Tomatoes are classic warm-season annuals with a tropical origin, making them extremely sensitive to cold conditions. They are not frost tolerant and lack the physiological defenses to cope with freezing temperatures, which can quickly lead to plant death. Success depends heavily on recognizing and mitigating the dangers posed by cold weather.
Critical Temperature Thresholds
The ideal temperature range for vigorous growth and fruit production is 70°F to 85°F (21°C to 29°C) during the day. Nighttime temperatures should remain between 60°F and 70°F (15°C and 21°C) for proper metabolic recovery.
A drop below 55°F (13°C) causes stress, slowing growth and impacting nutrient uptake. Below 50°F (10°C), the plant’s metabolism slows significantly, halting all growth, though the plant may survive. The fatal mark is 32°F (0°C) and below, which initiates the formation of ice within the plant tissue.
Understanding Chilling Injury Versus Frost Damage
Damage from cold occurs through chilling injury and frost damage. Chilling injury is a physiological disorder that happens at low, non-freezing temperatures, typically between 32°F and 59°F (0°C and 15°C). This damage results from cell membranes losing fluidity, disrupting metabolic functions over hours or days.
Symptoms manifest as stunted growth, yellowing or purpling of leaves due to impaired nutrient transport, and general wilting. The effects are not immediate but appear gradually as cell function degrades. This injury is common when young transplants are put out too early or during a prolonged cool spell.
Frost damage, in contrast, is an immediate event occurring when the temperature drops to 32°F (0°C) or below. Ice crystals form inside or between plant cells, causing the physical rupture of cell walls. Rapid dehydration as water turns to ice also contributes to tissue destruction.
The visual symptom is a sudden, water-soaked, or blackened appearance of the foliage and stems, often becoming mushy upon thawing. This cellular destruction is irreversible; while a light frost may only damage outer leaves, a hard freeze below 28°F (-2°C) can kill the entire plant.
Practical Steps for Cold Weather Protection
Gardeners should wait until the danger of the last expected spring frost has passed before transplanting tomatoes outdoors. Nighttime temperatures should consistently be above 50°F (10°C) for several days before planting to prevent chilling stress. Planting into cold soil will immediately stunt the plant, even if the air temperature is mild.
If an unexpected cold snap is forecasted, active protection is required. Water the soil deeply before the temperature drops, as moist soil retains and radiates more heat throughout the night than dry soil. This difference in ground temperature can sometimes be enough to prevent damage.
Covering plants traps the heat radiating from the ground and provides insulation. Use lightweight blankets, sheets, or commercial frost cloth, ensuring the material extends to the ground to seal in the warmth. The cover must not directly touch the tomato foliage, as this transfers cold and causes damage at the contact points.
For small plants, devices like a Wall O’ Water or a simple cloche can provide protection down to the low 20s Fahrenheit. After the frost danger has passed, any leaves or stems that have turned black or dark brown should be pruned away. Removing this dead material allows the plant to focus energy on new, healthy growth.