Tomato plants are classic warm-weather crops, originating in tropical and subtropical regions. This ancestry makes them tender annuals highly susceptible to cold exposure. While they thrive in heat, even a brief drop in temperature causes significant physiological stress and damage. Consequently, tomatoes generally cannot survive a hard frost event without intervention.
Understanding Tomato Vulnerability to Cold
Tomatoes experience chilling injury when temperatures drop below 50°F (10°C), leading to slow growth, reduced fruit set, and leaf discoloration. The true danger comes from freezing temperatures, which start at 32°F (0°C). Exposure to frost or a freeze at this point can be lethal.
The mechanism of cold damage is physical: water inside the plant cells freezes and expands. This ice formation ruptures the delicate cell walls, causing the tissue to collapse. The resulting damage is irreversible, and the softer, newer growth is the first to be affected. While a mature plant might briefly survive a light, short-duration frost if well-protected, sustained exposure below freezing will kill the plant outright.
Actionable Steps for Frost Protection
Proactive protection is the only way to save a tomato plant when freezing temperatures are forecasted. One effective preparation is to deeply water the soil a few hours before sunset. Moist soil retains heat better than dry soil, radiating warmth upward to create a warmer microclimate around the root crown.
Physical covers are the primary defense against frost and must be secured before dusk to trap the day’s warmth. Use materials like heavy blankets, burlap, or dedicated heavyweight row covers, ensuring the material drapes all the way to the ground. Since covers should not sit directly on the foliage, use tomato cages or stakes to create a supporting frame for larger plants.
To prevent cold air from entering and warm air from escaping, anchor the edges of the cover with rocks, bricks, or soil. For small plants, simple plastic cloches or gallon milk jugs with the bottoms cut out can be placed over individual seedlings. Remove the caps during the day to prevent overheating and condensation.
Supplemental heat can be used to raise the temperature under the cover by a few degrees. Place capped, dark-colored plastic jugs filled with water near the base of the plant, as they absorb solar heat during the day and slowly release it overnight. For an active heat source, a string of old-style incandescent holiday lights can be woven through the cage, providing gentle warmth under the covering. Container plants offer the simplest solution, as they can be moved entirely into a garage, shed, or covered porch to escape the cold air.
Identifying and Assessing Frost Damage
After a frost event has passed, wait until mid-morning or afternoon for a proper assessment. The full extent of the damage is often not visible until the frozen tissues have had a chance to thaw completely. The clearest signs of frost injury are blackened, water-soaked foliage and stems.
The leaves will appear wilted and limp, often turning dark gray or black as the burst cells release their contents. Inspect the stems, especially the main support stem, for discoloration, which may look dark brown or purple near the base. If only the outer leaves and growing tips show these symptoms, the damage may be superficial. If the main stem feels soft, mushy, or completely blackened near the ground, the plant has likely suffered catastrophic damage.
Salvaging Damaged Tomato Plants
If the frost was light and the main stem near the soil line appears firm and healthy, the plant has a chance at recovery. Resist the urge to immediately prune the damaged, blackened parts, as they can offer insulation to the underlying tissue. Wait until the threat of further frost has passed and new growth begins to emerge.
Once new shoots are visible, use clean shears to remove all the dead, frozen foliage and stems. Pruning the dead tissue encourages the plant to direct energy toward the remaining healthy growth nodes. If the main stem is frozen solid or completely compromised near the soil, the plant is typically beyond saving and should be removed.