Covering plants prevents irreversible cellular damage caused by freezing temperatures. This injury occurs when water within the plant tissue freezes, forming ice crystals. Extracellular ice crystals draw water out of the cells, causing severe dehydration and cell collapse. If temperatures drop too quickly, ice crystals can form inside the cells (intracellular freezing), resulting in immediate and fatal mechanical disruption. A physical barrier helps trap residual heat radiating from the soil, creating a warmer microclimate to mitigate these damaging effects. The decision to apply protection depends on meteorological factors and the specific cold tolerance of the plants.
Identifying Critical Cold Thresholds
The air temperature requiring protection is defined by the type and severity of the cold event. A “Light Frost” occurs when air temperatures drop between 33°F and 36°F. Although above freezing, ice crystals can form on plant surfaces due to radiative cooling, which causes foliage temperature to drop lower than the surrounding air. A “Light Freeze” (or “Hard Frost”) is defined by air temperatures between 29°F and 32°F, sufficient to damage or kill most tender plants.
Temperatures falling to 28°F or below for four or more consecutive hours qualify as a “Hard Freeze.” This level of cold is destructive, severely damaging or killing many perennial and semi-hardy plants. Air temperature reported by weather services is measured several feet above the ground, meaning the temperature at the plant level can be several degrees colder. Monitoring the dew point is also useful; a dew point above 45°F suggests enough moisture to limit radiative cooling, making frost less likely.
Assessing Plant Sensitivity to Cold
The temperature threshold for covering depends heavily on a plant’s natural cold tolerance, grouped into three categories.
Tender Plants
Tender plants, often tropical or warm-season annuals, are injured or killed by the slightest frost. Examples include tomatoes, peppers, basil, and impatiens. They require protection when temperatures are forecasted to drop to 40°F or below, as they are highly susceptible to chilling injury even above freezing.
Semi-Hardy Plants
Semi-hardy plants can withstand a brief light frost but are vulnerable to a sustained freeze. Cool-season vegetables like cabbage, lettuce, and broccoli tolerate dips into the high 20s (26°F to 32°F) with minimal damage. Protection is warranted when temperatures are expected to remain below 32°F for more than a few hours. Their moderate tolerance is due to accumulating solutes that lower the freezing point of cell water.
Hardy Plants
Hardy plants are naturally acclimated to cold environments and generally survive a hard freeze without covering. This category includes many perennials, trees, shrubs, and hardy cool-season vegetables like kale and spinach. They possess internal mechanisms, such as freeze-tolerant proteins, allowing them to survive temperatures well below 28°F. Protection is typically only required for newly planted specimens or during extreme, prolonged cold below 20°F.
Effective Methods for Applying Protection
The method of application dictates the effectiveness of cold protection. The goal is to trap heat radiating from the ground while preventing the covering material from resting directly on the foliage. If a cover touches the leaves, it can transmit cold air or moisture to the plant tissue, negating the insulation.
Use the following methods for effective protection:
- Lightweight frost fabric, breathable cotton sheets, or burlap are appropriate materials, as they allow air exchange while trapping heat.
- Use stakes, wire hoops, or tomato cages to create a frame that holds the material up and away from the plant surface.
- Group container plants tightly against a warm structure, such as a south-facing wall, and wrap the pots in bubble wrap or burlap for root insulation.
- For small, individual plants, use secured inverted containers or “hot caps.”
- In extreme cold, a string of incandescent holiday lights (not LED) placed under the covering provides supplemental heat.
The cover must extend to the ground and be secured with bricks or soil to fully seal the warm air pocket and prevent drafts.
Timing and Duration of Cold Protection
The timing of applying and removing covers is crucial for optimal plant survival. Covers should be applied in the late afternoon or early evening, ideally before sunset, to capture the heat the soil absorbed during the day. The ground continues to radiate this stored warmth upward throughout the night, and the covering acts as a blanket to prevent that heat from escaping.
Protection must remain in place until morning temperatures have risen sufficiently and any visible frost has completely melted. Leaving covers on too long after sunrise can cause rapid overheating, potentially damaging the plants. Overheating, combined with trapped moisture, can stress the foliage and create conditions favorable for disease. Covers should be removed when the air temperature stabilizes above freezing, typically mid-morning. If another cold night is predicted, reapply the covers before sunset, but remove them during the day to allow for light exposure and air circulation.