Blossom End Rot: Causes and the Simple Fix

Blossom end rot is that dark, sunken, leathery patch on the bottom (the flower end) of a tomato, courgette or pepper. It happens when calcium fails to reach the developing fruit — and that is almost always down to erratic watering, not poor soil. The good news: it is not a disease, it does not spread, and the fix is simply watering more steadily.

If you have just spotted it on your first ripening fruit, take a breath. This is one of the most common problems UK growers hit in their first season, and it is genuinely easy to sort out. Below is what it looks like, the real causes ranked by likelihood, how to tell it apart from actual diseases, and how to stop it coming back.

What it looks like

Blossom end rot starts as a small, watery or pale patch right at the base of the fruit — the end furthest from the stalk, where the flower originally was. Over a few days it grows, darkens to brown or black, and turns sunken, dry and leathery, almost like tanned hide.

A few tell-tale signs:

• It is always at the blossom end (the bottom), never near the stalk.
• It affects individual fruits, not the leaves or stems.
• The patch is flat and leathery, not fuzzy or mouldy.
• It often hits the first flush of fruit in early summer, then stops once the plant settles.

On tomatoes it is unmistakable — a flat brown crater on the underside. On courgettes you usually see the young fruit go soft, brown and shrivelled at the tip end, sometimes rotting before it has had a chance to swell. Peppers and aubergines get it too, in the same spot.

Most likely causes (ranked)

The underlying cause is always the same — calcium not reaching the fruit in time. But calcium moves through the plant dissolved in water, so anything that disrupts the water supply is what actually triggers it. Here are the culprits, most common first.

1. Inconsistent watering (the big one)

This is behind the vast majority of cases. When the compost swings between bone-dry and soaked, the plant cannot move calcium steadily to the fruit, and the blossom end is the first part to miss out because it is furthest from the stem.

Containers, growbags and hanging baskets are by far the worst affected. A growbag holds very little compost and dries out astonishingly fast in warm weather — sometimes twice a day at the height of summer. Plants in the open ground, with a big reservoir of soil around the roots, suffer far less.

The fix: water little and often enough that the compost stays evenly damp — never dust-dry, never waterlogged. In summer that often means a good soak every morning for pots and growbags, and a check again in the evening on hot days. Our guide to watering and feeding tomatoes walks through a routine that keeps moisture steady.

2. Letting plants dry out, then drenching them

A close cousin of the above, and worth calling out on its own because it catches people who think they water enough. If you let a plant wilt and then give it a huge drink to make up for it, that boom-and-bust cycle is exactly what causes the calcium to fail to arrive. The damage is often done during a hot spell or after a few days away.

The fix: aim for consistency over volume. A moderate amount every day beats a flood twice a week. If you are going away, set up a self-watering reservoir or ask a neighbour — growbags will not forgive a missed weekend in July.

3. Too much nitrogen feed

Overdoing a high-nitrogen feed pushes lush, fast leafy growth. The plant races ahead, the fruit grows quickly, and the calcium supply simply cannot keep pace with that speed. You sometimes see this on plants that look gorgeously green and vigorous but still rot at the fruit base.

The fix: once the first flowers set, switch to a high-potash tomato feed (the standard tomato fertiliser sold in every UK garden centre) rather than a general-purpose or high-nitrogen one. Follow the dilution on the bottle and do not be tempted to feed "extra strong" — more feed is not better here.

4. Damaged or restricted roots

If the roots have been damaged — by hoeing too close, by transplanting roughly, or by a pot that is simply too small and has dried into a hard root-bound lump — the plant cannot take up water and calcium properly even when there is plenty around.

The fix: give plants a generous pot (at least 30cm / 10 litres for a single tomato or courgette), avoid disturbing the soil close to the stem, and keep the root zone evenly moist so roots stay active. If you grow in containers, our guide to growing food in containers covers pot sizes and compost that hold moisture better.

How to tell it apart from disease

It is easy to panic and assume the worst, especially if you have heard of tomato blight. Here is how to be sure it really is blossom end rot and not something contagious.

• Location. Blossom end rot is always at the bottom of the fruit. Blight and most rots start anywhere — often on leaves and stems first. If your leaves are healthy and only the fruit bases are affected, it is blossom end rot.
• Texture. Blossom end rot is dry and leathery. Tomato blight produces brown, mushy, spreading patches with rapidly browning leaves, usually in warm, damp UK summers. Blight smells and spreads; blossom end rot does neither.
• Spread. Blossom end rot does not move from fruit to fruit or plant to plant. If you see one bad fruit and the rest are fine, that is reassuringly normal.
• Timing. It typically hits the earliest fruits and then eases off as the plant matures and your watering settles into a rhythm.

If the issue is yellowing foliage rather than rotting fruit, that is a different diagnosis — see why tomato leaves turn yellow instead.

How to prevent it

Prevention is almost entirely about keeping the water supply steady. Do these few things and you will rarely see it again.

Water consistently and deeply. The single most important habit. Keep the compost evenly moist at all times — check pots and growbags daily in summer, twice on hot days. A deep soak that wets the whole root ball beats a quick splash on the surface.

Mulch to hold moisture in. A layer of mulch (compost, grass clippings or bark spread over the soil surface) slows evaporation and smooths out the wet-dry swings that cause the trouble. It makes a real difference for plants in the ground and large pots through a UK summer.

Feed steadily, not heavily. Use a high-potash tomato feed once fruit has set, at the recommended strength. Resist the urge to over-feed with nitrogen-rich fertiliser, which drives growth faster than calcium can follow.

Use bigger containers and good compost. More compost means more of a moisture buffer, so the plant never lurches from drought to flood. A generous pot of decent peat-free multipurpose compost is far more forgiving than a thin growbag.

Don't disturb the roots. Plant carefully, keep your hoe away from the stem, and let the roots get on with the job of moving water and calcium where it is needed.

And yes — you can still eat them

Here is the reassuring part. Blossom end rot is cosmetic and physiological, not toxic. Simply cut away the affected end of an unripe tomato or pepper and the rest of the fruit is perfectly good to eat or cook with. Badly affected courgettes are usually best composted, but lightly affected ones can be trimmed and used.

So don't bin the whole crop. Tidy up your watering, mulch the surface, ease off the nitrogen, and the next flush of fruit will almost always come through clean. For the full picture on raising a healthy, productive crop from sowing to harvest, see our main guide to growing tomatoes — and if courgettes are your worry, the guide to growing courgettes covers keeping those thirsty plants happy through summer.