Why Do My Tomatoes Have Black Bottoms? Blossom End Rot Explained

You lift a truss and there it is, on the underside of a good-looking green tomato: a flat brown patch that looks scorched. The usual reflex is to reach for a spray, or for calcium. Neither, in most cases, will do anything at all.

What you are actually looking at

The patch forms on the bottom of the fruit, opposite the stem, exactly where the flower once was. It starts quietly, often as a slightly translucent or water-soaked area, then darkens, turns brown, and finally goes dry and leathery, almost papery. It is flat or slightly sunken, clearly outlined, and the tissue stays firm under your thumb. Gardeners call it black bottom; the textbooks call it blossom end rot.

Those details matter, because they are what separate this from other problems. Rot starting at the stem end, a soft area that collapses under pressure, white or grey fuzzy growth, a fermented smell, concentric rings: none of that fits blossom end rot. If what you are seeing looks more like one of those, set this explanation aside and look elsewhere. Nobody, this article included, can give you a firm diagnosis without seeing the plant. Treat what follows as a lens to hold up against what is in front of you.

Not a disease, and not a fungus

Blossom end rot is caused by no pathogen at all. Not a fungus, not a bacterium, not a virus. It is what growers call a physiological disorder: the plant is malfunctioning, but nothing is attacking it. The practical consequence is immediate. A fungicide, a copper spray, any treatment whatsoever will change nothing, because there is nothing to fight. It also does not spread from plant to plant. It is common to see one variety badly affected next to another that is untouched, or a single plant suffering in an otherwise healthy row. That is the signature of a growing-conditions problem, not of an infection.

Damaged tissue can later be colonised by opportunistic moulds, which is why an old patch sometimes ends up covered in a dark bloom. That fungus is a late passenger, never the cause.

Calcium travels with water, and never comes back

Calcium is a structural element: it holds cell walls together. In a swelling fruit, cells divide and then stretch very fast, and that is precisely when they need it. But calcium does not move freely around a plant. It rises from the roots in the transpiration stream, carried by the flow of water that the leaves pull upward as they lose moisture. And once it has been deposited in a cell wall, it stays there. The plant cannot recover it and send it somewhere else, unlike nitrogen or potassium, which it happily shuffles from one organ to another.

Two things follow. First, every organ has to be served while it is growing, with no chance to catch up later. Second, the organs that transpire heavily, the leaves, take most of the flow, while a young fruit, sealed by its cuticle and transpiring very little, sits at the end of the queue. When water supply becomes erratic, that flow contracts, and the worst-served tissue pays first: the base of the fruit, the point furthest from where it is attached. Cells there lose their cohesion, collapse, and the area browns and then dries out.

Put another way, calcium can be abundant in your soil and still be missing locally inside one fruit. This is not a stock problem. It is a delivery problem.

Why eggshells and lime fix almost nothing

This is where the most repeated piece of advice works against the gardener. Crushing eggshells around your tomatoes feels productive, but the calcium carbonate they are made of dissolves very slowly indeed; over the span of one season, the amount that actually becomes available is marginal. Garden lime does more, but what it mainly does is shift soil pH, and you do not raise pH by guesswork without knowing where you are starting from. A soil test answers that question. A hunch does not.

More importantly, these amendments aim at the wrong target. They slowly increase a reserve that was usually not the limiting factor in the first place. Plenty of garden soils, especially those over chalk or limestone, already hold far more than a handful of tomato plants could use. Adding calcium to soil that is not short of it does nothing to restore an interrupted flow of water, and that flow is what decides the outcome.

That leaves foliar calcium sprays, often sold as the quick fix. Reported results are genuinely mixed: calcium penetrates poorly, moves very little once it has landed, and struggles to reach the fruit, which is the organ that needs it. You can try it, but it would be dishonest to tell you it settles the matter.

Water regularly rather than heavily

The real lever is consistency. It is the swing that does the damage: soil that dries out properly, then a soaking, then another dry spell. Every lurch interrupts and then violently restarts the supply to the fruit. Smaller, more frequent watering beats one big weekly drench, and common practice is to adjust the rhythm to heat, wind and soil type rather than to a fixed day of the week. The simplest check is still a finger pushed into the soil, a couple of inches below the surface.

Mulch is the strongest ally of that consistency. It buffers the swings, slows evaporation and keeps moisture steadier around the roots.

Nitrogen, containers, and the first trusses

Too much nitrogen almost always makes things worse. It drives lush leafy growth, so more leaf area competing for the flow, while also pushing fruit to expand faster. You are asking for more, sooner, from a system that was already struggling to keep up. Balanced feeding beats generous feeding.

Container growing deserves particular attention. The volume of compost is small, and it dries out fast and abruptly, especially in hot weather or in a dark pot standing in full sun. That is exactly the boom-and-bust regime that triggers the disorder. A larger container, a mulched surface, and a daily check during hot spells make a real difference.

One last observation, and a reassuring one: it is usually the first trusses that suffer. The plant is still young, its root system limited, and early summer weather is rarely settled. As roots develop and your watering routine steadies, the problem often resolves itself on later trusses. An affected fruit, on the other hand, never recovers. The damaged area is permanent. Pick it off, which also stops the plant investing in it. There is no need to strip the plant or pull it out.

Connecting an incident to a period

The awkward thing about a disorder caused by inconsistency is that you see it weeks after the conditions that caused it. The fruit blackens today; the missed watering or the heatwave was some time ago. This is where a growing log helps, and it is what Marmanda keeps: a record of what you actually did and observed on each crop, waterings, feeds, incidents. It diagnoses nothing and does not read your photos. It simply lets you trace back and see what was happening while those fruits were forming.

And if the signs you are seeing do not match the ones described here, keep the question open and look elsewhere.