Tomatoes are warm-weather plants sensitive to temperature drops, which can quickly become a major concern for home gardeners. Knowing how to respond to cold weather forecasts directly impacts the harvest. The question of whether to cover plants at the 40°F mark is a common moment of urgency for anyone growing tomatoes.
Understanding Tomato Temperature Tolerance
Tomato plants are susceptible to chilling injury at temperatures well above freezing. The ideal growth temperature range is between 70°F and 85°F during the day, with nighttime temperatures above 50°F. When temperatures consistently drop below 50°F, the plant enters a state of stress, slowing its growth and development.
This slowdown occurs because the plant’s metabolism is inhibited, affecting processes like photosynthesis and nutrient absorption. Prolonged exposure to temperatures in the 40s can cause cold damage, visible as light tan or gray discoloration on the leaves, and stunt growth. Low temperatures are particularly damaging during the reproductive stage, causing poor pollen viability and weak fruit set, which reduces the final harvest. Lethal damage, involving cell wall rupture from ice crystal formation, occurs when temperatures fall below 32°F, making frost the most immediate threat.
Should You Cover Your Tomatoes at 40°F?
Forty degrees Fahrenheit is not an immediate death sentence for a tomato plant, but covering them is recommended as a precautionary measure. While the plant may survive a brief dip, it is operating in a danger zone where chilling stress is certain. Experts suggest covering tomato plants whenever nighttime temperatures are forecast to drop below 50°F to prevent growth stunting and stress.
The need to cover is more pressing if the 40°F low is accompanied by adverse conditions like wind or rain, which accelerate heat loss. A prolonged drop lasting all night also poses a greater risk than a brief dip near dawn. Young seedlings and plants actively setting fruit are more vulnerable to cold stress than mature, established plants. Covering plants at 40°F is a proactive choice that protects against microclimatic variations that could push the temperature lower.
Choosing and Applying the Right Cover
Effective cold protection involves trapping heat radiating from the ground and insulating the plant tissue. The best materials are fabric-based, such as lightweight row covers, old bed sheets, or blankets. Unlike plastic sheeting, fabric allows moisture to escape, preventing condensation from forming and freezing directly onto the leaves, which causes contact damage.
The application technique is important. Drape the material over the plants before sunset to capture the day’s accumulated ground heat. The cover must extend all the way to the ground and be secured with stones or soil to create a sealed pocket of warmer air. Remove the covering promptly the next morning once the temperature rises above 50°F. Leaving the cover on too long can cause excessive heat buildup and humidity, damaging the plant.