🐛✗ Tested and false“Blossom end rot is caused by a lack of calcium in the soil”
It is very rare for garden soil, growing bags or potting compost to actually be short of calcium; most gardens already hold enough. The real bottleneck is getting that calcium into the fruit, which depends on a steady flow of water through the plant rather than on how much calcium the soil contains. A dry spell followed by a heavy soak interrupts that flow and produces the same rot in soil that tested perfectly adequate the week before.
Our read RHS states plainly that soil deficiency is rare and frames blossom end rot as a water-flow problem; UC IPM independently describes it as a calcium and water balance issue in the plant, not a soil-level shortage.
What to do: Focus on consistent watering rather than adding calcium to the soil; only get a soil test done if watering has been genuinely steady and the problem persists.
In Scotland: Tomatoes, aubergines, peppers and chillies are mostly grown under glass or polytunnel in Scotland for warmth, and a growbag or pot under cover dries out fast on a sunny day with no rain to top it up accidentally, which is the actual trigger far more often than the soil itself.
2 sources
- RHS: Blossom end rotIt is very rare for soils, growing bags or potting media to actually lack calcium, and the disorder is caused by adverse growing conditions rather than a soil deficiency.
- UC IPM: Blossom-End RotBlossom end rot results from a low level of calcium in the fruit and water balance in the plant, not simply from a shortage of calcium in the soil.
“Adding crushed eggshells to the soil or planting hole prevents or cures blossom end rot”
Eggshells do contain calcium, but they break down far too slowly in soil to supply it on any timescale that matters to a fruiting plant in one season. Even where soil calcium is genuinely short, which is uncommon, eggshells are not a practical fix, and in the much more common case, where the soil already has enough calcium, adding more of it does nothing for a problem caused by inconsistent watering.
Our read Mississippi State University Extension addresses eggshells specifically and states they don't reliably prevent the disorder because the real driver is watering consistency, not calcium supply, and recommends a soil test rather than guessing before adding anything.
What to do: Skip the eggshells and put the effort into watering consistency instead; if a soil test genuinely shows low calcium, use a fast-acting source like gypsum rather than eggshells.
2 sources
“Blossom end rot spreads from one fruit to another, or from the ground, like a disease”
Blossom end rot has no pathogen behind it, so there is nothing for it to spread as. Each affected fruit develops the rot independently because of its own local calcium shortfall during a watering dip, not because it caught something from a neighbouring fruit or from soil contact. A truss with several rotten fruit usually reflects a watering lapse that hit all of them at once, not fruit-to-fruit spread.
Our read UC IPM states explicitly that the disorder is not associated with soil contact or with damage to other plant parts, and that it is not caused by a pathogen, with no pesticide or fungicide relevant to it. RHS separately confirms it is a physiological problem caused by growing conditions rather than a pest or disease.
What to do: Remove affected fruit for tidiness and to stop the plant wasting energy on it, but don't isolate the plant or treat it as a pest/disease outbreak; there's nothing to contain.
2 sources
- UC IPM: Blossom-End RotThe disease is not associated with soil contact or with damage to other plant parts, is not caused by a pathogen, and there are no pesticide solutions.
- RHS: Blossom end rotBlossom end rot is a physiological problem, caused by adverse growing conditions rather than a pest or disease.
“Adding Epsom salts to the soil, or watering tomatoes and peppers with an Epsom salt solution, prevents or cures blossom end rot”
Washington State University Extension's fact sheet on Epsom salt use in gardens singles out blossom end rot by name as one of the 'online recipes' for Epsom salt that has no scientific basis, and its own figure caption states plainly that blossom end rot on tomato is not related to magnesium deficiency. Magnesium and calcium are both cations that compete for the same uptake sites in roots and soil, so heavy, repeated Epsom salt use can if anything make calcium uptake harder, not easier.
What to do: Skip Epsom salts for blossom end rot. Focus on the things that actually govern calcium delivery to the fruit: even, consistent watering, avoiding root damage, and not overdoing nitrogen feed early on.
1 source
“Spraying the leaves with a calcium feed stops blossom end rot”
RHS and UC IPM both describe foliar calcium sprays as poorly suited to the job, because calcium moves through a plant mainly in the water stream to actively transpiring leaves, not from leaf to fruit, so spraying the foliage doesn't reliably get calcium to the fruit where it's needed. University of Maryland Extension takes a softer line, allowing that a calcium chloride spray on affected plants may bring some temporary relief, without claiming it addresses the underlying cause.
Our read RHS explicitly states that calcium is not readily absorbed by the fruit and that transport from leaves to fruit is poor, even warning that calcium nitrate feeds can worsen the problem by pushing leafy growth. UC IPM does not endorse foliar sprays as a control either. University of Maryland Extension is the outlier, describing a calcium chloride spray as offering possible temporary relief on fruit already affected.
What to do: Don't rely on a foliar calcium spray as the fix; treat watering consistency as the real lever, and if trying a spray at all, treat it as a minor extra rather than a substitute for steady watering.
2 sources
“Plants grown in pots or growbags get blossom end rot more often than plants in open ground”
Container growing genuinely raises the odds of blossom end rot, not because pots or growbags themselves cause it, but because a small volume of compost dries out and swings between dry and wet far faster than open border soil, which is exactly the pattern that interrupts calcium delivery to the fruit. The fix is the same steady watering that applies everywhere, just harder to maintain in a small container.
Our read RHS states that plants in containers or growing bags are most at risk of the irregular water supply that leads to blossom end rot, and recommends larger containers and automatic watering as a mitigation.
What to do: Use the largest practical container, mulch the surface to slow drying, and water little and often in hot weather rather than letting a growbag dry out between soaks.
In Scotland: Most Scottish tomato, pepper and aubergine growing happens under glass or polytunnel in growbags or pots for warmth, so this risk applies to a large share of Scottish growers by default rather than being an edge case.
1 source
- RHS: Blossom end rotPlants in containers or growing bags are most at risk of irregular water supply leading to local calcium deficiency in the fruits, and larger containers or border soil reduce the risk.
