A freeze occurs when air temperatures drop to 32°F (0°C) or below, causing water inside plant cells to freeze and expand. This ice crystal formation ruptures cell walls, leading to tissue damage that often appears as black or mushy foliage. Covering plants is a temporary measure designed to prevent this cellular destruction by trapping the heat naturally radiated from the soil. Protection success depends on timely action and knowing when to remove the covering to prevent secondary damage.
Factors Determining Cover Duration
The duration a plant can safely remain covered depends on the intensity of direct sunlight and the fluctuation of air temperature after the freezing event. Covers protect plants from the intense cold of night and radiative cooling by trapping ground heat, creating a warmer microclimate.
Once the sun rises and the air temperature climbs above freezing, the cover shifts from protection to a potential danger due to heat accumulation. For most opaque covers, removal is necessary by mid-morning, typically between 9:00 AM and 10:00 AM, as soon as temperatures stabilize above freezing.
Leaving covers on for longer than 12 to 48 hours consecutively introduces significant risk. If freezing conditions persist for multiple days, covers must be vented or temporarily removed during the warmest part of the day. This allows for light and air exchange, and the covers should be reapplied before sunset to trap the heat gained during the day.
Proper Techniques for Covering Plants
Selecting the correct material is the first step toward effective freeze protection, with breathable fabric being the most recommended option. Specialized frost cloth, old bed sheets, or blankets are ideal because they allow air and moisture exchange while trapping heat radiating from the soil. These materials are more effective than plastic, especially since plastic touching foliage can transmit cold directly to the plant tissue.
If plastic is used, or if layering for increased insulation, it must be supported by a frame or stakes. This ensures the plastic never makes direct contact with the leaves or stems. Protection relies on creating an insulating air pocket, and direct contact with a cold, non-breathable surface defeats this purpose.
For maximum benefit, the covering should drape all the way to the ground and be secured around the perimeter with bricks or heavy objects. This anchors the cover against wind and ensures the warm air rising from the soil is effectively trapped within the plant’s canopy.
Risks of Leaving Covers On Too Long
Exceeding the safe duration for covering plants introduces risks that can be as damaging as the freeze itself. The most immediate danger during daytime is rapid overheating, often referred to as solar gain, which is intensified by the greenhouse effect created under the cover. Even when the outside air is cool, the internal temperature of a covered plant can quickly spike above 80°F, essentially cooking the plant tissue. This thermal shock causes more harm than a mild frost.
A secondary risk is the buildup of excessive moisture and humidity within the confined space. When covers remain on for several consecutive days, the lack of air circulation creates an environment conducive to the growth of fungal pathogens and mold. This damp, stagnant condition can cause leaves to rot or yellow and weakens the plant. Proper management requires balancing protection from cold with the plant’s need for light, ventilation, and a stable temperature.
Post-Freeze Uncovering and Care
Once the threat of freezing temperatures has passed, the removal of protective covers should be managed carefully to avoid shocking the plant. If the plant has been covered for an extended period, remove the cover gradually, perhaps choosing a cloudy day or late afternoon. This prevents sudden exposure to intense sunlight, as a rapid transition can cause sunscald on tender foliage.
Upon uncovering, assess the damage, looking for discolored, blackened, or mushy tissue. Resist the urge to prune damaged parts immediately, unless the material is slimy and indicates active rot. The dead foliage and stems can serve as a layer of natural insulation, offering protection against subsequent light frosts. Pruning should be delayed until the plant begins to show new growth and the risk of the final frost of the season is over, typically in late spring.