Citrus trees are subtropical evergreens that thrive in warm, mild climates, making them uniquely vulnerable to freezing temperatures and frost. Unlike deciduous species, citrus trees do not drop their leaves or enter deep dormancy, leaving their foliage and fruit susceptible to cold injury throughout the winter. Protecting these trees requires precise timing; acting too late results in damage, while acting too early or leaving covers on too long can cause other issues. Understanding specific temperature thresholds and tree characteristics is necessary to determine when protective covering should be implemented.
Critical Temperature Thresholds
The need to cover a citrus tree is determined by the severity and duration of the expected cold event, categorized by specific temperature thresholds. A light frost, where temperatures dip between 28°F and 32°F, typically poses the greatest risk to the fruit rather than the tree’s established woody structure. For instance, fruit from sensitive varieties like Satsumas can sustain damage if temperatures drop to 30°F for two or more hours. Damage occurs when ice crystals rupture the fruit’s juice vesicles, causing the fruit to dry out and become inedible.
Protection becomes urgent during a hard freeze, defined as temperatures sustained below 28°F. The primary concern shifts to the tree’s leaves and wood, particularly for less cold-tolerant varieties such as lemons and limes, which suffer damage in the high 20s. For most common sweet oranges, significant leaf or wood damage can occur if temperatures fall below 26°F for an extended period. This prolonged exposure allows the cold to penetrate deeper into the plant tissue, causing ice formation in the cells that results in withered, darkened foliage.
Tree-Specific Variables That Affect Timing
While absolute temperature thresholds provide a general guideline, the actual timing for covering must be adjusted based on the individual tree’s characteristics. The variety of citrus significantly influences its cold tolerance. Kumquats and Satsuma mandarins are among the most cold-tolerant, with mature trees capable of withstanding temperatures as low as 20°F without severe wood damage. Conversely, acid citrus like lemons and limes are the most sensitive, necessitating covering at higher temperatures, often needing protection when the forecast is still in the high 20s.
The age of the tree is another important factor, as newly planted or young trees are much more susceptible to cold injury than mature specimens. Younger trees have less established root systems and thinner bark, making them less resilient to temperature fluctuations. Mature trees, having a larger, denser canopy, are better able to trap heat radiating from the soil, which offers a marginal degree of natural protection. This tolerance can also be influenced by a tree’s microclimate, such as a location near a south- or west-facing wall or pavement. These structures absorb solar heat during the day and re-radiate it at night, which can provide extra warmth and slightly delay the need for covering.
Practical Timing: From Forecasts to Uncovering
Operational timing requires close monitoring of weather forecasts to identify when temperatures are predicted to drop below the critical threshold for the tree’s specific variety. Covering should begin 24 to 48 hours before the expected cold snap to ensure the tree is fully protected before the temperature starts to fall. It is beneficial to secure the cover so that it extends all the way to the ground, trapping the heat naturally emitted by the soil.
Once the freeze threat has passed and temperatures begin to rise consistently above freezing, it is important to remove the protective cover promptly. Leaving a cover on too long, particularly a non-breathable material like plastic, can create a greenhouse effect when the sun comes out. The rapid buildup of heat and moisture inside the cover risks scorching the foliage or causing the tree to break dormancy. Removing the cover on a warm, sunny morning prevents this overheating and ensures the tree remains ready for any subsequent cold events.