While 40°F is generally above the freezing point of water, it represents a threshold where serious physiological damage, known as chilling injury, begins for many common garden and landscape species. For plants native to tropical and subtropical climates, this temperature is dangerously low and can cause significant harm. The key is that damage occurs not from ice formation, but from metabolic dysfunction. This article provides practical guidance for protecting vulnerable plants when temperatures dip to this level.
Why 40 Degrees Fahrenheit Poses a Threat
The primary danger at 40°F is chilling injury, a physiological disorder occurring at low, non-freezing temperatures. This cold shock affects the plant’s internal machinery, particularly the cellular membranes. As temperatures drop, the lipid bilayer of these membranes transitions from a flexible state to a rigid, gel-like structure.
This stiffening severely compromises the cell membrane’s function, leading to increased permeability and the leakage of solutes and electrolytes. The disruption extends to the plant’s metabolism, as enzymes anchored to these rigid membranes become less active or dysfunctional. Key processes like photosynthesis and respiration slow dramatically, effectively starving the plant of energy.
The root system is also highly sensitive; cold soil inhibits the roots’ ability to absorb and transport water, even when the soil is moist. This inhibition can induce physiological drought, causing the plant to wilt because it cannot take up enough water to compensate for transpiration. The combined effects of membrane damage and metabolic slowdown lead to a cascade of cellular failure that manifests as visible injury.
Determining Plant Vulnerability
Plant susceptibility to 40°F chilling injury varies widely based on native climate and genetic tolerance, dividing common species into distinct vulnerability groups.
Highly Vulnerable
These plants are typically from tropical origins, where chilling injury can begin at temperatures above 40°F. This group includes common warm-season annuals like impatiens, petunias, and basil, as well as vegetables such as tomatoes, peppers, and cucumbers.
Moderately Vulnerable
These species are often warm-season crops or semi-hardy vegetables that can tolerate brief dips, but not prolonged exposure to 40°F. Newly transplanted seedlings are also highly susceptible because their root systems are not yet established. Established warm-season vegetables, such as squash or melon plants, will likely experience severe stress, stunted growth, or flower and fruit abortion at this temperature.
Generally Safe
These are hardy, cool-season varieties, often tolerant of temperatures well into the 20s. This group includes established perennials, mature woody shrubs, and cool-season vegetables like kale, spinach, cabbage, and carrots. These plants have evolved mechanisms to acclimate to cold, such as accumulating cryoprotective solutes in their cells.
Practical Steps for Cold Protection
Immediate action is necessary when a temperature drop to 40°F is forecast, focusing on insulating the plant and conserving heat radiating from the ground.
One effective strategy is covering plants with a breathable, lightweight fabric, such as a frost cloth or an old sheet. The covering should extend all the way to the ground to trap the warmer air rising from the soil, creating a protective microclimate around the plant canopy. Ensure the covering does not directly touch the foliage, as this contact can transfer cold and cause localized damage.
For container plants, move them into a sheltered location, like a garage, shed, or close to a warm wall of the house that radiates residual heat. Clustering containers tightly together also helps them share warmth and shields exposed root systems.
Thoroughly water the soil the day before the cold snap is expected. Moist soil retains significantly more heat energy than dry soil and slowly releases that warmth throughout the night. This moisture also helps the plant avoid dehydration, a common stressor under cold conditions due to reduced root function.
Assessing and Recovering from Cold Stress
After the cold event passes, identify the visual signs of chilling injury. These can include dark, water-soaked spots on leaves and stems that eventually turn necrotic or brown, sudden wilting, and a bronzing or purpling of the foliage. These signs may not appear immediately, sometimes taking several hours or days to fully develop once temperatures warm up.
The most important step in recovery is to resist the urge to immediately prune away damaged tissue. Dead or dying leaves and stems can provide a layer of insulation to the underlying, healthy portions of the plant or the growing points. Wait for a few days to a week for the full extent of the damage to become clear before making any cuts.
Once the danger of cold has passed, move any protected plants back to their normal, bright environment gradually, especially if they were stored in a dark space. Continue to monitor the plant for new growth, which is the best indicator of recovery. Providing consistent, but not excessive, water and a balanced fertilizer can support the plant’s metabolism as it works to repair the cellular damage from the cold stress.