Blossom end rot in tomatoes: what causes it, and how I stopped it

By Joe, Founder of Dr Forest · June 2026 · Edited 28 Sep 2026 · 21 min read

The first truss sets, the fruit swell, and then a dark leathery patch creeps up from the bottom of each one. It looks like the plant is rotting from the floor up. It usually isn't, and the rest of the season is normally fine once you know what's going on.

Blossom end rot is a dark, sunken patch at the bottom of a tomato. It is not a disease. The fruit ran short of calcium because water flow was uneven. Garden soil and ordinary compost rarely lack calcium. Steady the watering, mulch, and ease off high-nitrogen and high-potash feeds first. Add calcium only if a soil test, or unbuffered coir, shows the supply is short. It's a physiological disorder, not a pest or infection — fungicides and pest sprays do nothing for it. Calcium travels in the water stream, and the fruit is a weak competitor against the leaves, so interrupted watering is the usual trigger.

First, the good news: this isn't a disease, and it won't spread to your other plants or fruit. The unaffected part of a marked tomato is still fine to eat. The most useful thing you can do today is even out your watering, so the soil never swings from bone dry to soaked. The rest of this guide explains why that works, and where calcium comes into it.

Get the watering right and the next trusses usually come through clean. In open ground the calcium is almost always already there, so the job is simply getting it to the far end of the fruit. Pots and grow bags are a different case: the supply itself can run short, and that is where a calcium feed earns its place alongside steady watering.

What blossom end rot looks like on tomatoes

It starts as a small water-soaked spot at the blossom end of the fruit, the end that carried the flower and sits farthest from the stem. The first damage is often internal, in the tissue just inside the skin, before anything shows on the surface. As it develops the spot enlarges, darkens to brown or black, sinks inward and turns leathery. Half the fruit can end up affected, with the bottom looking flattened.

It shows up first, and worst, on the early trusses, when the plant is growing quickly and hasn't built much root yet. The same disorder turns up on peppers, aubergines, courgettes and squash. It does not spread from plant to plant or fruit to fruit, and the firm flesh above the patch is safe to cut away and eat, provided no secondary mould has moved in.

Figure 1 · how it develops

From a faint spot to a leathery patch

The first damage is internal, in the tissue just inside the skin, before anything shows on the surface. Left to right, the blossom end darkens, sinks and turns leathery as the fruit keeps swelling.

Why the fruit runs short even when there's plenty in the ground

Calcium moves around a plant in one direction. It's carried up from the roots in the xylem, riding the stream of water that the plant pulls up and breathes out through its leaves. That stream is driven by transpiration, and leaves transpire far more than fruit. So the leaves get the lion's share of the calcium, and the fruit gets what's left. Once calcium has arrived somewhere, it stays put. It isn't pulled back out of the leaves and redirected to the fruit later. A tomato swelling quickly needs a steady delivery of its own, and the blossom end, farthest along the supply line, is the part most likely to be left wanting.

Figure 2 · how calcium moves

Calcium travels with water, and the fruit is last in the queue

Calcium rides the transpiration stream up the xylem. Leaves pull hard and take most of it. The fruit barely transpires, so its distal (blossom) end is first to run short when water flow stutters.

In open ground the Royal Horticultural Society is blunt about where the shortage isn't: garden soils very rarely lack calcium, so the trouble is delivery. When water flow stutters, the fruit's thin supply line is the first to fail, and the cells at the blossom end break down: their membranes leak, they lose structure, and the tissue collapses into the familiar dark patch.

Containers change the picture. A pot or grow bag holds far less in reserve than open ground, frequent watering washes nutrients through it, and some peat-free mixes and unbuffered coir can leave calcium supply short or poorly matched to potassium and sodium in solution — which is why coir is often pre-treated with calcium and why container growers reach for a cal-mag feed when a test or the media spec shows the risk. Do not treat every bagged compost as “holding calcium loosely.” So in pots the supply itself can run short, on top of the delivery problem. Watering still matters most, since small pots dry out and swing fastest, but the calcium side is a real second factor in pots and grow bags in a way it seldom is in open ground.

In open ground there is almost always plenty of calcium. The hard part is getting it to the far end of a fruit that is growing faster than the plant can keep up with. Paraphrasing RHS guidance on blossom end rot

The research is less settled than the gardening advice. The cell-level account favoured by Ho and White frames blossom end rot as a local calcium deficiency in the young fruit, where demand outruns supply. Saure has argued the opposite case over two papers: that the calcium shortage is a symptom of stress rather than its root cause, with the early damage driven by disrupted membranes and reactive oxygen, and calcium falling as a consequence. For a grower the practical lesson lands the same way either side of that debate. Stress on the plant, above all from water, is what tips a fruit into trouble.

More recent work sharpens the point. The de Freitas papers at UC Davis are largely abscisic-acid (ABA) and partitioning trials — they track how calcium is shared inside the plant under ABA treatments, not simple bag-rate trials. The fruit's portion is still set mainly by transpiration and water flow. The “too much calcium made it worse” shape belongs on Reitz et al. 2021 (excess calcium on whole plants vs excised fruit tissue), not on de Freitas as a rate study.

What actually triggers it

Uneven watering is the main cause, and everything else tends to work through the same bottleneck of disrupted water flow. The usual culprits:

• Wet–dry swings. Letting the compost dry out and then soaking it, over and over, is the classic pattern. Each dry spell starves the fruit of calcium for a while.
• Pots and grow bags. Limited root volume dries out fast and swings hard, and the compost holds less calcium in reserve, which is why container plants get it most.
• Hot spells. High heat and dry air make the leaves transpire even harder, pulling water and calcium away from the fruit just when it is growing quickest.
• Rapid early growth. The first trusses set while the plant is small and pushing on, so demand outpaces a young root system. Later trusses, on a bigger plant, usually fare better.
• Too much nitrogen. Heavy feeding, especially ammonium-based, drives leafy growth that competes with the fruit for calcium. Ammonium also competes with calcium directly at the roots.
• Feeding into dry soil. Concentrated feed in dry compost makes it harder for the roots to take up water at all, which only tightens the squeeze.

High potassium and magnesium in the feed can add to the problem too, since all three compete as positively charged ions for uptake at the root. None of this means the soil is short of calcium. It means the calcium that is there isn't reaching the fruit in time.

Figure 3 · competition at the root

Calcium queues with rivals at the same uptake sites

Calcium, potassium, magnesium and ammonium are all positively charged ions that compete for the same root uptake sites. Pile on potassium and the calcium gets crowded out.

How to stop it on the next trusses

You can't rescue a fruit that already has blossom end rot. What you can do is steady the conditions so the trusses still to come grow clean. The levers below are ranked by how well the evidence backs them.

Figure 4 · the single biggest lever

Steady watering keeps soil moisture out of the danger zone

Each time the compost dries out, the transpiration stream stutters and calcium delivery to the fruit stalls. Even watering holds moisture in range; wet–dry swings drop into the stress zone again and again.

Evidence synthesis drawn from RHS and university extension guidance alongside peer-reviewed trials (Hagassou 2019; de Freitas 2011, 2014; Haleema 2024), not a single source. "Strong" means consistently recommended; "supportive" means it helps under the right conditions; "weak" and "no" mean the method is unreliable or rests on a misconception.

Pick off the affected fruit when you spot it. It will not recover, and leaving it on only draws water and calcium that the healthy fruit could use. Then settle into a watering rhythm and hold it. Most plants grow out of the problem once the roots establish and the weather and watering steady.

Is it really blossom end rot?

A few other tomato troubles look alarming in the same way, and the fix for each is different. The quickest tell for blossom end rot: it is always at the blossom end, always firm and dry, and it never jumps between plants.

Figure 5 · quick diagnosis

Where each problem shows up on the fruit

Position is the fastest tell. Blossom end rot and catfacing sit at the bottom; blight blotches the shoulders and spreads to the leaves; sunscald bleaches the sun-facing side.

General horticultural guidance for distinguishing common tomato fruit problems. When the foliage is also affected and patches turn soft in wet weather, suspect blight rather than blossom end rot.

Why feeding hard on its own won't fix it

Here is the pattern that catches people out: they feed faithfully all season with a popular liquid tomato food and still get blossom end rot. The reason is in the bottle. Most high-potash tomato feeds are NPK feeds with a little magnesium and a few trace elements, and they carry no calcium at all. Tomorite, the best-selling one, declares NPK 4-3-8 with seaweed on the statutory declaration; calcium is not listed, and magnesium is not listed either unless a current declaration shows it. Most rivals are built the same way.

Figure 6 · what's in the bottle

A typical tomato feed carries no calcium

High-potash feeds drive fruiting with potassium and add a little magnesium and trace elements. Calcium is absent, because calcium and phosphate won't stay dissolved together in a concentrate.

There is a chemical reason for it. Calcium reacts with the phosphate in a concentrated feed to form compounds that won't stay dissolved, so a standard NPK concentrate can't hold much calcium in the bottle without it settling out. Calcium is usually supplied on its own for exactly that reason.

That works against you twice over with blossom end rot. The feed adds no calcium, so it does nothing for the supply side. And it is heavy on potassium, which competes with calcium for uptake at the root, so feeding hard can tighten the squeeze rather than ease it. None of this makes these feeds bad. They do the job they are built for, driving fruiting with potassium. It just means the calcium side is yours to cover separately.

Where calcium feeds actually help, and where they don't

This is the part most product labels get wrong, in both directions. A calcium feed does not override bad watering, and spraying a plain calcium salt straight at the fruit does little, because the waxy skin resists it and calcium barely moves once it has landed. Get the watering and mulching right first; that alone clears up most cases. The opposite mistake is just as common, though: assuming calcium nutrition never matters. It does, in the conditions where supply or uptake is the real limit.

There is a wrinkle worth being honest about, because the form of calcium matters. Calcium chelated with amino acids crosses the skin better than a plain salt, and most of the work showing this is in apple, where bitter pit is the calcium-deficiency disorder that mirrors blossom end rot. In one apple trial, Arabloo and colleagues found that repeated foliar sprays of amino-acid-chelated calcium raised fruit calcium against untreated trees. The honest caveats are that the gains are inconsistent, and a spray still lands on the skin and struggles to reach the flesh, which is where blossom end rot forms. So a chelated calcium spray can be a useful top-up if it wets the developing fruit early and often, but it sits behind steady watering and calcium worked into the root zone, not in front of them.

You don't need a lab test to spot those conditions. Growing in pots or grow bags, on light sandy soil that drains fast, or feeding hard for a heavy crop all push calcium supply or uptake to the edge, and that is where studies find calcium feeds reduce blossom end rot rather than do nothing. If a test or unbuffered coir shows supply is short, add calcium through the roots after watering is steady — not as a disease cure. One practical option is liquid gypsum (soluble calcium and sulphur without shifting pH). Keeping the main feed moderate — for example a plant-based organic tomato feed — helps avoid nitrogen and potassium spikes that crowd calcium out. No tomato feed should be sold or read as a “no blossom end rot” cure.

If you do add calcium in containers, keep watering first. A Cal-Mag can help where the media runs short, but boron evidence for blossom end rot is thin — Haleema et al. 2024 is one foliar pairing trial, not a reason to lead with boron in a root feed. Fulvic acid may aid uptake; it is not a BER fix on its own.

Think of all that as supporting the supply and uptake side while watering does the heavy lifting on delivery. Nutrition cannot override the water-driven sharing-out that sends most of the calcium to the leaves. Steady water is what carries it to the far end of the fruit; the feed is there so the calcium is present, balanced and easy to take up when the water moves it.

Related: Liquid gypsum · Cal-Mag · Garden calcium carbonate · blossom end rot collection

The first truss is the warning, not the verdict. Steady the watering, mulch the top, and the tomatoes that follow usually come through without a mark on them.

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