Tomato plants are botanically warm-weather perennials that are typically cultivated as annuals in most regions. Originating from subtropical climates, these plants require consistently high temperatures to function optimally and produce fruit. This tropical background makes them acutely sensitive to temperature fluctuations, establishing cold as the single greatest threat to their health and productivity throughout the growing season. A brief exposure to chilling temperatures can severely impact development, while freezing conditions are almost always fatal.
The Lethal Freezing Point
The absolute minimum temperature that causes immediate, irreversible damage to tomato plants is \(32^{\circ}\text{F}\) (\(0^{\circ}\text{C}\)). When the temperature drops to this freezing point and frost forms, the water within the plant’s cells turns to ice. This ice crystal formation physically expands, puncturing the cell walls and rupturing the membranes that hold the plant tissue together.
This process leads to the immediate destruction of the plant’s tender foliage and stems. Visible signs of severe frost damage appear quickly, characterized by leaves and stems that turn black, wilt, and become limp shortly after thawing.
A plant exposed to temperatures that cause a hard freeze, typically below \(28^{\circ}\text{F}\) (\(-2.2^{\circ}\text{C}\)) for several hours, has almost no chance of recovery. Even if the main stem survives, the damaged tissue will not regenerate, and the plant will be severely compromised.
Physiological Stress Below Optimal Range
Temperatures that remain above freezing can still inflict significant damage by causing chilling injury and stalling growth. This physiological stress begins when the air temperature, particularly at night, falls into the range of \(40^{\circ}\text{F}\) to \(55^{\circ}\text{F}\). Within this range, the plant’s metabolism slows down, disrupting critical functions necessary for reproduction and development.
A major consequence of prolonged cool temperatures is blossom drop, which occurs when nighttime temperatures consistently dip below \(55^{\circ}\text{F}\) (\(13^{\circ}\text{C}\)). The cold impairs the viability and development of the pollen inside the flowers, preventing successful fertilization and fruit set. Consequently, the plant aborts the unpollinated flowers, causing them to dry up and fall off, which directly reduces the overall yield.
The constant exposure to temperatures below \(50^{\circ}\text{F}\) (\(10^{\circ}\text{C}\)) also slows photosynthesis and inhibits the plant’s ability to take up and utilize certain nutrients from the soil. Phosphorus uptake is particularly affected in cold soil, which can lead to a deficiency that sometimes presents as a purplish discoloration on the undersides of the leaves and stems.
Although the plant may survive this non-lethal cold, the resulting stunted growth and compromised reproductive cycle negatively impact the quality and quantity of the final harvest.
Protecting Plants from Unexpected Cold
When cold temperatures, especially those nearing the freezing point, are predicted, a gardener can employ several short-term, actionable protection strategies.
Covering and Insulation
A simple and effective method is to cover the plants entirely with materials like old blankets, cotton sheets, or row covers. The covering should be draped over a frame or cage to avoid contact with the foliage, and it must extend all the way to the ground to trap residual warmth radiating from the soil.
Soil and Passive Heating
Prior to an impending cold snap, watering the area around the plants can also provide a small degree of protection. Moist soil holds and radiates heat more effectively overnight than dry soil, providing a warmer microclimate near the roots and lower stems. Gardeners can also use passive heating techniques by placing closed jugs of water that have warmed up during the day underneath the protective covers.
Specialized Devices
Specialized water-filled devices, sometimes called wall-o-waters or cloches, surround individual plants and absorb solar heat during the day, which they slowly release overnight. These measures are designed to raise the air temperature immediately surrounding the foliage by several degrees, potentially creating a buffer zone that keeps the plant safely above the damaging temperature thresholds. The covers should always be removed the following morning once temperatures rise to allow sunlight and air circulation.