Why do my tomatoes have black sunken bottoms?

Calcium moves through the plant in the transpiration stream, so it goes where water goes. When a plant dries out even briefly, the fastest-growing tissue — the end of a developing fruit — is the first to be starved, and the cells there collapse.

Step by step

1. Confirm what you are looking at.A flat, sunken, dark brown or black leathery patch on the blossom end of the fruit — the end away from the stalk. It is not a rot that spreads and it is not caused by an organism.
2. Look at the watering rhythm, not the amount.Cycles of bone dry then soaked are what cause it. A plant kept evenly moist rarely gets it, even in the same compost.
3. Water more often in hot weather.A fruiting tomato in a greenhouse in July may need watering twice a day. Once a day at a fixed time regardless of weather is what produces the problem.
4. Increase the pot size.Small pots and growbags dry too fast to be watered consistently. Two plants per growbag rather than three, or 15 litre pots, transforms it.
5. Mulch the surface.A mulch on the compost surface substantially evens out the moisture cycle in containers.
6. Consider whether an automatic system would help.This is a problem that drip irrigation on a timer solves almost completely, because consistency is the whole issue.
7. Do not over-feed.High potassium tomato feeds, applied heavily, can compete with calcium uptake. Feed at the label rate, not more.
8. Check the compost is not drying to the point of repelling water.Peat-free composts can become hydrophobic. If water runs straight through, the plant is dry despite the watering.
9. Remove affected fruit.It will not recover and it is drawing resources. The rest of the truss is usually fine.
10. Expect the next fruit to be clean.Once watering is consistent, newly setting fruit is unaffected. The existing damaged fruit does not heal.

Tips

• It is a watering problem in the vast majority of cases, not a calcium shortage.
• Peppers, chillies and courgettes get it too, and for the same reason.
• Consistency is everything. Twice a day in July is normal for a greenhouse tomato in a pot.

Common mistakes

• Adding calcium, eggshells or lime to the compost — There is nearly always plenty of calcium present. The problem is that the plant cannot move it to the fruit because of inconsistent water, so adding more changes nothing.
• Watering a lot once the problem appears — A sudden heavy soak after a dry period is exactly the cycle that causes it. Move to smaller, more frequent, more regular waterings instead.

If it doesn't work

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