Blossom End Rot on Tomatoes: Causes and How to Fix It

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The first time blossom end rot on tomatoes shows up in a garden, the usual reaction is panic, followed by spending money on calcium sprays. Both reactions are wrong. The black sunken patch on the bottom of an otherwise healthy-looking tomato is not a disease, is rarely caused by a soil calcium shortage, and almost never needs a special product to fix. Once you understand what is actually going on inside the plant, the fix is mostly free — and works.

This guide covers what blossom end rot actually is, why it usually is not a soil problem, the five real causes, what to do in the next two weeks, and the products and folk remedies you can safely ignore.

What Blossom End Rot Actually Is

Blossom end rot is a physiological disorder, not a disease. There is no fungus, no bacterium, no virus. The black sunken leathery patch on the bottom of the tomato — the end opposite the stem, where the flower originally was — is the visible result of cells in that part of the fruit dying because calcium failed to reach them while the fruit was sizing up.

Three facts shape the entire problem:

• Calcium moves with water through the plant. The roots take it up and the transpiration stream carries it to leaves and fruit. If transpiration breaks down for any reason, calcium delivery breaks down with it.
• The flower end of the fruit is the last stop on the calcium pathway. It is also the part that grows fastest in the first 7–14 days after fruit set. Demand is highest exactly where supply is most easily interrupted.
• Once a cell dies, it stays dead. The black patch will not heal. The rest of the fruit is fine; that cluster of cells is gone.

Which means: by the time you see the black spot, the cause already happened 10–20 days ago. Whatever you change now is for the next set of fruit, not the one in front of you.

Why It Usually Is NOT a Soil Calcium Deficiency

The most common bad advice is "your soil is low in calcium, add lime or gypsum." Across the vast majority of European garden soils this is wrong, for two reasons.

First, European garden soil is generally calcium-rich. Most soils outside specific peat-acidic areas (parts of the Netherlands, Scotland, Ireland, north-west Germany) sit between pH 6.5 and 7.5 and have far more calcium than a tomato plant will ever need. Adding lime to a calcium-rich soil can actually make the problem worse by tying up other nutrients.

Second, soil tests of beds where blossom end rot is severe almost always come back with adequate calcium. The problem is rarely supply at the root; it is delivery from the root to the fruit. Adding more to the soil cannot fix a transport problem inside the plant.

Add lime only if a proper soil test (any garden centre sells them; pH and base saturation) actually shows a low pH or low calcium reading. Otherwise, fix the transport, not the soil.

Five Causes That Really Trigger Blossom End Rot

In ranked order of how often they cause the problem in European home gardens:

1. Uneven watering. A 4-day dry spell followed by a heavy soak is the single most common cause. The plant shuts down transpiration during the drought, calcium delivery stops, fruit cells die. The water that arrives afterwards is too late.
2. High salt in the root zone. Over-feeding with tomato feed (or feeding a stressed plant the same as a healthy one) raises the salt level around the roots, which physically interferes with water uptake. The plant becomes a calcium-transport problem even when calcium is plentiful.
3. Damaged roots. Hoeing too close to the stem, transplanting too late, or compaction from foot traffic damages the fine root hairs that take up water and calcium first.
4. Cold soil at fruit set. The first trusses, set when soil is still under 14–15 °C, often show blossom end rot because the plant's root activity is slower than its leaf and fruit growth. Later trusses on the same plant are usually fine.
5. Container size and heat stress. Tomatoes in pots smaller than 20 litres are at constant risk. The pot dries out faster than the plant can adjust; a single hot afternoon does it.

Almost every case is one of the first three. The fix for the first three is the same: rhythm and protection.

Step 1: Fix the Watering Rhythm First

The single highest-leverage change is consistency. A tomato plant wants the same amount of water on roughly the same schedule, never the boom-bust of "I forgot for three days, now I'll catch up."

A working summer routine:

• Ground tomatoes: 8–10 litres per plant every 2–3 days in normal weather; daily in a heat wave once fruit is setting.
• Greenhouse / polytunnel tomatoes: 5–8 litres per plant per day in peak summer.
• Container tomatoes (20 L+): water until water runs out the bottom, daily, no exceptions. In 30 °C heat, twice a day.

Always water at the base, never on the leaves. Always in the morning, never the evening — wet leaves overnight invite blight and powdery mildew. For more detail on watering technique across crops, see our garden watering guide.

Step 2: Mulch and Protect the Roots

Mulch is the second-highest-leverage change. A 5–8 cm layer of straw, grass clippings, or compost around the base of each plant (not touching the stem) does three things at once:

• Evens out soil moisture between waterings — the swing from wet to dry shrinks dramatically.
• Keeps the surface soil cool, which keeps the fine roots alive and active during heat.
• Reduces splash-up of soil-borne fungal spores during heavy rain.

Mulch the moment the plants are tied to their stakes after transplanting, not in July. Adding mulch after the problem has started is still useful, but the benefit takes 7–10 days to show in the next set of fruit.

In Mediterranean Csa zones, mulch is non-negotiable. In Atlantic Cfb zones, a thinner layer is enough. Our mulching guide covers material choices and how thick to apply by climate.

Step 3: Check Container Size and Adjust Feeding

For container tomatoes, blossom end rot is almost always pot-size or feeding-driven. Two fixes apply:

• Go up a pot size. A 20-litre container is the minimum for a single indeterminate tomato. 30 litres is comfortable. Pots below 15 litres will give you blossom end rot in any year with one hot afternoon.
• Feed half-strength, twice as often. A weekly full-dose tomato feed in a small pot builds up salts in the root zone and triggers the disorder. Half-strength feed every 3–4 days delivers the same total but never spikes the salt level.

For ground tomatoes, over-feeding (especially with high-nitrogen feeds) can also push the plant towards leaf growth and away from balanced fruit growth. Stick to a balanced tomato feed once a week from first flowering, and use compost or comfrey tea as the everyday top-up. Our natural fertilising guide covers the rotation we recommend.

Step 4: Remove Affected Fruit, Save the Plant

The fruit that already has the black patch will not recover. Picking it off has nothing to do with disease control (there is no disease to spread) — it lets the plant put energy into the next truss instead of finishing a fruit that has lost its bottom third.

Pick affected fruit, compost it (it is not infectious), and turn attention to the next truss. The next set of fruit should be clean within 14 days if the watering, mulch, and feed changes have been made.

A plant that produces one truss with blossom end rot and then clean fruit afterwards is normal — it usually means the conditions early in the season caused it and have now improved. A plant where every truss has the problem is a plant in a too-small pot, in compacted soil, or on a feast-and-famine watering schedule.

For broader tomato care — pruning, staking, watering, and feeding — our how to grow tomatoes in Europe guide covers the season-long picture, and the tomato pruning guide covers the side-shoot decision that decides July yields.

Crops That Show the Same Problem

Blossom end rot is a tomato issue most famously, but it appears on other fruiting crops by the same mechanism and with the same fix:

• Peppers and chillies — dark sunken patch at the blossom end, especially on bell-pepper types.
• Aubergines — similar dark patch; less common but follows the same drought-rebound trigger.
• Courgette and summer squash — the small fruit goes yellow and rots from the flower end; often misdiagnosed as poor pollination.
• Watermelon and pumpkin — black sunken patch on the underside of the fruit, where it rests on the ground.

The fix on every one of these is the same: consistent watering, mulch, restraint with feeding, and patience.

What Does NOT Fix Blossom End Rot

A few products and folk remedies survive on the internet despite no measurable effect. Skip these:

• Calcium sprays on leaves. Tomato leaves barely absorb calcium through the leaf surface, and even if they did, the calcium would not be redirected to the fruit. The leaf calcium pathway is one-way upward, not down to the fruit.
• Eggshells in the planting hole. Eggshells release calcium so slowly (over years) that they cannot affect a fruit forming this week. Useful for very long-term soil building, useless for this season's tomatoes.
• Powdered milk in the soil. No mechanism of action. Wastes milk.
• Epsom salts (magnesium sulphate). Adding more magnesium to a magnesium-rich soil can actually interfere with calcium uptake. Net negative.
• "Tomato rescue" or "blossom end rot" branded sprays. Mostly water with a tiny calcium concentration; expensive solutions to a problem that is not chemical.

If a product is sold as a fix for blossom end rot and does not also change watering, mulch, or pot size in your garden — it is selling you a story, not a fix.

Summary

Blossom end rot on tomatoes looks alarming but is one of the most fixable problems in the summer garden, once you understand it is not a disease and almost never a calcium shortage. The cells on the bottom of the fruit died because calcium delivery broke down 10–20 days ago. Fix the watering rhythm, mulch heavily, go up a pot size for containers, and feed half-strength more often. The next truss will come in clean.