Frost damage occurs when air and plant tissues drop below the freezing point, usually 32°F (0°C). This causes ice crystals to form within plant cells, rupturing cell walls and leading to tissue death. Covering plants works by trapping the heat that radiates naturally from the ground throughout the night and creating a barrier against cold wind. The right cover establishes a temporary microclimate around the plant, which can raise the temperature by several degrees.
Essential Characteristics of Protective Covers
Cover effectiveness depends on three primary physical properties. The material must possess some insulating quality to slow the escape of thermal energy radiating from the soil. This helps maintain a warmer pocket of air around the plant, protecting the foliage from the colder outside air.
A cover also needs to allow for air and moisture exchange, a property known as breathability. Non-breathable materials trap moisture released by the plant, leading to condensation that can promote fungal diseases. Finally, the material must be light enough to rest directly on delicate foliage without causing physical damage.
The Best Materials for Frost Protection
The most reliable material is horticultural fleece, also known as floating row cover, a spun-bonded polypropylene fabric. This material is highly effective because it is lightweight enough to be laid directly over plants without the need for support frames. Depending on its weight, this fabric can offer protection ranging from 4°F to 8°F (2°C to 4°C).
Lighter-grade fleece allows significant light penetration, meaning it can be left over plants for several days without hindering photosynthesis. For more severe cold spells, a thick, heavy blanket, quilt, or comforter provides a higher degree of insulation. These heavy fabrics are useful for protecting larger shrubs and trees against temperature drops.
Unlike fleece, dense materials like blankets block all light and must be removed promptly once the sun is up. They are best used as a temporary, overnight defense against a predicted hard freeze. A specialized frost blanket is designed to allow air and moisture to pass through while still trapping radiant heat on clear nights.
Proper Techniques for Covering Plants
Timing is primary for successful plant protection. Covers should be placed over plants before sunset to trap the warmth the soil accumulated throughout the day. Removing the covers should be done after the sun rises and temperatures begin to climb, typically by mid-morning.
The material should not touch the foliage of the plant it is protecting. Wherever the cover contacts the leaves, it can transfer cold and cause localized damage. Using stakes, tomato cages, or wire hoops creates a tent-like structure that lifts the material. This ensures an insulating layer of air remains between the plant and the cover.
The cover must extend all the way to the ground and be securely anchored at the edges. Using bricks, rocks, or soil to seal the perimeter prevents wind from blowing the cover away. This seal ensures the warm air radiating from the soil is trapped beneath the barrier, maximizing the temperature differential.
Materials That Can Cause Damage
Thin plastic sheeting should be avoided. Plastic is a poor insulator and can transfer cold directly to the plant tissue if it makes contact. If left on during the day, clear plastic can rapidly create a greenhouse effect, causing the temperature inside to spike and potentially cause sunscald.
The non-porous nature of plastic prevents air circulation, leading to a buildup of condensation underneath. This trapped moisture can freeze directly onto the leaves, exacerbating frost damage. Similarly, materials like newspaper or cardboard are only suitable for low-growing plants and offer little protection if they become wet. Any cover that is non-breathable and heavy risks smothering or fostering disease if left in place for more than a single night.