Covering plants is a preventative measure that helps gardeners extend the growing season and safeguard sensitive vegetation from unexpected cold. This physical act is a form of passive heating, trapping warmth that radiates from the ground and soil. It also acts as a windbreak, maintaining a slightly warmer microclimate around the foliage. This protective layer prevents the internal formation of ice crystals within plant cells, which causes cellular damage leading to wilting, blackening, and tissue collapse. Effective protection hinges on correctly anticipating the temperature drop and understanding the environmental conditions that allow frost to form.
Defining the Critical Temperature Threshold
The absolute freezing point of water is 32°F (0°C), the temperature at which frost officially forms on surfaces. However, a safer and more practical temperature for taking action is often between 36°F and 40°F, depending on the plant variety. The National Weather Service issues frost advisories when air temperatures are forecasted to fall into the 33°F to 36°F range, acknowledging that cold air pockets and surface cooling can still cause damage.
Tender plants, such as most annual flowers, summer vegetables (like tomatoes and peppers), and tropical varieties, are highly susceptible. They should be protected when temperatures dip below 40°F, as they are often killed by even a light frost. Protection for these sensitive plants should be implemented proactively when the forecast hovers in the upper 30s.
Hardy plants, including established perennials, shrubs, and cold-tolerant vegetables like kale and spinach, can withstand temperatures lower than 32°F for short periods. Many can survive down to 28°F before suffering significant tissue damage. Using the 36°F mark as a trigger provides a necessary buffer against temperature variability and the tendency for cold air to settle in low-lying areas, creating localized “frost pockets” where the temperature is lower than the official reading.
Recognizing Weather Conditions That Precede Frost
Temperature alone is not the only indicator; the atmospheric context strongly predicts frost formation, especially during a radiation freeze event. This type of freeze occurs when the ground rapidly loses heat to the atmosphere, a process accelerated by clear, cloudless skies. Clouds act as a blanket, reflecting radiating heat back toward the earth, but their absence allows warmth to escape quickly.
A lack of wind is another key condition, leading to still air near the ground. Wind helps mix the air, preventing the coldest air from settling. However, on calm nights, cold, denser air settles close to the surface where plants reside. This pooling causes the temperature at plant level to be several degrees colder than the temperature measured higher up.
The dew point, the temperature at which air becomes saturated and dew forms, also plays a role in frost prediction. If the dew point is near the air temperature but still above freezing, the moisture released can slightly buffer the temperature drop. If the dew point is very low, however, the air is dry, and the temperature can plummet more rapidly. Dry air combined with any light breeze can lead to evaporative cooling, causing plant surfaces to drop below freezing even if the air temperature remains slightly above 32°F.
Optimal Timing for Application and Removal
The timing of cover application is dictated by the principle of trapping residual heat absorbed by the soil throughout the day. Covers should be placed over plants well before sunset to capture warmth radiating upward from the ground. Applying covers after dark risks missing the window to lock in the day’s warmth, significantly reducing the protection offered.
The lowest temperatures of the night typically occur just before sunrise, making this the period of peak danger. The covers slow the rate of heat loss until the sun begins to warm the air again. It is important to remove the covers once the temperature has clearly risen above freezing and all visible frost has dissipated from the surrounding area.
Leaving the covers on too long can be detrimental, particularly if the material is non-breathable, such as plastic sheeting. As the sun rises, trapped air beneath the cover can heat up rapidly, creating a greenhouse effect that stresses or scorches the foliage. Excess moisture trapped inside can also promote fungal growth, making prompt removal after the morning thaw necessary.