A temperature of 39°F (3.9°C) signals the approach of freezing conditions, but it is not an automatic death sentence for most plants. While plant tissues will not freeze at this temperature, it indicates atmospheric conditions are favorable for frost development. The decision to take action depends on factors beyond the thermometer reading. Effective cold protection requires understanding the difference between the air temperature and the temperature the plant’s leaves actually experience.
The Critical Temperature Threshold
The physiological danger zone for sensitive plant life begins at 32°F (0°C) and below, where water inside plant cells turns to ice, causing cell walls to rupture. The temperature reported by weather services is typically measured several feet above the ground, which is often warmer than the plant surface itself. This difference is especially pronounced during periods of radiational cooling, where heat radiates away from the earth and plant surfaces into a clear night sky.
Under these conditions, frost can occur even when the official air temperature remains just above freezing, perhaps at 36°F or 37°F. Plant leaves, particularly those exposed to the sky, can cool several degrees lower than the surrounding air due to this heat loss. Furthermore, tropical plants and new, tender growth can suffer chilling injury. This is physiological damage occurring above freezing, typically between 32°F and 50°F.
Factors Influencing the Decision to Cover
The need to cover plants at 39°F depends on several atmospheric and plant-specific variables. The primary consideration is the plant’s hardiness; a newly planted tropical annual, like a tomato, is far more susceptible than an established perennial shrub. Thin-leaved plants and those with high water content are particularly prone to damage.
The duration of the cold is a factor, though the depth of the temperature drop is generally more significant for short cold snaps. Clear skies dramatically increase the risk because they allow for maximum radiational cooling, accelerating heat escape. A strong wind accelerates the rate of heat loss from the plant’s surface, increasing the chances of desiccation and damage. Finally, damp soil holds and releases significantly more heat than dry soil, so dry conditions elevate the need for physical covers.
Practical Guide to Protecting Plants
For effective protection, the goal is to trap heat radiating from the ground and prevent direct frost contact with the foliage. The best covering materials are breathable fabrics like old bedsheets, blankets, burlap, or specialized frost cloths. These materials provide a layer of insulation while allowing air circulation and moisture to escape.
Avoid using plastic directly touching the plant foliage, as this can transfer cold and cause more damage where the plastic contacts the leaves. The cover must extend all the way to the ground and be secured, creating a temporary tent that traps the residual heat stored in the soil. Once temperatures rise above 45°F in the morning, the cover must be removed promptly to prevent the plants from overheating.
Identifying and Treating Cold Damage
After a cold event, signs of damage may not appear immediately, but they often manifest as discoloration and wilting. Common symptoms include leaves that appear blackened, water-soaked, or suddenly limp and translucent. This visual change is a result of the ice crystals forming and rupturing the plant’s cell walls.
The most important step following frost damage is patience; do not immediately prune away the damaged material. The dead foliage can insulate the lower parts of the plant and new growth points from subsequent cold snaps. Wait until the danger of frost has passed completely and new growth has begun to appear before cutting back the dead tissue. During this recovery period, avoid over-fertilizing and ensure the plant receives consistent water.