Protecting outdoor plants from unexpected temperature drops requires swift action to prevent irreversible damage. When water inside plant tissues freezes, it expands and ruptures cell walls, causing foliage to turn black and wilt. The primary goal of covering plants is to trap geothermal heat radiating from the soil and create a buffer against cold air and frost. Proper materials and structural methods are essential to prevent losing vulnerable plants entirely.
Determining When Coverage is Necessary
The need for protective measures is defined by the forecast temperature and the duration of the cold event. A light frost (33 to 36 degrees Fahrenheit) typically only damages tender, newly emerged foliage. A light freeze (29 to 32 degrees Fahrenheit) poses a threat to most unprotected, sensitive plants. If the temperature is expected to drop to 28 degrees Fahrenheit or lower for several hours, a hard freeze is likely, necessitating robust protection.
Preparation must begin hours before the temperature is predicted to drop below freezing. An effective preparatory step is deeply watering the soil around the plants during the afternoon. Water has a high specific heat capacity, meaning wet soil absorbs and retains more heat during the day than dry soil. The moist soil will slowly release this stored warmth overnight, which is then trapped by the covering material.
Pre-watering is only effective if the water is applied to the soil, not the foliage, as wet leaves are more susceptible to ice crystals. Wet soil also helps plants resist cold-induced dehydration, a major factor in freeze damage. Thoroughly soaking the ground ensures the heat-retaining property of the water is maximized beneath the cover.
Choosing and Utilizing Protective Materials
The selection of a covering material should be based on the degree of temperature protection required and the material’s breathability. Purpose-made floating row covers, constructed from spun-bonded polypropylene fabric, are rated by weight and offer varying thermal insulation. A medium-weight row cover provides a temperature buffer of 4 to 6 degrees Fahrenheit. Heavier-grade frost blankets can offer up to 8 degrees of protection, safeguarding plants down to about 24 degrees Fahrenheit.
Household materials like old sheets, blankets, or burlap provide a temporary 5 to 10-degree buffer for light frosts. These breathable fabrics are preferred because they allow moisture and air to pass through, reducing the risk of fungal disease and overheating. Plastic sheeting or tarps can offer significant thermal protection (8 to 10 degrees Fahrenheit), but must be used with caution. Plastic is non-breathable and tends to trap excessive moisture and heat when the sun rises.
A fundamental rule for all materials, especially plastic, is preventing direct contact with the plant foliage. The covering material will eventually cool to the outside air temperature. Any leaf touching it acts as a conductor, transferring cold directly to the plant tissue. This contact-induced freeze damage appears as a distinct line of dead tissue where the leaf rested against the cover.
Building the Protective Structure
A support structure is necessary to ensure the cover material does not rest on the plant’s leaves. Simple, temporary frames can be created using common household or garden items. Placing inverted tomato cages or tall garden stakes around the plant provides the necessary height and framework to drape the cover. For larger shrubs, patio chairs, sawhorses, or flexible PVC piping bent into hoops can form a tent-like structure.
Once the frame is in place, the chosen material must be draped over the structure, extending all the way to the ground. The cover’s edges need to be secured firmly to the soil to trap the warmth radiating from the ground beneath the canopy. This can be accomplished by weighing down the edges with rocks, bricks, or long pieces of lumber, or by burying the material’s perimeter in a shallow trench of soil.
Container plants require different strategies since their roots lack the insulating mass of the surrounding earth. The simplest solution is moving smaller pots into an unheated garage or shed. If containers are too large to move, group them tightly against a warm structure, such as the south-facing wall of a house. For additional root protection, the sides of the pots can be wrapped with insulating materials like burlap or bubble wrap.
Post-Freeze Management and Uncovering
Protective covers must be removed promptly once the temperature rises above freezing and the immediate threat of frost has passed. Leaving covers on too long, especially non-breathable plastic or heavy blankets, can lead to a rapid buildup of heat as the sun hits the material. This greenhouse effect can quickly overheat and stress the plants, causing more damage than the cold itself. Covers should be taken off in the morning, ideally before the temperature beneath them exceeds 40 degrees Fahrenheit.
After uncovering, assessing the extent of any freeze damage requires patience. Foliage that is blackened, brown, or appears mushy is damaged tissue, but it should not be pruned immediately. This dead material acts as a natural insulator, protecting the underlying stems and crown from future cold snaps.
For woody plants like shrubs, delay pruning until new growth emerges in the spring, which indicates the line between live and dead wood. A scratch test involves gently scraping the bark with a fingernail; green tissue underneath signifies life, while brown or black tissue is dead. Removing mushy herbaceous material is recommended to prevent rot or fungal disease.