When the forecast calls for a low of 32 degrees Fahrenheit (0 degrees Celsius), gardeners often wonder if they should cover their plants. The answer is generally yes, as this temperature marks the threshold where water freezes, leading to frost formation and potential plant tissue damage. Covering is a wise precaution, though the necessity depends on specific environmental and biological factors.
The Critical Decision: Is 32°F Always a Danger?
The temperature reported by weather services is typically measured several feet above the ground, differing significantly from the temperature at the plant surface. On a clear, still night, radiative cooling causes surfaces to lose heat rapidly. This means that while the official air temperature may hold steady at 32°F, the temperature on the leaves and the ground can drop lower, triggering frost formation.
Frost forms when the surface temperature reaches or falls below 32°F. If the air is dry (a low dew point), frost is less likely to form, even if the air temperature briefly dips to freezing. However, a slight breeze can prevent a sharp temperature inversion and keep the air mixed, reducing the risk compared to still air. Prolonged exposure below 32°F for more than a few hours greatly increases the risk of damage, even for hardy plants.
Identifying Vulnerable Plants
Plant vulnerability is categorized by natural hardiness and current growth stage. Tender annuals and tropical plants, such as tomatoes, peppers, basil, and impatiens, are the most susceptible. These plants can suffer damage or death even from a light frost because they lack mechanisms like antifreeze proteins to prevent ice crystals from forming within their cells.
Newly planted seedlings and soft, fleshy new growth are also sensitive to a light freeze. Containerized plants are at higher risk because their roots are less insulated than those in the ground, making the entire plant vulnerable to cold damage. In contrast, established, hardy vegetables like kale, carrots, and cabbage often tolerate a light freeze down to 28°F without permanent damage.
Essential Covering Techniques
Effective covering involves trapping the residual heat radiating from the soil and preventing direct contact between the cold air and the foliage. The best materials are breathable fabrics, such as frost cloth, old bed sheets, or blankets. These materials allow moisture to escape while providing insulation, and a single layer can offer several degrees of protection.
If using plastic, it must not touch the leaves, as it can transfer cold directly to the plant tissue. Gardeners should use stakes, hoops, or wire frames to create a supported structure and ensure separation. The covering should be draped down to the ground and secured with stones or soil to seal in the ground’s warmth. This creates a temporary, warmer microclimate, and using multiple layers of fabric increases insulation during a severe cold snap.
Timing and Duration of Protection
The timing of applying and removing the cover is just as important as the material used. Covers should be placed over the plants before sunset, which is when the ground begins to rapidly lose its stored heat through radiation. Applying the cover while the soil is still warm ensures that the heat is trapped underneath the barrier for the overnight period.
Covers must be removed promptly in the morning after the sun is fully up and the temperature has risen safely above freezing. Leaving covers on too long can be detrimental because the sun’s heat rapidly accumulates under the material, causing the plants to overheat. This rapid temperature increase, combined with trapped moisture, can lead to scalding or “steam damage,” which is particularly harmful to tender foliage.