A damaging freeze or hard frost severely threatens sensitive plant life. When temperatures drop below freezing, water in the plant’s tissues freezes outside the cells, drawing water out and causing dehydration. If temperatures drop too low, ice crystals can rupture cell walls, leading to tissue death. Protection requires creating an insulating barrier to slow heat loss and prevent this cellular destruction.
Selecting Protective Materials
The most effective material is specialized horticultural fleece, often called a row cover or frost cloth. This lightweight, permeable fabric allows air and moisture to pass through while trapping heat radiating from the soil, providing temperature moderation. For larger shrubs or trees, thicker materials like old blankets, quilts, or heavy-duty burlap offer superior insulation during severe cold events. The heavier the fabric, the more insulation it provides.
Common household materials like cotton sheets or towels can be used, though their protective capacity is less than specialized cloth. These materials succeed when draped over the plant and reaching the ground, sealing in the ground’s warmth. Plastic sheeting should only be used if suspended and does not directly touch the foliage. Plastic transfers cold quickly, and contact with leaves can concentrate the chilling effect, causing more damage.
Securing Covers and Providing Airflow
A structural framework is necessary so the protective cover does not rest directly on the plant’s leaves or branches. The cover’s insulating power comes from the layer of still air trapped between the material and the plant. Simple supports like garden stakes, tomato cages, or bent PVC pipes can create a tent-like structure. For smaller plants, upturned buckets or large opaque containers provide an immediate, rigid barrier.
Once draped over the framework, the material must extend to the ground to trap the heat released by the soil. The edges of the cover should be firmly secured using heavy objects like bricks, rocks, or a mound of soil. This anchoring prevents wind from blowing the cover off and seals the bottom, stopping cold air from funneling underneath. Container plants that cannot be moved indoors can be grouped and covered as a cluster, or the pot can be inverted with a secured cover for maximum soil insulation.
When to Cover and When to Uncover
The timing of applying covers is important for maximizing trapped warmth. Covers should be placed over plants in the late afternoon or early evening, just before the temperature begins its sharpest drop. This timing allows the material to capture the residual heat the soil has absorbed and radiated. Covering too early is unnecessary and can block needed sunlight.
Removing the cover is equally important to prevent damage. Covers must be taken off promptly the following morning after temperatures have risen above freezing. Leaving the material on during a sunny day can cause the enclosed space to rapidly overheat due to solar gain, effectively cooking the plant. Prolonged coverage prevents light absorption and creates a damp, humid environment that promotes fungal growth. If another freeze is predicted, remove the covers during the day to allow the soil to reheat before reapplying them.