Frost occurs when the air temperature drops to a point where ice crystals form on plant surfaces. The physical damage happens at the cellular level: when water inside the cells freezes, it expands, causing cell walls to rupture. This destroys the cell’s ability to function, leading to visual signs of frost damage, such as blackened or desiccated foliage. Protection measures work by creating a temporary microclimate that prevents the leaf temperature from reaching the freezing point.
Timing Your Protection
Taking action before the temperature reaches the freezing threshold is the most important step. Most tender plants are at risk when the forecast calls for temperatures dipping into the mid-30s Fahrenheit (around 1–3°C). Monitoring local weather forecasts is fundamental, especially during early spring and late autumn when temperatures fluctuate widely. To maximize the protective effect of any cover, the material must be applied before sunset. This timing traps the residual heat radiated from the ground that accumulated throughout the day.
Covering plants after dark means that much of the day’s warmth has already escaped into the atmosphere, making the cover less effective. The goal is to seal in this naturally warmer air pocket and prevent it from being displaced by sinking cold air. A proper covering acts as a thermal blanket, slowing the rate at which the ground-warmed air cools overnight. Waiting until the morning to cover the plants is often too late, as the coldest period typically occurs just before dawn.
Selecting Effective Cover Materials
The effectiveness of a cover is determined by its insulating capacity, weight, and breathability. Commercial frost cloth, often non-woven polypropylene, is recommended because it is lightweight and porous, providing a temperature increase of 5 to 8 degrees Fahrenheit beneath the material. This fabric allows for sufficient air and moisture exchange, necessary to keep the plant healthy during coverage. Heavier materials, such as thick blankets or burlap sacks, offer superior insulation for deeper freezes but must be supported to prevent them from crushing delicate foliage.
Household sheets and thinner fabric blankets provide several degrees of protection, but they should be removed daily to ensure the plants receive adequate light. Plastic sheeting or tarps can be used, but they present a greater risk if not applied correctly. Plastic is a poor insulator and will transfer cold directly to any foliage it touches, causing localized freeze damage. Furthermore, its non-porous nature traps moisture, which can condense and freeze directly onto the leaves, defeating the purpose of the cover.
Step-by-Step Covering Techniques
The method of covering depends largely on the size and structure of the plant being protected. For small, low-lying vegetables or individual annuals, a simple draping of a cloth material that extends to the ground is usually sufficient. The edges of the fabric must be secured tightly to the soil using heavy objects like rocks, bricks, or soil mounding. Ensuring a complete seal at the perimeter prevents the escape of the warmer air and maintains the beneficial microclimate.
For larger shrubs, trees, or tall vegetables, construct a support structure to hold the covering material off the foliage. This frame can be assembled using materials like wooden stakes, PVC pipe hoops, tomato cages, or patio furniture. The frame creates an insulating air space between the plant and the cover, which is important when using materials like plastic or heavy blankets that transfer cold or crush branches. Once the frame is in place, drape the chosen material over it, ensuring the cover extends all the way to the ground.
Securing the cover material to the frame itself with clips or clothespins can prevent it from collapsing onto the plant overnight. The final step is anchoring the bottom edges to the ground, sealing the protective barrier. This technique utilizes the ground’s warmth, which radiates upward and is trapped inside the tented structure. For container plants that cannot be moved, cover the entire pot and plant, then mound mulch or soil around the base of the container for additional root insulation.
Supplemental Protection and Post-Frost Care
In addition to covering, watering the ground before a freeze provides a significant layer of protection. Moist soil retains more heat during the day than dry soil because water has a higher specific heat capacity. This stored warmth is then released slowly overnight, contributing to the beneficial thermal pocket under the plant cover. Deeply watering the soil in the morning or early afternoon before a forecasted freeze is recommended, ensuring that the foliage itself remains dry to prevent ice formation.
Container plants are more susceptible because their roots are exposed to cold air, so they should be moved to a sheltered location, such as a garage, shed, or close to a warm exterior wall. This shelters them from wind and utilizes radiant heat from the structure. Once the threat of frost has passed, cover removal is important to prevent a second form of damage. Plants should be uncovered after the air temperature rises above freezing and the sun is no longer directly intense. Removing the cover too late in the morning, especially non-breathable materials, can cause rapid overheating and scorching of the foliage due to the greenhouse effect.