Tomatoes & Heat Stress : Climate Smart Farming



Tomatoes like a warm climate and a sunny location, but they do not like extremes. They suffer from heat stress, low humidity and sunburn and also from heavy rainfall, high humidity and waterlogging. Tomatoes can grow in a temperature range from 10 to 35°C but grow best between 20 and 30°C. In a heatwave plants will transpire more, blossoms won’t open and pollen will be destroyed, leaves will curl and fruit will not set until temperatures drop.

Climate predictions for south eastern Australia include more days above 35°C, more and longer heatwaves, longer dry periods with low humidity and also stronger winds and heavier rainfall events. It is the extreme events – heat waves, intense dry periods, heavy rainfall events, and hot drying winds, rather than average temperature increases, that will have the worst impact. To continue to grow tomatoes well we need to manage these extremes and to understand and reduce their impacts.

Understanding the impacts:

Roots: Most tomato roots feed in the top 10-20cm of the soil. Their preferred upper soil temperatures are about 25-27°C. When soil temperatures get above 35°C, roots will not spread and feed as widely and when they feed, they will be less able to take up the nutrients needed by the plants.

Fruits: Tomatoes produce fruit best between 20 and 30°C. At above 30°C fruit set is reduced and the fruits start to have problems ripening. Too much direct sunlight in hot weather will scald both green and ripening fruit.


Sun scald of tomatoes.

Photo Credit University of Maryland Extension


Pollination: Once there is a run of daytime temperatures above about 32°C, tomato blossoms don’t open and pollination is prevented.

Shoots and leaves: On hot days, the plants start transpiring more and need more water. When temperatures rise above about 29-30°C, leaves and upper shoots start to wilt.


Heat stress of tomato plants.

Photo Credit istockphoto


Too dry or too wet: Too much humidity can lead to fungal diseases and too little restricts pollination and can encourage pests such as Two spotted mites. Tomatoes prefer humidity between 40 and 70%.

Suggestions for reducing impacts:

Plant smart: Afternoon sun on a hot summer day is more likely to harm than help tomato crops. Consider mixed level plantings and permaculture designs. Plant in niches to suit the plant and the season. Make sure you have plenty of niches so you can rotate a variety of plants through each niche to reduce disease risk.

Shade the plant: particularly from afternoon sun in mid summer: use trees and vines, shade structures and buildings.

Design your plantings for your local conditions. For example:

  • If it is often dry you may need more plants around to increase the vertical layers of plants in your garden for shade and habitat.
  • If you live in a more humid area your tomato plants will need more space and good air flow.
  • Moist sea breezes can improve humidity and airflow on a summer afternoon as long as they are not so strong as to damage the plants.
  • Hot drying winds will increase heat and water stress and decrease humidity.

Care for the soil: Shade the soil by maintaining a leafy canopy and using a good cover of light coloured mulches but keep mulches 2-5cm away from the stem of the plants.

Ensure good drainage: Build soil organic matter, avoid areas with plough pans and heavy clay layers, plant in raised beds.

Care for the plants: Observe your plants and their environment. Are they wilting? Is the soil moist? How healthy are the leaves and the fruit? What weather are you expecting today and over the next few days?

Increase watering in hot weather. If you can, use drip line buried under mulch or in the topsoil, do not let water spray on shoots or leaves. Try to avoid water droplets bouncing off bare soil onto leaves or fruit. If it is very hot, you may need to water twice a day. Try to make sure that water is keeping the root zone moist, but not waterlogging the plant.

Keep water, mulch and other materials away from plant shoots, leaves and fruit to avoid fungal and pest attack. Remove diseased and damaged leaves, fruit and shoots.

If hot temperatures, heat waves or heavy rainfalls are predicted, harvest all fruit that is starting to colour.

Some tomato varieties are more heat tolerant than others, but before buying these, find out what varieties do well in your area and what seeds are available locally. Also consider the needs of different varieties, for example, some heat tolerant varieties need more water.


Please send suggestions and questions to Louise Rose (

More reading

Giri, Anju & Heckathorn, Scott & Mishra, Sasmita & Krause, Charles. (2017). Heat Stress Decreases Levels of Nutrient-Uptake and Assimilation Proteins in Tomato Roots. Plants. 6. 6. 10.3390/plants6010006, available at

SCPA wishes to acknowledge and pay our respects to the Traditional Custodians and Elders of this land.