Blossom end rot in tomatoes: a watering mistake

Blossom end rot in tomatoes, the sunken black patch on the bottom of the fruit, is caused by calcium not reaching that end, and uneven watering is nearly always behind it. Calcium travels in the water the plant draws up, so when the compost swings from dry to soaked, the fruit goes short. It won't spread to other plants. Marked fruit won't recover, so pick it off, then water evenly, mulch, add calcium for tomatoes, and go easy on high-nitrogen and high-potash feeds. The next trusses usually come through clean.

Joe's take · Joe Barrington, founder of Dr Forest

If you've got blossom end rot, the RHS advice is that you will have plenty of calcium. I don't fully agree with that. Use calcium, it is quite possible that is the issue.

"I fed the chillies my Cal-Mag supplement and I didn't get any more blossom end rot. It was right at the beginning of the chillies producing."

"Now whether I'm in pots or whether I'm in outdoor beds, I'll feed liquid gypsum at the planting and when flowers start to be formed."

The mistake I see most is feeding with a liquid fertiliser that has no calcium in it and never applying any calcium.

What blossom end rot looks like on tomatoes

Tomato blossom end rot starts as a small water-soaked spot on the blossom end, the end that carried the flower, opposite the stalk. The damage begins inside the fruit, in the fast-swelling cells, before anything shows on the skin [2]. The patch grows, darkens from greenish brown to black and sinks, leaving a flattened brown or black bottom on the tomato [1]. Some growers just call it bottom rot. It's worst on the first trusses, while the plant is growing fast on a small root system. Peppers, aubergines and courgettes get it too [1] [4].

Why the fruit runs short of calcium

Calcium only moves up a plant in the water stream, and once it's in a leaf it can't be moved on to the fruit [3]. Growing leaves and shoots pull far more water than fruit, so they take most of the calcium, and the gap widens in hot, dry weather [3]. When the water supply stutters, the blossom end, last in the queue, goes without, and its cells break down into the dark patch [2].

The RHS says it's very rare for soils, grow bags or potting compost to lack calcium, so the usual problem is getting it into the fruit [1]. I'd still use calcium, because it's quite possible that's the issue.

What causes blossom end rot in tomatoes

Uneven watering is the main cause of blossom end rot in tomatoes [3]. Everything else on this list makes the supply of water or calcium to the fruit worse.

• Compost left to dry out, then soaked, over and over.
• Pots and grow bags. Limited root space puts them most at risk, and plants in border soil are less likely to get it [1].
• Hot spells, when the leaves pull harder on the water and take more of the calcium [3].
• Heavy feeding. Too much nitrogen and potash pushes leafy growth that soaks up calcium, and ammonium, potassium and magnesium all compete with calcium at the roots [1] [3].
• Feeding into dry compost. The RHS says liquid feed should only go on moist soil [1].

How to stop blossom end rot on the next trusses

There's nothing you can do to save a fruit once it's marked [1], but you can steady the conditions for the trusses still to come.

What to change	Why it works	Worth doing?
Water evenly and never let the compost dry out	Keeps calcium moving to the fruit. In hot spells a pot may need water twice a day, which the RHS says beats one double dose. Dried-out grow bags are very hard to wet again [1].	Yes
Mulch the surface, at least 5 cm deep	Holds moisture around the roots between waterings [1].	Yes
Bigger pot, or a drip or self-watering system	Evens out the wet and dry swings that small containers cause.	Yes
Go easy on nitrogen and potash, and feed only moist compost	Less leafy growth competing for calcium [1] [3].	Yes
Top up calcium at the roots	Calcium goes in through the roots, so feed it there [3]. The RHS says soils rarely lack it, but I'd still add it, as it's quite possible that's the issue.	Yes, alongside even watering
Calcium spray on the leaves or fruit	The RHS says they aren't effective [1]. A calcium chloride spray cut blossom end rot from 17% to 7% of fruit in a field trial in Pakistan [5], but five calcium sprays made no difference in a Latvian greenhouse [6].	Rarely
Epsom salts	Magnesium sulphate, with no calcium, and magnesium competes with calcium at the roots [3].	No

Pick marked fruit off so it stops using water the healthy fruit needs. Later trusses usually come through clean once the roots fill out and the watering is steady.

Is it really blossom end rot?

Problem	How it looks	Spreads?
Blossom end rot	Sunken, leathery and dark at the base; firm and dry	No
Late blight	Greasy brown blotches that can go soft; leaves and stems marked too	Yes
Catfacing	Puckered scars and holes from cold or poor pollination, with no rot	No
Sunscald	Pale, papery, blistered patch on the side facing the sun	No

If you've also got yellow leaves on tomato plants, that's a separate problem with its own causes.

Why feeding hard on its own won't fix it

Most liquid tomato feeds carry no calcium. Tomorite's UK declaration is NPK 4-3-8 with seaweed, with no calcium listed [7], and most of its rivals are built the same way (I go through Tomorite NPK and calcium alongside four other feeds). The RHS warns that overfeeding with potassium-rich tomato feed can bring on calcium shortage in its own right [4], so feeding harder can make things worse.

Piling on calcium has limits too. In a UC Davis trial on processing tomatoes, a moderate calcium dose gave the least blossom end rot, a heavy dose made it more severe, and the fruit held no more calcium either way [8].

Where calcium feeds help, and where they don't

A calcium feed won't make up for erratic watering, because the calcium still has to reach the fruit in water [3]. When you add calcium, water a soluble source such as liquid gypsum into the root zone (my guide what is liquid gypsum covers what's in it and whether you can make your own). The fruit's waxy skin takes in very little that's sprayed on it [1] [3].

For the main feed, keep it moderate, such as the organic tomato feed I formulate and blend, so you avoid the nitrogen and potash spikes of a hard-driven liquid. The season's plan is in my guide to the best tomato food.

Related: Liquid Gypsum

Frequently asked questions

Q: Is blossom end rot caused by a lack of calcium in the soil? Not usually: the RHS says soils, grow bags and potting compost very rarely lack calcium, and uneven watering is what stops it reaching the fruit. I'd still add calcium at the roots alongside steady watering, because it's quite possible that's the issue.

Q: Can I still eat tomatoes with blossom end rot? Yes, as long as no mould has set in. It is a physiological disorder and is not infectious, so cut away the affected end and use the rest.

Q: Will affected tomatoes recover? No, once the patch appears the damage is done. Pick the fruit off and steady the watering to protect the trusses still to come.

Q: Do calcium sprays stop blossom end rot? Not reliably. The RHS says calcium sprays are not effective, because the fruit takes in very little through its skin.

Q: Do Epsom salts help blossom end rot? No. Epsom salts are magnesium sulphate with no calcium in them, and extra magnesium competes with calcium at the roots.

Q: Will a calcium-free liquid feed cause blossom end rot? Not on its own, because uneven watering is the main cause, but relying on one and never adding calcium is the most common mistake I see. Most liquid tomato feeds carry no calcium (see what's in Tomorite), and the RHS warns that too much potash-rich tomato feed can bring on calcium shortage in its own right.

Q: How often should I water tomatoes to avoid blossom end rot? Often enough that the compost never dries out: push a finger in each morning and water if it is dry a few centimetres down. In hot weather a pot may need it twice a day.

Q: 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.

Joe Barrington

Founder of Dr Forest in Stockport. Joe formulates and blends his own feeds, including the Tomato Fertiliser 3-4-6 and Cal-Mag, and stocks bought-in products alongside them.

Sources

1. Royal Horticultural Society. Blossom end rot. rhs.org.uk/problems/blossom-end-rot
2. 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 to 581. doi:10.1093/aob/mci065
3. Mayfield, J.L. & Kelley, W.T. (2009, last reviewed 2022). Blossom-end rot and calcium nutrition of pepper and tomato. University of Georgia Cooperative Extension Circular 938. fieldreport.caes.uga.edu
4. Royal Horticultural Society. Nutrient deficiencies. rhs.org.uk/prevention-protection/nutrient-deficiencies
5. Haleema, B., Shah, S.T., Basit, A. et al. (2024). Comparative effects of calcium, boron, and zinc inhibiting physiological disorders, improving yield and quality of Solanum lycopersicum. Biology 13(10): 766. doi:10.3390/biology13100766
6. Karlsons, A., Osvalde, A., Cekstere, G. & Āboliņa, L. (2023). Effects of Ca sprays on fruit Ca content and yield of tomato variety susceptible to blossom-end rot. Plants 12(8): 1640. doi:10.3390/plants12081640
7. Evergreen Garden Care. Levington Tomorite Concentrated Tomato Food, 2.5 L pack label, UK declaration: NPK fertiliser solution 4-3-8. lovethegarden.com
8. 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. doi:10.1016/j.scienta.2021.110514