Tomato problems and how to fix them (UK)

By Sophie Sparkes · Published August 2026 · 8 min read

Tomatoes are the crop most British gardeners try first, and the one that generates the most worried searching in late summer. The good news: most tomato problems look far more alarming than they are, and several of the scariest-looking ones aren't diseases at all. Here are the seven that actually come up, in the order people ask about them.

🍅 Why won't my tomatoes turn red?

This is the number one tomato question in Britain, and the answer surprises people: if your tomatoes have stopped ripening in a heatwave, it's because it's too hot — not too cold.

Tomatoes turn red by producing lycopene, and lycopene synthesis works best between roughly 10°C and 27°C. Above about 30°C it shuts down almost entirely. So a greenhouse baking at 35°C in August is actively preventing the thing you're waiting for. Fruit sits there stubbornly green while you assume it needs more sun, more heat, more feed — when what it needs is ventilation.

The other common cause is simply time. Tomatoes take around 6–8 weeks from the fruit setting to ripening, and if flowers set late in a cool British summer, the maths may just not work out before autumn.

What actually helps

• Ventilate. Open greenhouse doors and vents on hot days, and shade the glass if it's persistently above 30°C inside.
• Ease off the feed. High-nitrogen feeding pushes leafy growth at the expense of ripening. Once fruit has set, a high-potash tomato feed is what you want.
• Stop chasing more fruit. In late August, pinch out the growing tip and any new flowers. The plant then puts its energy into the fruit it already has instead of starting fruit it can't finish.
• Pick at "breaker" stage. As soon as a tomato shows the first blush of colour, it has taken everything it will ever take from the plant. It'll ripen just as well on a windowsill — and picking it takes the load off the ones behind it.

Curling leaves

Alarming, extremely common, and usually nothing to worry about. If the lower leaves are rolling upward and inward, keeping their normal colour and thickness, that's physiological leaf roll — the plant's response to heat, wind or irregular watering. It doesn't reduce your crop and it needs no treatment beyond steadier watering.

Worry only if the curling comes with distortion: new growth that is twisted, fern-like, unusually narrow or oddly pale. That points to either a virus or, more often in gardens, weed-killer drift — including from contaminated manure or compost. There is no cure for either; affected plants should come out.

Yellowing leaves

A few yellow leaves at the very bottom of the plant are normal. The plant is withdrawing nutrients from its oldest leaves and sending them to the fruit, which is exactly what you want it to do. Remove them and move on.

Widespread yellowing is a feeding problem. Yellowing between the veins on older leaves, with the veins staying green, is classic magnesium deficiency — very common in tomatoes fed heavily with high-potash feed, because potassium competes with magnesium uptake. A foliar spray of Epsom salts (magnesium sulfate) at roughly a tablespoon per 4.5 litres of water fixes it quickly.

If the yellowing doesn't fit either pattern, our general guide to why plant leaves turn yellow works through the other causes.

Brown patches on leaves — is it blight?

Late blight (Phytophthora infestans) is the one tomato problem that genuinely deserves the worry, and late August into September is exactly when it arrives in the UK.

What it looks like: irregular brown or grey-brown patches on the leaves, often with a paler halo and sometimes a faint white fuzz underneath in damp weather. Dark lesions follow on the stems, and the fruit develops firm brown leathery patches that spread fast. From first spots to a collapsed plant can be under a week.

Blight needs warmth and prolonged humidity — the classic trigger is two consecutive days with minimum temperatures of 10°C or above and at least six hours of very high humidity. That's a normal British late summer, which is why outdoor tomatoes suffer far more than greenhouse ones.

Not every brown mark is blight — see why plant leaves turn brown for the more mundane causes.

Splitting fruit

Tomatoes split when they take up water faster than the skin can stretch — typically a dry spell followed by heavy watering or heavy rain. The fruit swells, the skin can't keep up, and it cracks.

The fix is consistency rather than quantity: water little and often rather than letting pots dry out and then drenching them, and mulch the surface to even out the moisture. Split tomatoes are perfectly edible — pick them straight away and use them within a day or two, because open splits let rot in fast.

Black, sunken patches on the bottom of the fruit

This is blossom end rot, and it's the problem most often misdiagnosed. It looks like a disease — a dark, flat, leathery patch on the base of the fruit — but nothing has infected the plant.

It's a calcium delivery failure. Calcium moves through the plant in water, so when watering is erratic the fastest-growing tissue — the far end of a developing fruit — gets starved of it, even in soil with plenty of calcium. That's why it's so common in grow bags, containers and hanging baskets, which dry out fastest.

So adding calcium usually doesn't help. Consistent watering does. Remove affected fruit, get the watering steady, and the fruit that sets afterwards will be fine.

Lots of flowers but no fruit

Tomatoes are self-fertile — each flower can pollinate itself — but the pollen needs movement to shift. Outdoors, wind and insects do it. In a still greenhouse, nothing does, and the flowers drop.

Tap or gently shake the flower trusses every couple of days when they're open, ideally around midday. Keep the greenhouse ventilated: pollen becomes unviable above roughly 32°C, and very humid air makes it clump rather than fall. And check your feed — too much nitrogen produces a magnificent leafy plant that never gets round to fruiting.

Purple-tinged leaves

Usually a phosphorus problem, and usually early in the season rather than now. Cold roots can't take phosphorus up efficiently, so young plants in a chilly spring go purple on the undersides of the leaves and along the veins. It normally corrects itself as the soil warms. If it persists into summer, feed and check the plant isn't sitting in cold, waterlogged compost.

Tomato questions

Should I remove leaves to help tomatoes ripen?

Removing the leaves below the lowest ripening truss improves airflow and makes picking easier, and that's worth doing. But stripping leaves does not make fruit ripen faster — ripening is driven by temperature and time, not sunlight on the fruit. Take too many off and you remove the leaves feeding the plant, which slows everything down.

Can I still eat tomatoes that have split?

Yes, as long as the split is clean and fresh with no mould or souring. Pick them immediately and use them within a day or two. Cut out the cracked section if it's started to discolour.

What do I do with green tomatoes at the end of the season?

Pick them all before the first frost. Ripen them indoors in a drawer or paper bag with a banana or a ripe tomato, which releases ethylene. Fruit that has reached full size will usually ripen; very small immature fruit won't, and that's what chutney is for.

Is tomato blight contagious to other plants?

Yes — to potatoes especially, since it's the same organism as potato blight, and the two crops pass it back and forth. Don't grow them next to each other, and never compost blighted material.

Do Epsom salts help tomatoes?

Only for magnesium deficiency — the yellowing-between-green-veins pattern. They're often recommended as a general tonic or as a fix for blossom end rot, and for both of those they do nothing. Epsom salts supply magnesium, and blossom end rot is a calcium and watering problem.

Why are my tomatoes small?

Most often overcrowding on the truss, under-feeding once fruiting starts, or simply a variety that produces small fruit. Thin the truss to five or six fruits on large varieties, feed weekly with a high-potash tomato feed once the first fruits have set, and don't let the plant dry out between waterings.