When sensitive plants face a sudden drop in temperature, frost damage can rapidly destroy new growth. Frost occurs when ice crystals form on the plant surface, often when air temperatures are still slightly above freezing, causing plant tissue to cool below that point. Protection involves creating a temporary microclimate to trap warmth and block the direct formation of ice.
Effective Materials for Plant Coverage
Selecting the right material is the first step in successful frost protection, prioritizing insulation and breathability. Commercial frost blankets, often made from spunbond polypropylene, are lightweight enough to rest directly on foliage without causing damage and allow some light and air to pass through. These specialized fabrics are rated by weight, with heavier versions offering greater temperature protection, sometimes down to 28°F (-2°C).
Common household items like old sheets, towels, or burlap also work well because they are permeable, allowing trapped moisture to escape and preventing wet foliage from freezing solid. Thicker blankets or quilts provide better insulation but should only be used over a frame or on sturdy plants to avoid crushing branches. Plastic sheeting and tarps are non-breathable and must be used with caution, as they can trap excessive moisture that freezes or cause rapid overheating when the sun rises.
The most important rule for any non-breathable material, including plastic or heavy canvas, is that it must not touch the plant foliage. These materials conduct cold and can transfer freezing temperatures directly to the leaves, causing tissue damage where contact is made. Clear plastic can quickly heat up in the sun, creating a miniature oven effect that scorches the plant. Therefore, plastic is a poor choice unless it is tented well off the plant and promptly removed in the morning.
Proper Application Methods
The way a cover is applied is just as consequential as the material itself, as the goal is to capture heat radiating from the soil. The cover must extend all the way to the ground on all sides, creating a thermal seal to maximize the warmth retained within the temporary structure. Securing the edges with rocks, bricks, soil, or heavy clips prevents wind from lifting the material and allowing cold air to funnel inside.
For taller, sturdier plants like shrubs, a simple draping application may be sufficient, provided the material is light and breathable. Delicate or newly sprouted plants require a tenting method, which uses stakes, tomato cages, or other supports to hold the fabric well above the foliage. This tenting prevents the material from crushing the plant and ensures that any moisture or cold transfer from the cover does not damage the leaves.
The effectiveness of the cover relies on trapping the warmth the earth absorbed during the day. By creating a fully enclosed area, the radiant heat from the soil surface is captured. This elevates the air temperature immediately surrounding the plant by several degrees, which is often enough to protect the plant from a light or moderate frost.
Essential Pre-Covering Steps
Maximizing the effectiveness of any cover begins with specific preparations preceding the cold event. The most impactful action is thoroughly watering the soil around the plant a day or two before the predicted frost. Moist soil holds significantly more heat than dry soil because water has a high specific heat capacity, allowing it to absorb and slowly release warmth throughout the night.
The water fills the air pockets in the soil, preventing heat from escaping and creating a dense, warm reservoir near the roots. A well-hydrated plant is also less susceptible to cold damage because the water within its cells helps maintain structural integrity and reduces the risk of cellular dehydration from freezing. It is important to avoid watering the foliage itself, as surface moisture will simply freeze and cause damage.
For any potted plants, the best defense is to move them to a more sheltered location, such as under a porch eave, against a warm exterior wall of a house, or inside a garage. Grouping container plants together reduces the amount of surface area exposed to the cold air, helping the pots insulate each other. If the plant cannot be moved, insulating the container itself with burlap or blankets helps protect the vulnerable root ball from freezing solid.
Knowing When to Cover and Uncover
The timing of deployment and removal is crucial for successful frost protection. Covers should be placed in the late afternoon or early evening before the temperature begins to drop, ideally before sunset, to trap the residual warmth of the day. Tender annuals and vegetables need protection when temperatures are forecast to drop to 32°F (0°C) or below, while sensitive tropical plants may require covering when temperatures dip into the mid-30s.
The protection must remain in place overnight, as the coldest temperatures usually occur just before dawn. Once the sun rises, however, the covers become a liability, and prompt removal is crucial to prevent the plants from overheating. The air trapped underneath the cover can rapidly warm up, especially if a non-breathable material is used, and this quick temperature increase can scorch or “cook” the foliage.
Covers should be removed as soon as temperatures rise above freezing, typically mid-morning, to allow for air circulation and sunlight exposure. If the cold snap is expected to last for multiple days, the covers must still be completely removed or at least vented during the day and then reapplied each evening. Leaving the cover on too long, even if breathable, can inhibit the plant’s access to necessary light and air.