Blossom End Rot And How To Avoid It – Part One

I seem to spend a lot of my time, at this part of the season, looking at my tomatoes from underneath – the blossom end!

Discovering that your tomatoes are affected by Blossom End Rot (BER) is one of the most frustrating experiences when growing tomatoes.
They look great from above, but just as they are about to be picked and eaten, you find that the fruit are suffering from black/brown leathery patches on the underside – what a big disappointment!

Blossom End Rot and How To Avoid It - Blossom End Rot in Tomatoes

The tomatoes above are so badly effected, they are useless! I’ve had seasons when almost every tomato on a plant was affected by BER.

Hopefully, this article should help youunderstand Blossom End Rot and how to avoid it next season!

So what is Blossom End Rot?
BER is a calcium deficiency. As a tomato grows, calcium is required to develop the fleshy wall inside the skin.
If calcium runs out or the supply of calcium is interrupted, a dark leathery patch will form in the growing area where calcium was absent. This is normally the underside or blossom end of the tomato.

Availability of calcium is the key to avoid BER
In order to avoid the problem, we have to make sure that calcium is available at all times, without interruption, when the fruit are swelling.
This can be quite difficult to achieve given the many different circumstances in which tomato plants are grown and unpredictable weather conditions and temperatures – all of which can effect calcium uptake.

Translocation – Moving nutrients from lower leaves to the growing tip and fruit
When more magnesium is needed by the growing tip or fruit of a plant, than can be absorbed by the roots, extra magnesium can be taken from other parts of the same plant – usually the lower leaves.

Some nutrients are mobile
Some nutrients move around a plant’s system quite freely such as nitrogen and magnesium and are able to be moved to the areas of greatest need – usually the growing tip. These are called mobile elements.

Calcium is immobile
However, nutrients such as calcium and Iron are immobile and are unable to move from one part of a plant to another.

The only place that a plant can obtain calcium, when more is needed, is directly from the soil.**

Plants cannot move calcium from lower leaves to growing tip if required, as nitrogen or magnesium is able to be moved.

This means that even a short interruption of calcium from the soil may result in BER.

This process of moving nutrients around a plant’s system is sometimes referred to as translocation. Calcium cannot be translocated.

Dry soil is nutrient barren
Roots can only absorb nutrients when nutrients are dissolved in water.
If the soil is dry, a plant will be unable to absorb nutrients and calcium intake will be interrupted. However, the mobile elements such as magnesium may be moved from lower leaves to the growing tip when there is a shortage, but this cannot happen with calcium because, as already explained, calcium is immobile.
A good reason to keep soil moist and not allow it to dry out. When the calcium supply is stopped, even for a short time when the tomatoes are swelling, BER is likely to appear.

Some advice doesn’t explain why!
When I first started growing tomatoes I was bugged by the advice: “Keeping the soil moist helps avoid Blossom End Rot”. It seems a rather unclear piece of advice!

It was some years later before I understood that BER is a calcium difficiency and calcium is an immobile nutrient or element.

Still, had it not been for calcium, and my desire to understand and pass on the information, I don’t think I would have started the Tomato Growing website and Newsletter.

Next week, in part two, there’ll be tips and suggestions on how to foliar feed** to help avoid BER.

I hope that many of you are tasting the results of your hard work (or soon will be!).



See also: Blossom End Rot – Part Two


11 Responses

  1. Robert Smith
    | Reply

    Hi Nick Firstly a big thank you for another very informative newsletter, long before I started seeing your bulletins I was blissfully unaware of the many and varied pitfalls and potential problems associated with growing tomatoes, now though I realise how lucky I must have been in my first couple of years of attempts. When I read some of the responses from your other recipients I think how knowledgeable they appear, however I have in my little 8 by 6 greenhouse a number of plants all of which have quite a lot of trusses of fruit, just waiting for them to begin to ripen, so once again and with the benefit of your weekly newsletters, it looks as though I am going to be quite fortunate and have a reasonable crop.
    If I may, while I have the opportunity, could you advise me please, if and when I should stop adding nutrients to my Quadgrows, I seem to remember reading once upon a time somewhere that you should stop feeding after the fruits have formed !
    Many thanks again Nick, I don’t suppose we shall have the pleasure of too many more newsletters, how unfortunate that will be.

    • Nick
      | Reply

      Hi Robert, usually we change from a balanced feed (or feed already in the compost) to tomato – high potash feed when the tomatoes are around pea size. You would stop feeding once the tomatoes on the top truss have ripened.
      I’m pleased that you find the newsletter useful.

  2. Sue Heyer
    | Reply

    Hi Nick
    I feed my toms once a week with a specified tomato fertiliser. Inbetween feeds I water them with tapwater (via the hose). Our water is very hard, i.e. has lots of calcium (have to descale the kettle at least twice a week!). Do you think this calcium content is enough to protect against BER or is it the wrong type of calcium? BTW haven’t seen any sign of BER yet (touch wood!)
    Regards, Sue

    • Nick
      | Reply

      Hi Sue, I think you should be OK, with calcium in the soil and in the tap water. If you are growing cherry tomatoes they are rarely affected by BER but the larger or fleshier the tomatoes, the more prone they are to BER.

  3. Anonymous
    | Reply

    hi Nick,
    i used to suffer from BER often as you know, but since i switched to using the hoselock growbag watering system 3 years ago i have had none, but be carefull using these watering systems not to fill to the max water level, only keep half topped up

    • Nick
      | Reply

      Thanks for the tip on half filling the reservoir!

  4. Michael Johnson
    | Reply

    Hi-Nick, and all, f you want to avoid and do awahy with all the problems of B.E.R, here is what you have to do, (Tried and tested) it works for 99% of the time,
    When the flowers are just forming earlier on , and before the strt to open, water them with a 3 to 5 litre can of water which has added to it 3 tablespoonfuls of (Calcium Chloride) which can be obtaeine quite cheaply from either e-bay or other places on the internet, I used to use calcium Nitrate but after trials find that Calcium chloride is better for results, this stops b.e.r from ever forming in the forst place and need only be repeated about twice during the early part of the season until the little tomatoes are about as big as marbles or walnuts,
    Then another very good method is to add some Zeolite to your potting mix or you can add it at a later stage if you wish and just mix it into the top three inches of compost, zeaolite is a sort of compound rare earth tht can also be bought from e-bay cheaply or other places on the net, what it does is keep the compost just nicely damp or moist and never lets it dry out completely, and gives you another eight days or so on top of normal compost before you have to water it, this goes a long way to prevent and stop B.E.R. -try it you will be amazed, I use both at the same time and so far not one single case of B.E.R has been seen and the tomatoes are now as big as clementine oranges, giant or very large varieties and also elongated are much more prone to it than others.

  5. Rhys Jaggar
    | Reply

    My experiences with BER say that three things contribute:

    1. The tomato strain – some seem very sensitive, others relatively immune.
    2. The type of pot used – breathable pots dry out quicker and sensitive strains succumb more easily in e.g. Rhizopots if they dry out. I have never seen a tomato with BER grown in a quadgrow yet.
    3. Bursts of extreme heat. We had 5 days of 30-34C here in June and despite watering once or twice a day, sensitive strains still succumbed. It so happened that I had not at that point started treating plants with Epsom salts as I assumed the compost still had nutrients in it at that stage….

    I have seen no BER on Super Marmande despite growing them in a rhizopot, whereas San Marzano, fed and watered identically in same Rhizopots, saw a raft of fruit removed due to BER in late June.

    I have grown Zenith in Rhizopots in the past and had some BER, but in Quadgrows they never succumb.

    My solution for traditional pots in hot weather is standing pots on saucers and maintaining a reservoir in the saucer at all times. This is superior to drenching from above for some reason. It seems to work well for Shirley, Alicante (which does get BER in extreme heat if it dries out), Black Russian and Tigerella. Obviously, you do not stand roots in water in cool weather….

    I tend to only grow certain strains in Rhizopots over several seasons if they resist BER well. Maskotka, Sungold and Super Marmande are my best three for that.

    I am still struggling with the challenges of Black Krim and BER, whereas I have never yet managed to impose BER on Black Cherry no matter what I do to them….

    • Nick
      | Reply

      Hi Rhys, Thanks for your experiences of BER – very helpful information and advice to follow!

  6. Rob
    | Reply

    Hi Nick, Thank you for another interesting and informative newsletter.

    My indoor tomatoes (ring culture) suffered BER for many years until, thanks to one of your previous years’ newsletters, I discovered I had been overfeeding them. If I remember, it was the supply of excess feed to the plant that prevented the uptake of Calcium. Since then, I have taken your advice that underfeeding is always better than overfeeding. This year, my plants look the healthiest ever with no sign of BER. Cheers!


    • Nick
      | Reply

      Hi Rob, it’s good to know that the advice works! Overfeeding can block the uptake of calcium but it is often difficult to track down the cause of BER until you’ve have a few seasons to experiment – as you obviously have.

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