Blossom end rot in tomatoes: what causes it, and how I stopped it
Tomato growing
Blossom end rot in tomatoes: what causes it, and how I stopped it
By Joe, Founder of Dr Forest · June 2026
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 in tomatoes is a calcium problem inside the fruit, and only rarely a shortage of calcium in open ground. It's a physiological disorder, not a pest or a disease, which is why fungicides and pest sprays do nothing for it. The dark sunken patch at the blossom (bottom) end appears because the fast-growing fruit ran short of calcium at the wrong moment. Calcium travels through the plant in the water stream, and the fruit is a weak competitor for it against the leaves. Anything that interrupts steady water flow, mainly uneven watering, brings it on.
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.
From the founder · my experience
How I stopped it on my own plants
I've been on both sides of this. A couple of seasons back I grew chillies in pots and got blossom end rot on the first flush. I added a cal-mag supplement, the problem stopped, and once it was part of the regular feed it didn't come back. One garden proves nothing on its own, but it matched the research I'd been reading.
There are two camps on this, and I don't fully buy either. The line that there's "almost always enough calcium" is the over-cautious end, and the marketing that says to pour on calcium is the other. I think the truth, as ever, is actually in the middle. In pots, especially small ones with fast-growing heavy feeders like chillies and tomatoes, plants do run short of calcium, and a high-potash mineral or liquid feed carrying little or no calcium makes that more likely, not less.
So here's what I do now. In pots, a calcium-magnesium supplement stays in the feed as a precaution, and I've not had it back. I've not had blossom end rot in open ground myself, but if I did, I'd sort the watering first and still add calcium alongside it. At roughly 20p a plant, there's very little to lose.
My experience is also backed by research. In a controlled greenhouse trial, Reitz and colleagues at UC Davis found that a moderate calcium application cut blossom end rot against no calcium at all, while overdoing it backfired. That's the shape of it: enough calcium helps, too much doesn't, and steady water ties the two together. The rest of this guide fills that in.
What it is
A localised calcium shortage in the fruit, brought on by uneven water supply. The first trusses and pot-grown plants are hit hardest. An affected fruit won't recover, but later ones can be protected.
What to do first
Steady the watering and mulch the surface so the soil never dries out. Go easy on nitrogen. Reach for a calcium feed where supply or uptake is the limit, in pots, sandy soil or under heavy feeding, and work it into the root zone, not onto the leaves.
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 the popular peat-free and coir composts hold calcium loosely. Coir in particular tends to bind calcium and magnesium while letting potassium and sodium dominate the solution, which is why it is often pre-treated with calcium and why container growers reach for a cal-mag feed. 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. Studies by de Freitas and colleagues at UC Davis traced how calcium is shared out inside the plant and found the fruit's portion is set mainly by transpiration and water flow, not by how much calcium sits in the soil. Raising the calcium supplied to the roots did not reliably raise calcium in the fruit, and in a later greenhouse trial from the same group, pushing it too high made blossom end rot worse rather than better. The shortage the fruit feels is one of delivery and timing inside the plant, not usually one of stock in the ground.
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.
| What to change | Why it works | Evidence |
|---|---|---|
| Water evenly, never let it dry out | Keeps the transpiration stream steady so calcium keeps reaching the fruit. In hot weather that can mean watering twice a day. | Strong |
| Mulch the surface | Slows evaporation and buffers soil moisture between waterings. | Strong |
| Bigger pot, or a drip or self-watering system | Removes the sharp wet–dry swings that small containers cause. | Strong |
| Go easy on nitrogen; feed only moist soil | Stops lush leaf growth and concentrated salts from out-competing the fruit for calcium and water. | Supportive |
| Top up calcium where supply or uptake is the limit | In pots and grow bags, fast-draining sandy soil, or under heavy feeding, plant-available calcium can run low. Field and greenhouse trials show calcium nutrition cuts blossom end rot under those conditions. Feed it to the root zone, not the leaves. | Supportive |
| Pair calcium with boron | Boron helps the plant take up calcium and use it to build cell walls and membranes. The lowest blossom end rot in trials came from calcium and boron together, not calcium alone. | Supportive |
| Foliar calcium spray on leaves or fruit | Aims to add calcium directly, but the fruit's waxy skin barely absorbs it and leaf calcium doesn't travel to the fruit. Nitrate-based sprays can make things worse by pushing leafy growth. | Weak |
| Epsom salts | Epsom salts are magnesium, not calcium, and the extra magnesium can compete with calcium at the root. | No |
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.
| Problem | How it looks | Spreads? |
|---|---|---|
| Blossom end rot | Sunken, leathery, dark; firm and dry | No |
| Late blight | Greasy brown blotches, can go soft; leaves and stems affected too | Yes |
| Catfacing | Puckered scars and holes from cold or poor pollination, not rot | No |
| Sunscald | Pale, papery, blistered patch from exposure | No |
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, is NPK 4-3-8 with magnesium, seaweed and trace elements; calcium is not on the list, and most of its 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 that's how you grow, a calcium feed is worth adding before trouble starts: our blossom end rot collection gathers the soluble calcium feeds and the Cal-Mag for exactly these conditions. The trick is to get the calcium in through the roots, where the plant can move it, not onto the leaves. Our liquid gypsum puts soluble calcium and sulphur into the root zone without shifting pH and without the ammonium that makes blossom end rot worse. Where the pH itself is low and locking calcium up, a fine garden calcium carbonate lifts pH and adds calcium together. And keeping the main feed moderate, with a balanced slow-release option like our tomato fertiliser, avoids the nitrogen and potassium spikes that crowd calcium out.
Calcium's weak point is uptake and mobility, so the supporting nutrients earn their keep. Our Cal-Mag carries calcium and magnesium with boron and fulvic acid for that reason. Boron helps the plant take calcium up and bind it into cell walls and membranes, and in field trials calcium paired with boron gave less blossom end rot than calcium on its own. Fulvic acid is a biostimulant that chelates nutrients and supports their uptake and use; the direct evidence on blossom end rot specifically is thin, so read it as helping the calcium get in and get used, not as a 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.
From the Dr Forest range
Calcium feeds for tomato growers
Our blossom end rot collection brings together the soluble calcium feeds, the calcium-with-boron supplement and the pH correctors that cover the nutritional side, made with organic ingredients in small batches in the UK. They earn their place where supply or uptake is the limit. Use them alongside steady watering, not instead of it.
- Liquid gypsum · soluble calcium and sulphur to the root zone, pH-neutral
- Cal-Mag · calcium and magnesium with boron and fulvic acid to support calcium uptake
- Garden calcium carbonate · raises a low pH and adds calcium
Browse the full blossom end rot collection for the calcium range.
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.
Frequently asked questions
Is blossom end rot caused by a lack of calcium in the soil?
It depends where you're growing. In open ground, almost always no: garden soil rarely lacks calcium, and blossom end rot is a delivery problem, the fruit going short because uneven watering interrupted the water stream that carries calcium. In pots and grow bags it is less clear cut. The buffer is small, frequent watering leaches calcium, and peat-free and coir composts hold it loosely, so the supply itself can run short, especially under a high-potash feed. Steady the watering first either way, and in containers a calcium feed often earns its place too.
Can I still eat tomatoes with blossom end rot?
Yes, as long as no mould has set in. Blossom end rot is a physiological disorder, not an infection, so the firm flesh above the patch is fine. Cut away the affected blossom end and use the rest.
Will affected tomatoes recover?
No. Once a fruit has blossom end rot the damage is done and it won't reverse. Remove it so the plant isn't wasting water and calcium on it, then fix the conditions to protect the trusses still to come.
Do calcium sprays stop blossom end rot?
They are unreliable. Calcium is poorly absorbed through the fruit's waxy skin, and calcium that lands on the leaves doesn't travel to the fruit. Calcium-nitrate sprays can even make it worse by pushing leafy growth. Consistent watering does far more than any spray.
Will a calcium feed like liquid gypsum stop blossom end rot?
It depends on why the fruit is short. Where calcium supply or uptake is the limit, in pots, sandy soil or under heavy feeding, a soluble calcium feed worked into the root zone reduces the risk, and studies back that up. Where the cause is uneven watering, the feed won't override it, so steady the water first. Apply calcium through the roots, not as a spray on the fruit.
Do boron and fulvic acid help with blossom end rot?
They help on the uptake side. Boron supports the plant in taking calcium up and using it to build cell walls and membranes, and calcium paired with boron reduced blossom end rot more than calcium alone in field trials. Fulvic acid is a biostimulant that chelates nutrients and aids their uptake. Neither replaces consistent watering; they make the calcium that is present easier to take up and use.
Does Tomorite contain calcium?
No. Tomorite is a high-potash NPK feed, around 4-3-8, with magnesium and trace elements but no calcium. Most concentrated liquid tomato feeds are the same, because calcium and phosphate won't stay dissolved together in the bottle. A feed like that drives fruiting well, but it won't cover the calcium side of blossom end rot, so supply calcium separately and water steadily.
Why do I still get blossom end rot when I feed my tomatoes regularly?
Because a standard tomato feed doesn't address it. These feeds are high in potassium and carry no calcium, and the extra potassium can compete with calcium for uptake at the root. Blossom end rot comes down to getting calcium to the fruit, which means steady watering plus, where supply is short, a separate calcium feed worked into the root zone. Feeding harder with a high-potash food won't change that.
Do Epsom salts help blossom end rot?
No. Epsom salts are magnesium sulphate, and the problem is calcium, not magnesium. Adding magnesium does nothing for blossom end rot, and too much can compete with calcium for uptake at the root.
How often should I water tomatoes to avoid blossom end rot?
Keep the soil or compost evenly moist and never let it dry out completely. In hot weather a pot may need watering twice a day; as a rough benchmark, plants want somewhere around 25 to 50mm of water a week from rain or irrigation. Mulching the surface helps hold that moisture steady.
Does blossom end rot spread to other plants?
No. It isn't infectious and won't pass from plant to plant or fruit to fruit. Even if your earliest tomatoes are affected, the later ones on the same plant can be perfectly fine once the watering steadies.
Sources
- Royal Horticultural Society. Blossom end rot. rhs.org.uk/problems/blossom-end-rot
- Royal Horticultural Society. Fertilisers: types and uses. rhs.org.uk/garden-jobs/fertilisers
- Royal Horticultural Society. Nutrient deficiencies. rhs.org.uk/prevention-protection/nutrient-deficiencies
- Saure, M.C. (2001). Blossom-end rot of tomato (Lycopersicon esculentum Mill.): a calcium- or a stress-related disorder? Scientia Horticulturae 90(3–4): 193–208.
- Saure, M.C. (2014). Why calcium deficiency is not the cause of blossom-end rot in tomato and pepper fruit – a reappraisal. Scientia Horticulturae 174: 151–154.
- Ho, L.C. & White, P.J. (2005). A cellular hypothesis for the induction of blossom-end rot in tomato fruit. Annals of Botany 95(4): 571–581.
- Topcu, Y. et al. (2022). Blossom-end rot: a century-old problem in tomato (Solanum lycopersicum L.) and other vegetables. Molecular Horticulture 2: 1.
- Hagassou, D., Francia, E., Ronga, D. & Buti, M. (2019). Blossom end-rot in tomato (Solanum lycopersicum L.): a multi-disciplinary overview of inducing factors and control strategies. Scientia Horticulturae 249: 49–58.
- de Freitas, S.T., Shackel, K.A. & Mitcham, E.J. (2011). Abscisic acid triggers whole-plant and fruit-specific mechanisms to increase fruit calcium uptake and prevent blossom end rot development in tomato fruit. Journal of Experimental Botany 62(8): 2645–2656.
- de Freitas, S.T., McElrone, A.J., Shackel, K.A. & Mitcham, E.J. (2014). Calcium partitioning and allocation and blossom-end rot development in tomato plants in response to whole-plant and fruit-specific abscisic acid treatments. Journal of Experimental Botany 65(1): 235–247.
- Reitz, N.F., Shackel, K.A. & Mitcham, E.J. (2021). Differential effects of excess calcium applied to whole plants vs. excised fruit tissue on blossom-end rot in tomato. Scientia Horticulturae 290: 110514.
- Haleema, B., Shah, S.T., Basit, A., Hikal, W.M., Arif, M., Khan, W., Said-Al Ahl, H.A.H. & Fhatuwani, M. (2024). Comparative effects of calcium, boron, and zinc inhibiting physiological disorders, improving yield and quality of Solanum lycopersicum. Biology 13(10): 766.
- Canellas, L.P., Olivares, F.L., Aguiar, N.O., Jones, D.L., Nebbioso, A., Mazzei, P. & Piccolo, A. (2015). Humic and fulvic acids as biostimulants in horticulture. Scientia Horticulturae 196: 15–27.
- Arabloo, M., Taheri, M., Yazdani, H. & Shahmoradi, M. (2017). Effect of foliar application of amino acid and calcium chelate on some quality and quantity of Golden Delicious and Granny Smith apples. Trakia Journal of Sciences 15(1): 14–19.
- UConn Home & Garden Education Center (2025). Put an end to blossom-end rot: water and nutrient management is key.
- University of Georgia (CAES) Field Report. Blossom-end rot and calcium nutrition of pepper and tomato.
- Michigan State University Extension. Blossom-end rot of tomato tip sheet.