such as wood chips or straw, around the base helps regulate soil temperature and protects the root system. The mulch should be spread several inches deep but kept a few inches away from the trunk itself to prevent moisture buildup that could lead to rot. Removing turf or weeds around the tree’s base also allows the bare, moist soil to absorb and release more solar heat.
Post-Winter Assessment and Care
Once the threat of freezing temperatures has passed, protective covers should be permanently removed to allow for full sunlight and air circulation. A careful assessment of damage is required, though injury may not be fully apparent for weeks or months. Symptoms of cold injury can include darkened, water-soaked leaves, bark splitting on the trunk, and dried-out fruit.
Resist the urge to prune immediately following a cold event, as this can worsen the damage or encourage new, tender growth susceptible to later frost. Damaged leaves and small stems can actually provide a measure of insulation for the wood beneath them. Delay pruning until the spring growth flush has expanded, typically late spring or early summer.
Waiting allows the tree to clearly demarcate the truly dead wood from the damaged but viable tissue. New growth will emerge from healthy wood, and any new shoots that quickly wilt indicate that the wood behind them is dead. Pruning should then involve cutting back several inches into healthy, green wood to ensure all damaged tissue is removed. Maintaining healthy trees through good cultural practices, such as proper watering and nutrition, is the best long-term strategy for increasing their cold tolerance.