Tomato plants are sensitive to temperature extremes, requiring a narrow thermal range for successful fruit production. While warmth is necessary for growth, high temperatures, especially those sustained over multiple days and nights, can halt development and severely reduce yield. Understanding the precise temperature thresholds where stress begins and the symptoms to look for is the first step in protecting a harvest from summer heat.
The Critical Heat Thresholds for Tomato Plants
The optimal daytime temperature range for tomato plants is between 70°F and 85°F (21°C to 29°C), allowing for peak photosynthesis and fruit development. Stress begins when daytime temperatures consistently exceed 85°F (29°C). Once temperatures climb above 90°F (32°C), plants shift from reproductive growth to survival mode, causing biological machinery to slow down and reproductive functions to suffer first.
Nighttime temperatures are an equally important factor in tomato heat stress. The plant requires a cool period to recover and process sugars produced during the day, ideally between 60°F and 70°F (16°C to 21°C). Nighttime temperatures remaining above 70°F to 75°F (21°C to 24°C) prevent this recovery and can cause greater pollen damage than daytime heat alone. A combination of daytime highs above 90°F and nighttime lows above 75°F creates a severe stress environment, making successful fruit set highly unlikely.
Specific Effects of Excessive Heat on Plant Development
When air temperature rises too high, the most immediate consequence is the failure to produce fruit, beginning with pollen sterility. Heat damages the delicate pollen grains, especially during their development three days before the flower opens, reducing viability and preventing fertilization. The flower, unable to be pollinated, will then drop from the vine, a symptom known as blossom drop.
High heat coupled with intense, direct sunlight can damage the developing fruit, leading to a condition called sunscald. This damage appears as white, yellow, or leathery patches on the side of the fruit most exposed to the sun. The intense heat kills the tissue under the skin, resulting in an unmarketable area.
The plant has a physical defense mechanism known as physiological leaf roll or curl, which is a direct response to water stress. The leaves curl inward and upward, reducing the total surface area exposed to the sun and wind. This protective measure decreases the rate of transpiration, the process of releasing water vapor through the leaves. By minimizing water loss, the plant attempts to maintain its internal moisture balance against heat.
Mitigation Strategies to Protect Tomatoes During Heatwaves
The primary defense against heat stress involves managing soil moisture and providing temporary shade during the hottest parts of the day. Deep, consistent watering is crucial, as it keeps the root zone hydrated and slightly cooler. Soak the soil thoroughly instead of frequent, shallow watering, encouraging deeper root growth that is more resilient to surface heat.
Applying a layer of organic mulch, such as straw or wood chips, around the base of the plant helps stabilize the soil temperature. A layer two to three inches thick acts as an insulator, reducing heat transfer into the root zone and slowing the evaporation of soil moisture. This practice can reduce moisture loss by up to 70%.
For daytime temperatures consistently above 90°F, providing shade can reduce the ambient temperature around the plant by several degrees. A shade cloth rated for 30% to 50% light reduction is recommended, as anything denser may hinder photosynthesis. The cloth should be positioned to block the intense afternoon sun, generally from 10 AM to 4 PM, providing relief without eliminating the light source.
Careful pruning decisions also play a role in heat management by maintaining adequate foliage. Leaves provide natural shade to the developing fruit, offering the best protection against sunscald. While some light pruning is beneficial for air circulation, removing too many leaves exposes the fruit to direct sun, counteracting the plant’s natural cooling and protection.