Blossom End Rot: Why Your Tomatoes Turn Black Underneath

It's one of the most demoralising sights in the summer garden: your first proper flush of tomatoes ripening nicely, and then you turn one over. The bottom — the blossom end — has sunk into a dark, leathery, rotten-looking patch. This is blossom end rot, and the first thing to know is that it isn't a disease. Nothing is infecting your plants, nothing will spread from fruit to fruit, and the fix is free. But the folklore around it wastes more gardeners' money than almost any other tomato problem, so let's take it apart properly.

What blossom end rot looks like

It starts as a small, water-soaked spot at the blossom end of the fruit — the end opposite the stalk — usually while the tomato is still green and about half grown. The spot darkens, sinks, and turns tough and leathery, sometimes covering a third of the fruit. Secondary moulds may move into the damaged tissue and turn it black and fuzzy, which is what makes it look like a disease. Tomatoes are the classic victim, but peppers, aubergines, and courgettes and other summer squash get their own version too. Paste and plum tomatoes are notoriously the most susceptible.

The real cause: a calcium delivery failure

The damaged tissue is genuinely short of calcium — that part of the old advice is true. Calcium is the mortar plants use to build cell walls, and a developing fruit that can't get enough of it literally falls apart at the far end. The catch: in the vast majority of gardens, the soil contains plenty of calcium. The shortage is in delivery, not supply. Calcium only moves through the plant dissolved in water, carried by the transpiration stream, and it moves preferentially to the leaves, which transpire far more than fruit. Any interruption in water flow — a dry weekend, a flooded week, damaged roots — and the fruit is the first place calcium stops arriving. The blossom end, at the very end of the supply line, collapses.

What actually triggers it

• Uneven watering — the classic cause. Drought followed by soaking creates exactly the stop-start flow that starves fruit of calcium.
• Container growing: pots dry out fast and swing between wet and dry, which is why blossom end rot hits container and grow-bag tomatoes hardest.
• Too much nitrogen: heavy feeding pushes lush leaf growth, and those leaves outcompete the fruit for calcium.
• Root damage from hoeing or transplant shock — fewer working roots means less water uptake.
• A first heavy fruit set in early-to-mid summer, when demand suddenly spikes — which is why the first trusses of the season are hit most often.

Do calcium sprays and eggshells work?

The fix, step by step

• Remove affected fruit; it won't recover, and the plant will redirect its energy.
• Water deeply and on a schedule — vegetable beds want roughly 2.5–4 cm of water a week, from rain or you, delivered evenly rather than in occasional floods.
• Mulch 5 cm deep around each plant with straw, compost, or grass clippings to buffer soil moisture.
• Check containers daily in hot weather; in a heatwave a grow bag may need water morning and evening.
• Ease off high-nitrogen feeds; switch to a balanced tomato fertiliser once fruit sets.
• Don't hoe deeply near the root zone — tomato feeder roots sit close to the surface.

Pro tip: later trusses on the same plant usually escape entirely once watering steadies — the plant hasn't 'caught' anything, and a bad first flush says nothing about the rest of your season.

Warning: a soil test is worth doing before you add any calcium product. Only genuinely calcium-poor or very acidic soils benefit from lime or gypsum, and unnecessary additions can lock up other nutrients.

Can you still eat them?

Yes. Blossom end rot is physiological damage, not an infection — cut away the leathery patch generously and the rest of the tomato is fine to eat, as long as secondary mould hasn't moved in. If the flesh under the patch smells off or has gone soft and grey, compost it.

If you're not certain whether you're looking at blossom end rot or something nastier like late blight, photograph the fruit in PlantMe — the app's diagnosis engine covers 500+ plant diseases and disorders and will tell you in seconds whether it's a watering problem or a fungus that needs urgent action.