The tomato plant (Lycopersicon esculentum) is a warm-season annual crop highly valued for its fruit production. Originating from tropical and subtropical regions, its development is tightly governed by ambient temperature. Temperature is the most significant environmental factor determining a successful harvest, dictating everything from germination rates to final fruit quality. The plant operates within a narrow thermal window, and deviations outside this range quickly lead to physiological stress and reductions in yield.
Optimal Temperatures for Vegetative Growth
Tomato plants exhibit their most robust vegetative growth—the production of stems, leaves, and overall biomass—when air temperatures are maintained within an ideal range. Optimal daytime temperatures generally fall between 70°F and 82°F (21°C to 27°C). Within this warm zone, the plant maximizes its photosynthetic efficiency, converting sunlight into sugars necessary for rapid development.
A slight but consistent temperature drop during the night is important for maintaining plant health and fruit production. Nighttime temperatures should ideally settle between 62°F and 64°F (17°C to 18°C). This cooling slows the plant’s respiration rate, the process of consuming stored sugars for energy. By reducing the metabolic burn rate overnight, the plant retains more carbohydrates produced during the day, allocating them toward stem growth and fruit development.
Low Temperature Stress and Chilling Injury
When temperatures fall below the optimal range, tomato plants begin to experience low-temperature stress, which can lead to metabolic disruption. Chilling injury, a non-freezing injury, typically occurs when temperatures drop below 50°F (10°C) for several hours. This threshold can be slightly higher for fruit, where damage is seen below 55°F (12.5°C).
Visible symptoms of prolonged cold exposure include stunted growth and the purpling of leaf undersides and stems. This discoloration is often a sign of inhibited nutrient uptake, particularly phosphorus and potassium, which the plant struggles to absorb from cold soil. Fluctuating day-to-night temperatures can also induce a physiological leaf curl, where the plant attempts to conserve moisture and regulate its internal pressure.
Chilling injury is distinct from frost damage, which occurs when air temperatures drop to or below the freezing point of water, 32°F (0°C). At this temperature, water within the plant’s cells turns to ice, causing crystals to expand and rupture the cell walls. This cellular destruction results in the immediate collapse and blackening of the tissue, leading to permanent damage and often plant death.
High Temperature Stress and Fruit Set Failure
High temperatures pose a major threat to tomato production, primarily by disrupting the reproductive phase of the plant. Sustained daytime temperatures above 90°F (32°C) can cause significant physiological stress, with the most noticeable effect being fruit set failure. This failure is not due to the plant lacking energy but rather to a direct impact on the pollen itself.
Exposure to excessive heat sterilizes the pollen grains, preventing successful fertilization. High temperatures disrupt the plant’s carbohydrate metabolism, preventing the necessary accumulation of starch in the developing anthers. Without viable pollen, the flower cannot be fertilized, leading to non-pollinated blossoms drying up and falling off the plant, a phenomenon known as “blossom drop.”
High heat also affects the fruit directly, even after successful fertilization. When fruit is exposed to intense, direct sunlight, sunscald can develop. This injury appears as a large, pale, blistered area on the side of the fruit facing the sun, often leading to a tough, dry patch susceptible to cracking and secondary infections. Furthermore, temperatures consistently above 86°F (30°C) inhibit the production of lycopene, the pigment responsible for the red color, resulting in tomatoes that ripen with a pale orange or yellow hue.
Techniques for Temperature Regulation
Gardeners can employ several techniques to buffer tomato plants against temperature extremes. During periods of intense summer heat, installing a lightweight shade cloth is effective, reducing the ambient temperature around the plant by 10°F to 15°F. The cloth should block the intense afternoon sun, allowing the plant to continue photosynthesis without heat stress.
Managing soil temperature is equally important. Applying a thick layer of organic mulch, such as straw or wood chips, helps insulate the root zone. Mulch keeps the soil cooler during the day and stabilizes moisture levels; deep, consistent irrigation also provides evaporative cooling for the entire plant.
For protection against cold, especially during early spring or late fall, temporary structures are necessary. Placing row covers or plastic tunnels over the plants can trap residual heat and shield the foliage from direct frost exposure, providing insulation against overnight drops.