Tomato plants originate from warmer climates and are exceptionally sensitive to cold. A frost event is highly likely to be fatal to the plant tissue because this warm-season annual lacks the natural defenses of hardy plants. Understanding the specific mechanisms of cold injury and knowing the temperature thresholds are necessary for protecting your plants. This information helps determine when to take action to save your garden from an unexpected cold snap.
The Immediate Answer: Why Tomatoes are Frost Sensitive
Tomato plants contain a high percentage of water, which is the primary reason for their vulnerability to freezing temperatures. When the temperature drops below freezing, the water inside the plant cells begins to form ice crystals. These crystals take up more volume than the liquid water they replace, causing the cell walls to rupture.
The damage often occurs from ice forming in the spaces between cells (extracellular freezing), rather than inside the cells. As ice forms outside, it draws water out through the cell membrane, leading to severe cellular dehydration. This process collapses the cells, turning the affected tissue dark, wilted, and eventually black.
Defining the Danger Zone: Specific Temperatures
The threshold for frost damage is defined by the freezing point of water, \(32^\circ\text{F}\) (\(0^\circ\text{C}\)). Exposure to temperatures at or below this point risks a light frost that can damage the foliage. Even temperatures slightly above freezing, between \(32^\circ\text{F}\) and \(40^\circ\text{F}\) (\(0^\circ\text{C}\) and \(4.4^\circ\text{C}\)), can cause chilling injury, which stunts growth and affects fruit quality.
A hard freeze, considered temperatures below \(28^\circ\text{F}\) (\(-2.2^\circ\text{C}\)), is lethal to the entire plant. The duration of the cold affects the extent of the damage; a brief dip to \(31^\circ\text{F}\) is less damaging than several hours below freezing. Factors like wind, humidity, and cloud cover influence whether frost forms, even if the air temperature is slightly above the freezing point.
Protecting Plants Before a Freeze
The most effective preventative measure is to physically cover the plants to trap ground heat and prevent frost from settling on the leaves. Use blankets, sheets, or specialized frost cloth draped over the plants, ensuring the material extends to the soil line to seal in warmth. It is important to support the cover with stakes or a frame so the fabric does not directly touch the foliage, as contact can transfer the cold and cause localized damage.
For tomatoes growing in containers, the simplest action is to move them temporarily into a garage, shed, or covered porch for the night. Watering the soil heavily late in the afternoon before the expected freeze can also provide a small degree of protection. Moist soil holds and releases heat more effectively than dry soil, helping to warm the air immediately surrounding the plants overnight. Furthermore, placing strings of old-style incandescent Christmas lights within the covering structure can generate a small amount of heat that may raise the temperature by a few degrees.
Assessing and Treating Frost Damage
After a frost event, wait for three to five days before fully assessing the plant’s condition. The extent of the internal damage is not always immediately apparent, and compromised tissue will eventually turn black and wilted. If the entire plant is blackened down to the soil line, it is likely dead and should be removed.
If you observe green tissue remaining on the main stem, especially near the base, the plant has a chance of recovery. Any damaged, blackened leaves or stems should be carefully pruned away to encourage new growth and prevent disease from entering the dead tissue. Applying a water-soluble fertilizer that is relatively high in nitrogen can help stimulate the plant to produce new foliage quickly.