A freeze warning signals a significant temperature drop, typically indicating that air temperatures will reach 32°F (0°C) or lower. This threshold is dangerous because water within plant cells can freeze, expanding and rupturing the cell walls, which leads to tissue death. Preparing for these events requires immediate steps to insulate vulnerable plants and minimize the severe damage that can occur within just a few hours. The goal is to protect the plant’s moisture content and trap the latent heat present in the soil near the plant’s root zone.
Identifying Vulnerable Plant Types
Tender annuals, such as petunias, impatiens, and warm-season vegetables like tomatoes and basil, are among the most susceptible to frost damage. These plants are genetically programmed for warmer climates and lack the cellular adaptations needed to withstand freezing conditions.
Tropical and sub-tropical specimens, including citrus, hibiscus, and bougainvillea, also require covering because their foliage and stems are easily damaged by even a light freeze. They lack the ability to enter dormancy fully, making their tissue prone to freezing and subsequent mushy decay. Newly planted trees and shrubs, even those considered cold-hardy, are also vulnerable since their root systems are not yet fully established in the surrounding soil mass.
Plants situated in containers are another group needing immediate protection, as their roots lack the insulation provided by the surrounding earth. The entire root ball is exposed to the cold air, which can freeze quickly, often resulting in more severe damage than to in-ground plants. Grouping potted plants together in a sheltered area, such as against a warm exterior wall, before covering them provides a greater chance of survival.
Timing the Protection
The decision to cover plants depends on the severity and duration of the expected temperature drop, distinguishing between a light frost and a hard freeze. A light freeze typically involves temperatures dipping briefly between 32°F and 29°F, which is sufficient to kill highly tender plants like basil and tomatoes. A hard freeze, however, occurs when the temperature falls below 28°F for several hours, which can severely damage or kill most frost-sensitive plants.
Timing the application of protective covers is equally important, as the goal is to trap the heat absorbed by the soil during the day. Covers should be placed over plants in the late afternoon or early evening, well before the air temperature begins to plummet. The earth releases stored solar heat throughout the night, and a properly secured cover creates a warm microclimate around the plant canopy. For maximum effect, the covering must be secured to the ground to prevent the escape of the warmer air trapped underneath.
Essential Covering Materials and Methods
Breathable fabrics are the most effective materials for plant protection. These materials allow for air exchange while trapping radiant heat rising from the soil, preventing ice crystals from forming on the foliage. Using a double layer of lighter material can often provide better insulation than a single, heavier layer.
Breathable fabrics include:
- Frost cloth
- Burlap
- Old sheets
- Blankets
Avoid letting plastic sheeting come into direct contact with plant foliage. Plastic is not breathable and can trap moisture, which then condenses on the leaves; when the temperature drops, this moisture freezes directly onto the plant tissue, causing cold burn. Any leaves touching the plastic also risk direct cold transfer, which can immediately damage the sensitive tissue.
A proper covering method involves creating a tent-like structure to support the material above the plant canopy. This can be accomplished using wooden stakes, tomato cages, or other simple frames. The cover should drape over this structure, extending all the way to the ground, where the edges must be secured with bricks, rocks, or soil to seal in the ground heat. For container plants that cannot be moved indoors, grouping them tightly together and then covering the entire cluster with a thick blanket provides communal insulation.
Post-Freeze Recovery and Uncovering
After the freeze event has passed and temperatures rise above the freezing point, the protective coverings should be removed promptly. Leaving covers on during the day, especially when the sun is out, can cause temperatures beneath the material to rise rapidly, leading to overheating or “cooking” of the plant tissue. Removing the covering in the mid-morning allows the plant to receive sunlight and air circulation, preventing the buildup of humidity and heat.
If damage is visible, such as blackened or mushy foliage, resist the urge to prune immediately. The damaged leaves and branches provide a layer of insulation to the plant’s core and root system. Pruning too early can expose healthy tissue to subsequent cold snaps, so wait until the last expected freeze of the season has passed before cutting back the dead growth. Watering the plants after the freeze assists in recovery by rehydrating the tissues, as cold wind and freezing conditions often cause desiccation.