Saturday, July 15, 2006

Blossom-End Rot: Scourge of the Tomato Hordes

Composed July 15, posted July 25, Blogger used to let ya change the date when you had a draft saved up!

Just when you think that nothing stands between you and more tomatoes than you possibly know what to do with, disaster can strike, in the form of blossom-end rot. A brownish or blackish patch appears on the very bottom of a tomato, and spreads. If only a small patch, the tomato can be harvested and used-- just cut the patch away. It's not guaranteed to spread, but it can do so. A really bad case will spread up to the whole bottom half of the tomato, which will quickly become an oozy smelly mess. Heartbreaking!

Here is a small paste-type tomato with a minor case of blossom-end rot. Fortunately, the other tomatoes on the plant seem unaffected. Its brethren on the same cluster are also normal, so this particular tomato must have been having a bad day. I'm definitely going to be keeping an eye out, though.

If you'd like to minimize your chances of finding out about this first-hand, there are a lot of things you can and should be doing. Blossom-end rot often appears first when the weather gets very hot and dry, and the tomato is going into its heaviest bearing phase. For many Bay Area gardeners, especially in the South to mid-Bay, that time is about now.

First, let's look at the factors that go into blossom-end rot. It's fundamentally caused by an inability to get minerals, water, and nutrients in the right ratio to the right place. However, the situation leading to this could be any one, or combination, of the following:

  • Simple lack of water. Lack of water can start blossom-end rot on a day to day basis, but once it starts, it will continue on affected fruits.
  • Lack of available minerals, specifically calcium, and to a lesser extent, magnesium, due poor soil or competition from other plants.
  • Dry, hot winds that evaporate water from the plant faster than the plant can replace it.
  • Over-fertilization, leading to sudden growth spurt demanding more nutrients than readily available. Especially easy if foliar feeding with too rich a mix.
  • Using blossom-set sprays which encourage the plant to start more fruits than it can currently support.
  • Sudden change in watering, such as trying 'drought watering' during heavy fruiting, or having someone over-water plants while you are on vacation.

How to keep this situation from arising?

  • Keep a regular watering schedule of deep waterings, and adjust them appropriately as the plant grows and bears. If you've been watering shallow and often, rather than deep and seldom, you are in for a long summer: your tomato plants probably have not bothered to root deeply, and you'll have to keep watering them frequently to get them through the summer.
  • Get a simple, under $10, moisture meter from the local hardware store, the kind with prongs that you press into the soil. On days which are supposed to get hot, dry, and windy, check your plants before you go into work, or in mid-morning if you have the flexibility. If the soil does not read 'moist' at the 6 - 8 inch layer, and you are due to water within a day or two, do it now.
  • If you see your plants drooping when you get home, and/or when there is not bright sun overhead, mist them heavily to help normalize the water in the plant. They can take it in through the leaves. Look at the top few inches of new growth to decide what's 'drooping'; most tomato varieties have straight tips. A water-starved plant will let the top couple of inches droop over, like someone slumping after a tough day.
  • When you plant your tomatoes, in addition to compost you should add bone meal or a cal-mag supplement. If the supplement is 'prilled', that means it's pelleted: make sure the box says it's safe for veggies. Some iron supplements are prilled with material containing arsenic, for instance, and are unsafe to use on citrus or veggies. The manufacturers shrug and say "the product is labelled for lawns only". Anyway.
  • Mulch your tomato plants with at least an inch of compost, cocoa hulls, straw, or other plant material. The feeder roots near the surface are responsible for accomodating changing conditions in moisture and nutrients to deal with day-to-day growth. The mulch will prevent the top layer of soil from drying out. I prefer compost, though if I had a bigger garden, I'd use straw over compost. The compost gives the plants nutrients directly, whereas cocoa hull or straw will take long enough to break down that it won't contribute significantly to this year's garden, only to next year's when you rake it into the beds after clearing them out. Note that if you are mulching for the first time, water first, then mulch, then give the mulch a soak-down. Otherwise you will interrupt your watering cycle, as the mulch will suck it up. If you are watering under the mulch via soaker or drip, ignore that last, and just spray down the mulch a bit to keep it in one place and avoid having it sponge up water from the soil, away from your plants.
  • Full-grown plants bearing fruit will need an additional dose of nutrients and minerals as they begin to settle down to ripen the fruit. They need this at least a couple weeks, ideally longer, before they REALLY need it, as these things take time to break down to a useable form. Watering on top of compost mulch will take care of the nutrients, by and large, but minerals will need to be sprinkled around the plant (6 - 12 inches from the stem, where the feeder roots are greatest) and scuffled or raked into the compost/mulch. If you are using a drip or soaker that is under the mulch, you'll need to scrape back some mulch and work the mineral supplement lightly into the top half-inch or so of soil. Then re-cover it with the mulch you scraped back.
  • Be cautious about using foliar feeding after your plant is about half-set with fruit. You could push it into overcommitting. If you are a very experienced gardener (much more so than myself), you probably know enough not to overdo it. I'm not sure about it myself, so I don't want to lead anyone astray. I first did foliar feeding this year, while trying to save some tomatoes which had to be moved as almost-mature plants (long story, involving neighbors' construction project). I tried it on some of the non-moved plants, too, but didn't want to go crazy with it. :-)
  • Don't try calcium sprays and the like. From what I have read, those are not going to do you any good, unless possibly you spray them on the soil around the base of the plant. Nutrients fed into the leaves tends to stay in the leaves, giving them more 'excess' sugar production to send to fruits. Dissolved minerals, on the other hand, especially calcium and magnesium, form layers of mineral salts on the leaves if used heavily, and can interfere with normal photosynthesis. Think about the white stuff that gets all over your nice shiny plumbing in places with really 'hard' water-- same kind of thing. Not what you want on leaves.

I hope this helps you with your tomatoes. If somebody from your local Master Gardener extension program says something different, listen to them and not to me! Otherwise, I think I've got the basics here. Good luck, and good gardening.


Blogger Genie said...


This is an amazing and really useful post -- thank you so much for sharing it!

The Inadvertent Gardener

6:52 PM PDT  
Blogger Carol said...

I think you did a good job of covering the basics of blossom end rot. Thanks for an informative post.

6:19 PM PDT  
Blogger gardenbuff said...

Thanks for the comments. If I may add - a product I have used for blossum end rot and other problems is a liquid calcium product called Nutri-Cal and can be purchased at The calcium is absorbed directly into the fruit.

It is safe and really easy to use - I just spray it directly on the fruit twice a week.

You might want to try it along with the other excellent suggestions.

7:10 AM PDT  
Blogger david appelman said...

Let me first say I love that you are growing a victory garden and helping other do the same! I have some friends in the Bay area that are having difficulties tearing themselves away from their lives and letting their crop die.

You should consider getting a professional soil sample, I know that here in Florida they cost anywhere from zip to 12 bucks. I normally pay 8 for mine. Once I get the results I just sprinkle a bit of lime and some nitrogen, and all my plants are happy again!

Here is a guide that explains how to do it, and where to send it based on your area. I hope it helps! (long link I know)

7:49 AM PDT  

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