Tomatoes originate from warm regions and require plenty of sunshine to thrive and produce fruit. While this often leads gardeners to assume that hotter weather is better, the relationship between the plant and high temperatures is complex. Once temperatures climb too high, especially for prolonged periods, the plant’s ability to reproduce and develop quality fruit is significantly compromised.
Defining High-Temperature Limits
Tomato plants operate most efficiently within a relatively narrow temperature band for optimal growth and fruit production. The ideal daytime air temperature range is between 70°F and 82°F, encouraging strong photosynthesis and overall plant health. A slightly cooler environment at night is also beneficial, ideally between 60°F and 65°F.
Problems begin to emerge when temperatures consistently exceed these optimal thresholds. Daytime temperatures above 85°F to 90°F induce heat stress. High nighttime temperatures, sustained above 70°F to 72°F, are even more disruptive, causing significant physiological strain and impacting the reproductive cycle. These upper limits define the point where the plant shifts from productive growth to a survival state.
How Extreme Heat Affects Pollination and Fruit Set
The most immediate and noticeable effect of extreme heat is the failure of the plant’s reproductive process, leading to a lack of fruit set. High temperatures interfere directly with the delicate development and viability of the pollen grains. A few hours of temperatures exceeding 104°F on successive days can result in complete fruit set failure.
Nighttime temperatures are particularly damaging because the pollen tube, which must grow down the style for fertilization to occur, is inhibited when temperatures remain above 75°F. High heat can also cause the developing anthers to suffer from altered carbohydrate metabolism, which is necessary for healthy pollen development. This collective failure of the pollen and the flower’s structure often results in “blossom drop,” where the seemingly healthy flowers simply dry up and fall off the plant without forming fruit. When a heat wave passes, the plant may resume flowering, but all flowers developed during the period of extreme heat are often lost.
Physiological Damage to Plants and Developing Fruit
Heat stress also causes physical damage to the plant and diminishes the quality of any fruit that does manage to set. One common issue is sunscald, which appears as pale white or yellowish blotches on the exposed shoulders of green or ripening fruit. This damage occurs when the fruit is subjected to intense direct sunlight, especially after protective foliage has been lost due to disease or pruning.
High temperatures severely impact the fruit’s ability to develop its characteristic red color. The production of lycopene, the red pigment responsible for the fruit’s color, is inhibited when temperatures exceed 86°F. This results in fruit that is yellow, orange, or unevenly ripened, often with a tough, white internal core. Inconsistent moisture availability caused by rapid water loss can also lead to fruit cracking or splitting, as internal pressure exceeds the skin’s capacity to stretch.
Actionable Steps for Heat Mitigation
Gardeners can take several specific actions to help tomato plants survive and produce during periods of high heat.
Adjusting the watering routine ensures the root zone remains sufficiently hydrated and cool. Deeply soaking the soil early in the morning allows the plant to access moisture throughout the hottest part of the day while minimizing water loss to evaporation.
Providing shade is a highly effective mitigation strategy to keep the plant’s internal temperature down. A lightweight shade cloth (blocking 30% to 50% of sunlight) can be temporarily draped over plants during peak heat hours (noon to 4 p.m.). This localized shade reduces stress on foliage and developing fruit, helping to prevent sunscald and preserve pollen viability.
Applying a thick layer of organic mulch, such as straw or shredded wood chips, is important for regulating soil temperature. A 2- to 3-inch layer helps to insulate the ground, keeping the root system cooler and reducing moisture evaporation. Keeping the soil cool is important because the roots are the plant’s primary mechanism for taking up the water needed to counteract heat stress.