Growing Tomatoes – ‘Blossom End Rot’

A common problem that has been affecting this year’s crop of tomatoes (for allotmenteers who have greenhouses – or those, like me, who are experimenting with outdoor varieties) is ‘blossom-end rot’.

What does this look like? – The condition can easily be recognised by the base of the fruit turning black as it matures and ripens.

What are the causes? It is important to realise this is not a bacterial disease nor is it caused by pests. It is actually a calcium deficiency in the plant. (Calcium is necessary for forming the skin of the fruit, and the blemishes occur when the plant is starved of calcium as the fruit is developing).

Why does it occur?  Whilst it is possible that the lack of calcium could be a problem with your compost or insufficient feed applied to the plants at the right time, it is not difficult to work out why this condition is more prevalent this year. By far the most likely cause is erratic watering as the fruit develops – often unavoidable in the kind of drought conditions we have been experiencing. If the plant is allowed to dry out, it cannot absorb the calcium it needs. Be careful, though – over zealous watering can also have the same effect.

What can you do? Not much, I’m afraid – other than adjusting the watering. The good news is that the condition doesn’t affect the taste of the rest of the fruit – just cut off the affected parts and enjoy the remainder of your labours. The other upside is that it doesn’t necessarily mean that later fruit on the plant will be affected. If the plant gets the right amount of water, fruit now forming could well be fine.

How to avoid the problem in the future? Good quality compost will help but, as I’ve already indicated, not allowing the plant to dry out is the surest way to avoid the problem. Other things that might assist:

  • Choose a variety that is resistant to the condition
  • Plant the tomatoes out at the right time – planting too early when the soil is still cold can affect root growth which will exacerbate the problem
  • Mulch the soil at the base of the plants to help retain moisture as the plants grow
  • Feed the growing plants with a product designed for tomatoes – and resist the temptation to overfeed (this can be as bad as under-feeding!)
  • Add crushed eggshells (high in calcium content) to your compost – most soil will not be short of calcium – but this won’t do any harm and could help if the condition persists

Hopefully, though, next year will not see such prolonged spells of high temperatures – with apologies to those who like it that hot!


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