Heat Stress and Tomatoes

It only takes small increases in temperature to affect living systems. We can look to humans for an example. Our optimal body temperature is 98.6˚F. When our temperature rises 3˚F to 101.6, we don’t feel well. We're at the emergency room if it increases another 3˚F to 104.6.

The optimal daily air temperature range for tomatoes is 70–75˚F. When temperatures are within this range, the weather is otherwise favorable, and other factors like fertility and pests are managed well, you’ll have high yields of high-quality tomatoes. Plants show the effects of high heat when daily temperatures reach 82˚F, and heat stress occurs when temperatures are between 88 and 102˚F. Night-time air temperatures above 70˚F also result in heat stress. Under high-temperature stress, you’ll see changes in your tomato plants.

During June and July 2024, tomato plants in the State College, Pennsylvania area faced significant heat stress due to high temperatures. Over the two months, there were 14 days when daytime temperatures reached 88˚F or higher—6 occurred in June and 8 in July. Additionally, there were 12 nights with temperatures above 70˚F— 4 in June and 8 in July, contributing further to the stress on the plants. The heat made those 61 days one of the hottest periods on record, with June and July ranking as the 9th warmest in the last 131 years.

Heat tolerance in tomato plants varies by cultivar, with some being more tolerant than others. High temperatures and light levels affect mechanisms important in photosynthesis. This decreases photosynthesis and leads to smaller plants and lower yields.

Flowers and fruit are more sensitive to high temperatures than leaves and stems. When high temperatures occur 7–15 days before flowering, you’ll likely see aborted and dropped flowers like in the picture below. Keep in mind that other factors, such as uneven soil moisture and inadequate fertility, can also result in aborted and dropped flowers. When temperatures are high, several plant mechanisms are altered, leading to flower abortion and drop, including an increase in the hormone abscisic acid and abnormal flower development, which affects pollination and fruit set. In addition to seeing fewer fruits per plant, fruit quality is also affected.

Aborted and dropped flowers on a tomato plant. Photo: Elsa Sánchez

At temperatures above 84˚F, fruit weight per plant and the number of seeds per fruit will likely be lower than when temperatures are within the optimal range. You also may notice more uneven ripening and softer fruit. This can translate into more culled fruit, shorter shelf life, changes in flavor, and difficulty transporting fruit to market.

Fruits exposed to high heat can develop disorders, including tomato pox, and high heat with direct sunlight can lead to sunscald.

You may also notice that fruits ripen quicker than expected. We’ve been hearing from farmers that they’ve been seeing earlier-than-expected ripening on their tomatoes and other crops.

It’s tough to address heat stress in the field. Start by looking for tomato cultivars with heat tolerance that fit your markets before the growing season begins. Use appropriate plant spacings to ensure good air circulation in the planting. This can help reduce temperature and humidity levels around the plants. Recommended spacings for tomatoes are 18 to 24 inches between plants in a row and a minimum of 5–6 feet between rows for staked culture; 24 inches between plants in a row and a minimum of 4–5 feet between rows for determinate tomatoes without stakes; and 24 to 36 inches between plants in a row and a minimum of 5–6 feet between rows for indeterminate tomatoes without stakes. It’s also essential to keep your plants well-watered. Installing shade cloth over the plants is another option.

References

Hazra, P., S.H. Ansary, D. Sikder, and K.V. Peter. Breeding tomato (Lycopersicon esculentum Mill.) resistant to high temperature stress. International Journal of Plant Breeding, 2007, 31-40.

Mulholland, B.J., R.N. Edmondson, M. Fussell, J. Basham, and L.C. Ho. Effects of high temperature on tomato summer fruit quality. The Journal of Horticultural Science and Biotechnology, 2015, 365-373.

Alsamir, M., T. Mahmood, R. Trethowan, N. Ahmad. An overview of heat stress in tomato (Solanum lycopersicum L.). Saudi Journal of Biological Sciences, 2021, 1654-1663.