The thermometer reading of 45°F commonly triggers concern among gardeners about protecting their plants. This temperature is not lethal for established plants, but it serves as a warning sign requiring an assessment of your garden’s specific vulnerabilities. The decision to cover plants at this relatively mild temperature depends entirely on the type of plant you are growing and the environmental conditions accompanying the cold night. A temperature of 45°F signals the possibility of frost.
Understanding the Critical Temperature Threshold
While 45°F itself is safely above the freezing point of water, it falls within the range where certain plants begin to suffer physiological strain known as chilling injury. For the most sensitive species, metabolic damage can begin when temperatures drop below 50°F to 59°F, long before a freeze occurs. Chilling injury is not caused by ice crystals forming, but rather by the disruption of cellular membranes and enzyme function at non-freezing temperatures.
The true danger to most garden plants comes from frost, which occurs when the plant’s surface temperature drops to 32°F or lower. Air temperature reported by weather services is typically measured several feet above the ground. On clear, calm nights, plant surfaces lose heat rapidly through radiation, causing their temperature to drop several degrees below the recorded air temperature. This means a reported air temperature of 40°F can easily lead to a ground frost, making 45°F a useful threshold for preparedness.
Plant Hardiness and Vulnerability by Type
The need to cover plants is directly related to their natural tolerance, classifying them into three main groups based on their sensitivity to cold.
Tender Plants
Tender or tropical plants are the most susceptible to injury and require protection well above the freezing mark. Species like tomatoes, peppers, basil, and impatiens can suffer irreversible damage or death from chilling injury when exposed to temperatures below 50°F for extended periods. Their warm-weather origins mean their cells are not adapted to withstand the cold-induced stress.
Semi-Hardy Plants
Semi-hardy or frost-tolerant plants possess a moderate degree of cold resistance and can generally tolerate air temperatures that dip down to 32°F, or even a light, brief frost around 28°F. This group includes popular cool-season crops such as lettuce, carrots, beets, and pansies. While a light frost may damage the outer leaves of these plants, the main structure or root system usually remains intact, allowing them to recover quickly.
Hardy Plants
The most resilient group is the hardy plants, which can withstand heavy frost and temperatures consistently below 28°F. Vegetables like kale, spinach, cabbage, and garlic are genetically equipped to survive severe cold, often tasting sweeter after a hard frost. For these plants, a 45°F temperature poses no threat and covering is unnecessary unless an exceptionally severe freeze is forecast.
Environmental Factors That Increase Risk
A temperature of 45°F is more concerning when other atmospheric factors suggest a sharp drop is imminent. The dew point, which is the temperature at which water vapor in the air condenses, is a telling indicator of frost potential. When the dew point is close to the air temperature, it signals high humidity, which increases the likelihood of a visible, white frost as the temperature nears 32°F.
A large difference between the air temperature and the dew point often signals very dry air, which can lead to a more dangerous “black frost.” Dry air cools more rapidly at night, and while no visible ice forms, the internal moisture of the plant can freeze. Clear night skies also increase the risk, as the absence of clouds allows heat radiated from the ground to escape directly into the atmosphere, accelerating the drop in surface temperature. Wind can also exacerbate the cold by stripping away the insulating layer of air surrounding the foliage, leading to increased heat loss.
Practical Guide to Protecting Plants
Once the decision to cover is made based on plant type and environmental risk, selecting the correct material and technique is paramount. Lightweight fabric coverings, such as old bedsheets, blankets, or horticultural fleece, are the most effective materials. These options allow the plant to breathe while trapping the heat that radiates from the soil.
It is advisable to avoid placing clear plastic sheeting directly onto the foliage, as it conducts cold, and any moisture trapped beneath it can freeze and cause direct contact injury. When covering plants, ensure the material extends all the way to the ground and is secured with soil, rocks, or bricks to effectively seal in residual warmth. For potted plants, the easiest solution is to move them temporarily against a warm wall of the house or into a protected area like a garage or shed.
The timing of the covering is as important as the material used; covers should be placed over plants before sunset to capture the heat stored in the soil throughout the day. It is equally important to remove the protective layers shortly after sunrise, or once the temperature rises above 40°F. Leaving covers on too long can cause the temperature underneath to spike rapidly under direct sunlight, leading to overheating and damaging the plants.