When temperatures drop below freezing, unprotected plants face frost damage. Frost occurs when the temperature of a plant’s surface falls to 32°F (0°C) or lower, causing water vapor to freeze onto the foliage. The greater danger is freeze injury, where water inside the plant’s cells freezes, dehydrating the cell and causing it to collapse. This damage appears as blackened or water-soaked tissue and necessitates quick coverage to create a protective microclimate.
Identifying When Protection is Necessary
Plant damage generally begins when air temperatures approach 32°F (0°C). Tender plants, such as tomatoes or annual flowers, are vulnerable to a light frost, often predicted when temperatures are forecasted to dip into the 33°F to 36°F range. This is because the temperature at the ground level can be several degrees colder than the official air temperature recorded a few feet higher.
A light freeze (29°F to 32°F) will kill most warm-weather crops, while a hard freeze (28°F and below for several hours) is destructive to all but the most cold-hardy varieties. Clear, calm nights with little to no wind and a dew point below 45°F are the atmospheric conditions most likely to result in frost, as heat radiates quickly away from the earth into the atmosphere.
Preparation must begin before sunset, as the ground starts losing heat immediately after the sun goes down. Applying protection in the late afternoon ensures the cover is in place before the coldest hours of the night, which typically occur just before dawn. Acting quickly upon a frost advisory provides the necessary lead time to prepare materials.
Selecting the Right Protective Material
The best materials for frost protection offer both insulation and breathability to manage temperature and moisture effectively. Dedicated frost cloth, or row cover fabric, is made from lightweight spun-bonded polypropylene. This material allows light and water to penetrate while trapping heat. Heavier weights of this fabric, such as 2.5 oz or 4.0 oz, provide greater temperature protection than lighter versions.
Old sheets, blankets, and towels are excellent alternatives that provide effective insulation for a short-term cold snap. Thicker materials, like heavy blankets or comforters, offer better protection, especially when temperatures fall into the mid-20s. Burlap is another natural, breathable option, functioning well to block wind and cold while preventing moisture buildup that can lead to fungal issues.
Avoid using plastic sheeting directly over foliage. Plastic is non-breathable and an efficient conductor of cold; any leaf touching the plastic will suffer frost burn. Plastic also creates a greenhouse effect on sunny mornings, causing temperatures beneath the cover to spike rapidly and stress the plant.
Step-by-Step Covering and Securing Methods
Proper application requires creating a protective tent over the plant to prevent the cover from touching the foliage and maximize heat retention. For small or medium plants, install support structures, such as tomato cages, wooden stakes, or flexible PVC pipe hoops, before draping the material. This frame ensures an insulating air pocket surrounds the plant, preventing direct contact with the cold cover.
The material must extend all the way to the ground to capture the residual heat radiating up from the soil. This trapped thermal energy is the primary source of warmth that keeps the air temperature inside the covered space above freezing. For larger shrubs or trees, loosely wrap the trunk and secure a sheet or burlap over the canopy, ensuring the cover seals the space around the base.
Securing the cover firmly is necessary to keep the warm air pocket intact and prevent wind damage. Anchor the edges of the fabric or sheet to the ground using bricks, heavy rocks, soil, or landscape staples. For covers placed over cages or hoops, use strong clips or clothespins to fasten the material to the support structure, creating a sealed enclosure.
Post-Frost Removal and Plant Assessment
Removing the protective covering at the correct time is as important as putting it on. Remove the covers promptly in the morning once temperatures rise above 32°F and the sun is fully out. Leaving the material on, especially if it is non-breathable or dark, can cause the temperature beneath to soar, rapidly overheating and scalding the foliage.
If the cold snap is expected to last several days, temporarily remove the covers during the warmest part of the day to allow for air circulation and sunlight exposure, then re-cover the plants before sunset. After the frost danger has passed, assess the plants for damage, which typically appears as limp, blackened, or brown foliage.
Resist the immediate urge to prune away the damaged material. The dead or damaged leaves and stems serve as a natural insulating layer, protecting the healthy inner tissue from subsequent late-season cold snaps. For woody plants, wait until new growth emerges in the spring to identify and selectively remove only the wood that is clearly dead.