Why have my tomatoes stopped ripening in hot weather?
Above roughly thirty degrees a tomato stops making the pigment that turns it red, so fruit sits green or blotchy orange for weeks. Cooling the plant, not feeding it, is what restarts it.
- Difficulty
- beginner
- Time
- 12 min
- Read
- 5 min
Short answer
Tomatoes ripen best in the low to mid twenties. Above about thirty the plant stops producing lycopene, the red pigment, while still producing the yellow and orange ones — so fruit stalls, or ripens to a blotchy orange with yellow shoulders. Shade and ventilate to bring the temperature down, or pick at the first colour change and ripen the fruit indoors.
This is one of the most misdiagnosed problems in a British greenhouse. Everything looks healthy, the fruit is full size, and it simply will not turn. People feed, water more, remove leaves and worry about disease, when the plant is doing exactly what its biochemistry requires at that temperature. The fix is temperature, and if you cannot change the temperature you can change where the fruit ripens.
Step by step
- Confirm it is heat and not immaturity.Fruit that has reached full size and is still hard green, with no colour change at all after weeks, on plants that are otherwise healthy in a hot greenhouse, is the classic pattern. Fruit still growing is simply not ready.
- Check the temperature where the fruit actually is.A greenhouse thermometer at head height under the ridge reads considerably higher than the fruit trusses lower down, and a max-min thermometer beside the trusses tells you what the fruit is experiencing. This is worth knowing before changing anything.
- Get the temperature down.Open every vent and the door early, before the sun is on the glass. Get shading on the outside of the glass. Damp down the floor in the morning and again at midday to raise humidity and lower the air temperature. The greenhouse heatwave guide covers the full routine.
- Do not strip the leaves to let sun onto the fruit.This is a widespread and counterproductive habit. Tomatoes do not need light on the fruit to ripen — they need warmth within the right range, and the ripening is triggered internally. Removing leaves exposes fruit to direct sun, which causes sunscald: a pale, sunken, leathery patch on the shoulder that never ripens. Leaves also shade and cool the fruit.
- Remove only the leaves that are genuinely in the way.Taking off the lower leaves that are yellowing or touching the ground improves airflow and reduces disease, and that is worth doing. Defoliating the plant around the trusses is not.
- Pick at the breaker stage and ripen indoors.Once a tomato shows the first blush of colour at the blossom end, it has stopped drawing anything from the plant and will ripen perfectly well off it. Picking at that point and finishing them in a bowl indoors at normal room temperature is the reliable answer in a hot spell, and it takes the load off the plant.
- Never ripen tomatoes in the fridge.Below about twelve degrees the ripening process stops and the texture and flavour are permanently damaged. A cool room, not a cold one, and out of direct sunlight — a sunny windowsill overheats them and repeats the original problem.
- Reduce the load on the plant.Removing some of the smallest and latest trusses concentrates the plant on ripening what is already there. Late in the season, pinching out the growing tip stops the plant starting fruit it will never finish.
- Keep watering steady rather than increasing it.Erratic watering causes splitting and blossom end rot, which are separate problems with their own guides. It does not cause the ripening stall, and watering more will not fix it.
Tips
- A ripe tomato in the fridge loses flavour compounds within days. Keep them in the fruit bowl and refrigerate only if they are about to spoil.
- A banana or a ripe tomato in the bowl releases ethylene and speeds the others along noticeably.
- Blotchy ripening with hard yellow shoulders is the same heat mechanism, sometimes with a potassium shortage on top. A high-potassium tomato feed is worth using through the season, but it will not overcome a heat stall.
- Outdoor tomatoes in a British summer rarely hit this problem — it is overwhelmingly a greenhouse and polytunnel issue where the air is not moving.
- At the end of the season, green fruit ripens indoors over weeks in a drawer, or makes chutney. Do not leave it on the plant hoping for late sun.
Common mistakes
- Removing leaves to let the sun get to the fruit — Ripening is triggered by temperature, not by light on the fruit. Stripping leaves causes sunscald and removes the shade that was keeping the fruit within its ripening range.
- Feeding more to push ripening — The stall is a temperature effect. Extra feed does nothing for it and excess nitrogen actively pushes the plant into leaf rather than fruit.
- Leaving the greenhouse shut to keep it warm — The problem is that it is too hot, not too cold. In a British heatwave the greenhouse needs every vent open from early morning.
- Putting picked tomatoes in the fridge to keep them — Cold stops ripening permanently and destroys texture and flavour. Ripen at room temperature out of direct sun.
Questions people ask
What temperature do tomatoes ripen at?
Best in roughly the low to mid twenties Celsius. Below about ten to twelve degrees ripening effectively stops, and above about thirty the plant stops making lycopene, the red pigment, so fruit stays green or turns a blotchy orange. The window is narrower than most people assume.
Will they ripen if I pick them green?
Fully grown green fruit will ripen indoors, though a mature green tomato picked before the breaker stage never develops quite the same flavour. Once there is any colour at the blossom end, it will ripen just as well off the plant as on it.
Why are the shoulders staying yellow and hard?
Greenback and blotchy ripening, driven mainly by high temperature and strong direct sunlight on the fruit, sometimes with low potassium contributing. Shading the greenhouse and keeping leaf cover over the trusses is the main answer, alongside a high-potassium feed through the season.