Protecting plants from frost involves covering them to trap the heat radiating from the ground, creating a warmer microclimate around the foliage. This protective measure is typically applied when air temperatures are forecasted to drop to 32°F (0°C) or below, especially for tender or tropical varieties. Uncovering plants too soon risks exposing them to a lingering frost, but leaving them covered for too long can cause a different kind of damage. The timing of cover removal is necessary to avoid harming the vulnerable plant tissue.
Understanding Why Timing is Crucial
Uncovering a plant immediately after sunrise, while ice crystals are still present on the leaves, can be more damaging than the initial cold itself. Plant cells that have frozen contain ice, and rapid thawing causes the cell walls to rupture, leading to irreversible damage. This rapid temperature shift is a form of thermal shock, which stresses the plant’s metabolic processes.
The exposure of frozen tissue to bright, direct sunlight can result in a condition known as sun scald. Sun scald causes the rapid warming and thawing of the frozen cells, which damages the internal membranes. This is why damage often appears as bleached or scorched spots on the side facing the morning sun. Plants covered for multiple days are particularly sensitive to this sudden change in light and temperature.
The Critical Temperature Threshold
Wait until the air temperature has risen consistently above freezing, ideally reaching 40°F (4.5°C). While a temperature just above 32°F (0°C) means the frost is technically over, the extra margin allows the ground to begin warming and reduces the risk of sun scald on the sensitive tissue. Checking the local forecast is paramount to ensure the cold event is definitively over. If a second night of frost is predicted, the coverings must still be removed during the day to prevent moisture buildup, which can promote fungal growth.
Wait until the sun has been up long enough to warm the area substantially, typically mid-morning. If a non-breathable material like plastic was used, removing it promptly after the temperature rises above freezing is important. Leaving a plant under plastic on a sunny day can quickly cause lethal overheating and excessive condensation.
Safe Removal and Gradual Acclimation
The process of removing the covers should be gradual, effectively “hardening off” the plant to the renewed sun and air. If the plant has been under cover for more than one day, a full, immediate uncovering can lead to severe shock. Begin by partially opening the cover on the side that is sheltered from the strongest morning sun, usually the north or west side, to introduce gentle light and air circulation.
For large row covers or tents, lift the material on the east or south side to allow light to filter in, then return a few hours later to fully remove it. This slow introduction over a few hours allows the plant’s internal systems to adjust to the light intensity and air movement. Full removal should occur when the air temperature is at its highest point for the morning, typically between 9:00 a.m. and 11:00 a.m.
Breathable fabrics, such as horticultural fleece, are generally safer for longer periods of coverage than materials like plastic sheeting. If plastic was used, ensure it is completely removed once the danger is past, as direct contact with cold plastic can transfer chilling injury to the leaves. After removal, monitor the plants closely throughout the day for signs of stress, such as wilting or drooping.
Immediate Post-Frost Assessment
Once the covers are safely off, the gardener should immediately assess the plants for visible damage. Look for signs of injury such as leaves that appear limp, translucent, or have turned a dark green, black, or brown color. These are all indications of tissue death from freezing.
While the sight of damaged foliage may prompt a desire to prune, resist the urge to cut off the blackened parts right away. The dead tissue acts as a natural insulator, providing a layer of protection to the healthy stems and buds beneath it in case of an unexpected later frost. Pruning also forces the plant to divert energy to new growth, which would be even more vulnerable to a subsequent cold snap.
Finally, check the soil moisture levels, especially for plants that were covered with non-breathable materials. Plants under cover can dry out quickly in the wind, or conversely, the trapped moisture can lead to soggy soil. Water deeply if the soil is dry, but ensure the area has good drainage to avoid root rot. Wait until the danger of frost is entirely past before considering any corrective pruning or fertilization.