The temperature of 38 degrees Fahrenheit (3°C) is a concern for many gardeners. While safely above the freezing point of 32°F, 38°F falls within a range that can still cause significant physiological damage, especially to tropical or subtropical plants. Whether this temperature is “too cold” depends entirely on the plant’s genetic makeup and natural environment. The potential for harm at 38°F highlights the difference between two distinct types of cold stress.
Understanding Chilling Injury Versus Freezing Damage
The risk plants face at 38°F is primarily known as chilling injury, a form of damage that occurs at low but non-freezing temperatures, typically between 32°F and 50°F. This damage does not involve the physical rupture of cells by ice crystals, but instead affects the plant’s internal, metabolic processes. Chilling-sensitive plants, generally native to tropical regions, have cellular membranes composed of lipids that become rigid and less fluid when exposed to these cooler temperatures.
This structural change in the cell membrane disrupts its function, leading to metabolic imbalance and enzyme dysfunction. Symptoms of chilling injury can include water-soaked spots, surface pitting, wilting, and internal browning, which may take several days to appear. Unlike chilling injury, freezing damage occurs when temperatures drop to 32°F or lower, causing water in the plant’s intercellular spaces to turn into sharp ice crystals. These crystals physically dehydrate the cells and destroy the cell walls, leading to immediate tissue death, which often appears as blackening and a mushy texture.
Categorizing Plant Vulnerability to Near-Freezing Temperatures
A plant’s response to 38°F is determined by its inherent cold tolerance, allowing categorization into three vulnerability groups. The Tender/Tropical Plants group faces the highest risk, as these species are highly susceptible to chilling injury. Plants like basil, tomatoes, impatiens, pothos, and peace lilies can begin to suffer damage when temperatures consistently fall below 50°F to 55°F. Prolonged exposure to 38°F will almost certainly result in irreversible damage or death due to the severe disruption of their cellular membranes.
The Semi-Hardy Plants can tolerate a brief dip to 38°F without immediate failure, but they are not fully immune. Examples include pansies, cabbage, Swiss chard, and lettuce, which can often withstand temperatures down to the low 30s or even 20s if hardened off. However, if the 38°F temperature persists for many hours, tender new growth on these plants may still be damaged.
The final group, Hardy/Cool-Season Plants, is at the lowest risk, possessing genetic adaptations that allow them to endure temperatures far below 38°F. Established trees, shrubs, garlic, kale, and other winter vegetables can handle temperatures well into the teens or single digits. These plants have mechanisms, such as increasing soluble solids in their cells, which lower the freezing point and protect them from cold stress.
Immediate Protective Measures When Temperatures Drop
When a forecast calls for temperatures near 38°F, immediate action can prevent chilling injury, especially for cold-sensitive species. The simplest measure for tender container plants is to move them indoors into a garage, enclosed porch, or basement. This relocation protects them from cold air and potential frost.
For plants that cannot be moved, physical insulation is necessary to trap heat radiating from the soil. Covering plants with commercial frost cloth, old blankets, or burlap sheets can provide several degrees of protection. The covering must extend all the way to the ground to be effective, and the material should not directly touch the foliage, as cold can transfer through the fabric.
Another preparation is to thoroughly water the garden beds before the temperature drop, ideally 24 hours in advance. Moist soil retains heat more effectively than dry soil and releases that warmth slowly throughout the night. This radiating heat can keep the air immediately surrounding the plants a few degrees warmer. Additionally, moving potted plants close to a south-facing wall or solid fence utilizes radiant heat absorbed by these structures, offering a localized pocket of warmth.