The tomato plant is a warm-season crop native to the Andes Mountains, thriving in heat and bright sunshine. While warmth is necessary for vigorous growth and sweet fruit production, there is a point where the temperature becomes detrimental. When ambient temperatures exceed the plant’s physiological limits, its focus shifts entirely to survival, halting the processes that yield a harvest. Understanding the precise temperature boundaries that trigger this stress response is essential for successfully growing tomatoes, especially in climates experiencing prolonged heat waves.
Critical Temperature Thresholds
The temperature at which a tomato plant begins to struggle involves two distinct thresholds: one for the daytime high and one for the nighttime low. During the day, the optimal temperature range for the plant to conduct photosynthesis and sustain vegetative growth is between 70°F and 85°F (21°C and 29°C). Once daytime temperatures consistently rise above 90°F to 95°F (32°C to 35°C), the plant’s internal functions start to slow down drastically.
The second, and often more significant, boundary is the nighttime temperature. If the air temperature remains above 70°F to 75°F (21°C to 24°C) after sunset, the plant cannot recover from the day’s heat. This prolonged warmth forces the plant to maintain a high metabolic rate, burning up the sugars through respiration instead of allocating them toward fruit development. High nighttime temperatures are a major cause of fruit set failure, even if daytime temperatures are manageable.
How Heat Stress Affects Tomato Production
The most immediate consequence of excessive heat is the failure of flowers to develop into fruit, known as blossom drop. Temperatures above 90°F (32°C) during the day, or sustained high temperatures overnight, cause the pollen to become sterile. The plant may still produce flowers, but the pollen grains cannot successfully pollinate the stigma, and the flowers will simply dry up and fall off the vine.
Heat also severely compromises the quality of any fruit that manages to set. High temperatures inhibit the production of lycopene, the carotenoid pigment responsible for the red color in tomatoes. When this process is disrupted, the fruit often develops yellow shoulders, where the top portion of the tomato near the stem remains yellow or green while the rest of the fruit ripens. The fruit may also be smaller, softer, and ripen more quickly than usual, leading to a reduced shelf life.
Direct sun exposure during extreme heat can result in sunscald, which appears as white, leathery, or blistered patches on the side of the fruit facing the sun. This damage is essentially a sunburn, and the affected tissue is non-recoverable. To protect itself and conserve moisture, the foliage of the plant will often show visible symptoms, such as leaf curling or rolling. While this wilting action is a survival mechanism to reduce the surface area exposed to the sun, prolonged severe heat stress can lead to the desiccation and browning of leaves.
Strategies for Cooling Tomato Plants
When high temperatures are forecasted, the most effective defense is to reduce the heat load on the plant and maintain moisture within the soil. One of the best ways to reduce direct sun exposure is by using a shade cloth over the plants during the hottest part of the day, typically from noon until 4 p.m. A shade cloth that provides 30% to 50% shade is sufficient to lower the ambient temperature without significantly impeding photosynthesis. It is beneficial to suspend the cloth a few feet above the canopy to allow for airflow, preventing heat from becoming trapped underneath.
Water management becomes paramount in survival conditions, and plants require deep, consistent watering, ideally done in the early morning. Deep watering encourages the roots to grow downward, making the plant more resilient to surface heat and drought. The soil should be kept uniformly moist, and a simple test involves probing the soil two to three inches down to ensure it is not completely dry.
A thick layer of organic mulch, such as straw or shredded wood chips, is a powerful tool for mitigating heat stress. Applying a layer that is two to three inches deep helps to shade the soil, which keeps the root zone cooler than exposed earth. Mulch also slows the rate of water evaporation from the soil, helping to maintain the consistent moisture levels necessary during heat waves.