Why are my tomatoes not ripening?

Ripening is driven by ethylene and temperature, in a fairly narrow band around 20-25°C. Direct sun is not what ripens a tomato — this is why removing leaves to 'let the sun in' does not work and often makes things worse by exposing fruit to scald.

What you'll need

• Secateurs (optional)
• A paper bag and a banana (optional)

Step by step

1. Check the temperature range rather than the light.Ripening slows below 10°C and stalls above about 30°C. A hot greenhouse in a heatwave stops ripening entirely.
2. Stop feeding with high-nitrogen fertiliser.Nitrogen pushes leafy growth at the expense of fruit. Switch to high-potassium tomato feed once fruit has set.
3. Reduce the number of trusses late in the season.Remove flowers and very small fruit that will not have time. The plant then concentrates on what can actually ripen.
4. Pinch out the growing tip.Once you have four or five trusses on a cordon variety, remove the top. It stops the plant investing in new growth.
5. Remove leaves shading the fruit only lightly.A few lower leaves improve airflow and reduce disease. Stripping the plant is harmful — leaves make the sugars that ripen the fruit.
6. Reduce watering slightly as fruit matures.Slight stress concentrates flavour and encourages ripening. Not drought — just less than during the growing phase.
7. Ripen off the plant at the end of the season.Pick the whole truss and keep indoors at room temperature. A banana in a paper bag with them releases ethylene and accelerates it markedly.
8. Use the ones that will not ripen.Green tomato chutney, fried green tomatoes, or roasted. A frost-threatened crop is worth harvesting whole.

Tips

• Different varieties ripen at very different rates. If a variety consistently runs out of season where you are, choose an earlier one next year.
• Do not put unripe tomatoes on a sunny windowsill — the heat and light cause uneven ripening and tough skins. A shaded room is better.
• Never refrigerate a tomato you still want flavour from. Below about 12°C the flavour compounds are permanently damaged.

Common mistakes

• Stripping leaves to let sunlight in — Sunlight does not ripen tomatoes, temperature does. Removing leaves reduces the sugars the plant can make and exposes fruit to scald.
• Continuing high-nitrogen feed — It drives leaf growth instead of fruit development. Switch to potassium-rich feed once fruit has set.
• Leaving every truss on a late plant — The plant divides its energy between fruit that cannot possibly ripen. Removing the surplus concentrates it.

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