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Tomatoes Not Turning Red? Heat Is Usually the Reason

Above about 30°C tomatoes stop making lycopene and stall green or orange. Ripening is temperature-driven, not sun-driven — so don't strip the leaves.

By Zakaria 3 min read Save to Pinterest — opens Pinterest in a new tab
Green tomatoes vine
Photo: HBH-MEDIA-photography, Pixabay Content License, via Pixabay
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Usually heat. Above about 30°C a tomato stops making the pigment that turns it red, while ripening in every other way.

Working out which cause you have

Match the symptom to the cause
What you seeLikely cause
Orange or yellowish, soft, won’t redden, hot weatherHeat. Over 30°C stops lycopene
Green and hard for weeks, cool weatherToo cold — under ~10°C ripening stalls
Yellow or green shoulders on ripe fruitPotassium shortage, or heat on the shoulder
Whole truss stuck, plant loaded with fruitToo much fruit. The plant is rationing
Patchy, blotchy ripeningUneven water, or potassium
Pale leathery patch on the sun sideSunscald — usually after over-pruning leaves

The myth worth killing

“Strip the leaves so the sun gets to the fruit.”

Ripening is a chemical process run by temperature and ethylene. A tomato ripens in a closed paper bag with no light at all — which is the whole basis of the end-of-season method below.

Meanwhile the leaves are the factory producing the sugars that make the fruit taste of anything, and bare fruit in strong sun gets sunscald: pale, leathery, ruined patches.

Remove only leaves that have a problem

Diseased, yellowing, or touching the soil — those go, for airflow and disease. Healthy leaves shading fruit are doing their job.

What to do in a heatwave

You cannot make the plant produce lycopene above its ceiling, so the options are to lower the temperature or stop waiting:

  • Shade cloth at 30–40 per cent over the plants through the worst of it
  • Water consistently — dry roots make every stress worse
  • Pick at the breaker stage (first blush of colour) and finish them indoors, where it is cooler than the greenhouse
  • Feed with potassium if the shoulders are yellowing and the plant has had nitrogen-heavy feed

End of season

  1. Pick anything at mature green or beyond. Full size, skin slightly glossy, seeds sliding away from the knife when cut rather than sliced
  2. Leave the small hard immature fruit — it will not ripen properly. Chutney, or fried green
  3. Indoors at 18–21°C, out of direct sun. A spare room beats a windowsill
  4. A paper bag or closed box with a banana or apple speeds it up, both being ethylene sources
  5. Check every couple of days and remove anything rotting before it spreads
  6. Never the fridge. Below about 10°C the ripening mechanism is permanently damaged and the fruit ends up mealy and tasteless

A whole plant hung upside down in a frost-free shed works too, and is mostly a convenience — the fruit is no longer drawing anything from the plant at that stage.

Common questions

Why are my tomatoes not turning red?

Heat is the most common reason. Tomatoes produce lycopene, the pigment that makes them red, within a temperature band of roughly 10 to 30°C, and above about 30°C production shuts down while the fruit keeps ripening in other respects. The result is fruit that softens and goes orange or yellowish but never reddens. The other common causes are too little potassium, very heavy leaf cover combined with cool weather, and simply having too much fruit on a plant for it to ripen at once.

Does removing leaves help tomatoes ripen?

Almost never, and it usually makes things worse. Ripening is driven by temperature and the plant's internal ethylene, not by sunlight falling on the fruit — a tomato in a dark drawer ripens perfectly well. Leaves are what produce the sugars that make fruit taste of anything, and stripping them exposes fruit to sunscald. Remove only leaves that are diseased, yellow, or lying on the soil.

How do I ripen green tomatoes at the end of the season?

Pick them at the mature green or breaker stage and bring them indoors to a spot at about 18 to 21°C, out of direct sun. A paper bag or a closed box with a ripe banana or apple speeds it up, because both release ethylene. Do not refrigerate unripe tomatoes — below about 10°C the ripening process is damaged permanently and the fruit never develops flavour. Fully green immature fruit will not ripen properly and is better used for chutney or fried green tomatoes.

What is the mature green stage?

The point at which a tomato has reached full size, the skin has changed from matt to slightly glossy, and the jelly around the seeds has set — cutting one open shows seeds that slide away from the knife rather than being sliced through. Fruit at that stage will ripen fully off the plant with no loss of quality, which is what makes end-of-season picking worthwhile. Anything smaller and harder will not.

Will tomatoes ripen after the plant is pulled up?

Yes. Hanging a whole plant upside down in a frost-free shed or garage is a traditional end-of-season method and works reasonably well for fruit at or near the mature green stage. The fruit draws on nothing from the plant at that point, so it is really just a convenient way to store and watch them. Picking the fruit and ripening it in boxes indoors is tidier and makes it easier to remove any that rot.

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