When temperatures drop below freezing, the water inside plant cells expands, forming ice crystals that rupture the cell walls and destroy tissue, leading to frost damage. Gardeners often use readily available plastic sheeting as a quick solution for safeguarding vulnerable foliage during a sudden cold snap. This common practice aims to create a temporary, sheltered environment to help plants survive the cold.
Plastic as a Frost Barrier
Plastic functions effectively as a temporary frost barrier by managing the heat already present, not by insulating the plant directly. The ground retains residual heat absorbed during the day, releasing it as long-wave infrared radiation throughout the night. Plastic sheeting placed over the plant traps this radiant heat, preventing it from escaping into the cold night air. This trapped heat establishes a microclimate immediately surrounding the plant that is warmer than the ambient outside air. While plastic is a poor insulator, its value lies in creating this enclosed air space, and a plastic cover supported by a frame can provide 3°F to over 6°F of frost protection.
Essential Rules for Safe Plastic Use
Using plastic sheeting successfully requires adherence to certain rules to prevent damaging the plants you intend to save. The most serious risk is allowing the plastic to make direct contact with the plant’s foliage. If the plastic touches the leaves, it conducts the external cold directly to the tissue, negating the protective microclimate. Plastic also traps moisture released by the plant, which condenses on the inside of the cover. If this condensation is touching the plant tissue when temperatures fall, the moisture freezes onto the leaf surfaces, causing localized cell death, often called “frost burn.” To prevent this, you must erect a simple structure, such as hoops, stakes, or tomato cages, to maintain a critical air gap between the plastic and the plant. The plastic must also be anchored securely to the ground around the perimeter to seal the bottom and effectively trap the ground’s warmth inside the enclosure.
Comparing Plastic to Alternative Covers
While plastic is highly accessible, specialized materials often offer superior performance and fewer risks. Horticultural row covers, frequently called frost blankets, are made from spun-bonded, breathable fabric. This breathability allows moisture and excess heat to escape, reducing the risk of condensation and subsequent freezing damage. Unlike plastic, thick materials like blankets or burlap can often be laid directly onto the foliage without causing localized damage. These fabric covers generally offer better insulation, or R-value, than a single layer of plastic, providing 2°F to 6°F of protection depending on the material’s weight. For protection against severe freezes, the best strategy is layering, where a plastic sheet is used as a second, outer layer draped over a breathable blanket to achieve maximum heat retention.
Timing the Cover and Uncover Process
The timing of covering and uncovering plants is just as important as the material selection itself. Covers must be placed over the plants before sundown to capture the heat that the soil retained during the day; applying the cover too late means the ground has already lost most of its stored heat, making the barrier less effective. The cover must be removed promptly the following morning, particularly if the day is sunny. Plastic creates a greenhouse effect, and even on cool days, temperatures inside the cover can quickly rise to damaging levels. Leaving the plastic in place can cause rapid overheating, which can stress or “bake” the plants, leading to wilting and death, so removing the cover allows the plants to ventilate and prevents the accumulation of excessive heat and humidity.