Tomato Versus The Volcano

Harvesting a healthy tomato crop in the fall is the Holy Grail for Prairie Gardeners. To get to that end, we have to navigate all the great obstacles that nature puts in our way, including extreme heat!

Extended periods of high heat (above 30C) and sun can wreak havoc on your tomatoes. 

In June, most tomatoes are in Stages 1 and 2 of development. If greenhouse temperatures exceed 30C during Stage 1, fertilization will fail and you will get less fruit set. If temperatures exceed 30C during Stage 2, the fruit can develop physiological disorders like cracking, green shoulder, sunscald, and gold spot. 

Heat stress drives up your plant’s respiration rate and reduces photosynthesis. High temperatures and direct sunlight impact chromoplasts in cells (where lycopene is stored). Lycopene is what gives tomatoes their red colour. Between 30C and 40C, the sun and heat will convert the lycopene into β-carotene (yellow colour). 

Temperatures above 40C are lethal to plant cells and the exposed tissue will die, turn white, dry out and flake away like parchment.

Fruits that are directly touched by extended strong sunlight with no shade can be more than 10 °C higher than the surrounding air. The extent of damage to the fruit depends on the spectral quality of the light, temperature, and length of exposure. 


Many of the physiological disorders discussed here can be avoided by shading your greenhouse and/or fruit in the summertime. This can be done by installing a shade cloth or promoting the upper leaf development of your plants to shade the developing fruit below. Shading the fruit reduces its temperature, which promotes cell division and growth, and reduces cracking, yellow shoulder, and sunscald. 

Make sure your plant also has enough calcium and magnesium. It will prevent blossom end rot and make your fruit more elastic and less prone to cracking.

Growing tomatoes is incredibly challenging. Take some of these steps to get through the hot season and give your plants enough time to mature to harvestable size. And feel free to boast about your success!