Preventing Blossom-End Rot on Tomatoes

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A severe case of blossom-end rot. Photo: http://www.smallfootprintfamily.com

Blossom end rot is a common deficiency disease seen in tomatoes, but also peppers and squashes. It’s characterized by a lesion that forms on the tip of a young fruit ready to ripen, on the opposite side to where it is attached to the plant. This is the point where the flower was originally found, thus the name “blossom-end” rot.

The lesion is light brown, small and watery at first, then grows and becomes dark brown or black, sunken and hardens. The lesion may eventually cover more than half of the fruit and can be invaded by other organisms.

Blossom-end rot occurs when fruits are growing rapidly and therefore have a high need for calcium, yet are unable to get enough. The solution might seem to treat the plant with a calcium-rich fertilizer, such as chicken manure or almost any other organic fertilizer (nearly all contain calcium), and in fact, applying calcium is often recommended as a treatment. However, studies show that simply applying calcium has no significant effect. Even if the calcium-rich fertilizer is sprayed directly on the plant’s foliage and no other treatment is applied, the calcium tends to remain in the foliage and very little reaches the fruit.

The Real Culprit: Moisture Stress

Keep tomato plants well-watered and you’ll never see blossom-end rot. Ill.: hubpages.com

In fact, blossom end rot is almost never due to the absence of calcium (calcium is abundant in most garden soils), but to the inability of the plant to absorb calcium from the soil. And that is most often due to irregular watering. If the plant lacks water during the critical period of fruit formation, less sap reaches the fruit which will therefore not receive its share of calcium and voilà! Blossom end rot sets in. Typically, blossom-end rot occurs when the plant is repeatedly stressed by irregular watering or rainfall, going from very dry to moist to very dry again. It tends to occur more often in container-grown plants … because they dry out very quickly.

Blossom end rot almost never occurs when tomato plants are mulched. Photo: http://www.tomatodirt.com

The solution? Always ensure constant moisture to the roots of tomatoes, peppers and squashes and blossom-end rot is unlikely to occur. Applying mulch to the soil at the base of the plant is ideal because it helps keep the soil evenly moist.

Epsom Salts Aren’t Helpful… At All!

My dad used to treat blossom-end rot by watering his tomatoes with a solution of Epsom salts … and it worked! But not because of the salts! Epsom salts are simply magnesium sulfate. They can add sulfur and magnesium to the soil. But, as you’ve read, blossom-end rot is due to a lack of calcium, a very different mineral. Watering with a solution of Epsom salts can therefore help tomato plants … not because of the salts themselves, rather because of the H2O they were diluted in. Read more about Epsom salts in the garden here: Garden Myth: Read Epsom Salts as a Cure-All.

Other factors to consider are:

  • Adjusting the soil’s pH to close to 6.5. Calcium tends to remain insoluble and thus unavailable in soil that is either too acid (pH below 6) or too alkaline (pH above 7).
  • Avoiding the excessive use of nitrogen-rich fertilizers (those with a higher first number, such as 15-10-10). They cause overly rapid green growth, draining calcium to the plant’s foliage rather than its fruit.
  • Avoiding hoeing at the foot of the plant. This severs plant roots and thus disrupts the flow of calcium-bearing sap to the fruit. Here again, mulch can come to the rescue! A good mulch prevents weeds from growing, so there will be no need to hoe the soil around the plant and blossom end rot will therefore be less likely.
  • Some varieties, like ‘Big Boy’, ‘Fantastic’ and ‘Whopper’ and most paste tomatoes (‘Roma’, for example), are more sensitive to blossom-end rot than others.

Adapted from an article originally published on August 5, 2015.

Garden Myth: Epsom Salts as a Cure-All

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If you search the Internet, you will see many sites that advocate the use of Epsom salts in gardening. Some claim that it stimulates extraordinary growth, others that it makes tomatoes bigger, encourages roses to bloom more heavily, makes foliage greener, strengthens plant roots and even keeps slugs at bay! Some of these sites are actually sponsored by Epsom salts producers (it’s soooo easy to hide one’s identity these days!), but others come from sincere gardeners who really think they have found a miracle product and want to share it with the world.

Epsom salts got its name from the town of Epsom in Surrey, England, because it was originally produced by boiling down mineral water taken from the town’s springs.

I have to confess: I myself spread the false information about Epsom salts for many years. I’d learned about it from my father, who used them regularly on his tomatoes and roses, and just never thought to check into it. Then one day a reader questioned me about whether Epsom salts were really as useful for tomatoes as claimed and I started to check into things. Wow! I really was way off base. Mea culpa! I’ll now try to save my reputation by sharing the correct information.

Certainly Not a Miracle Product

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Epsom salts. Photo: Chemicalinterest, en.wikipedia

If you want to understand what Epsom salts can and cannot do, you have to understand what they are. They are actually just a simple chemical: magnesium sulfate (MgSO4·7H2O). They contain magnesium and sulfur as well as oxygen and water. Yes, you could use them as a fertilizer … but they’ll only be useful if the soil is lacking in magnesium or sulfur. And few soils are.

Their utility is therefore limited to those few cases where either the soil is deficient in either magnesium or sulfur or where those minerals are in some way locked up and unavailable to plants. Those are the cases where plants would benefit from treatment with Epsom salts.

Personally, though, if I suspected a plant was suffering from a mineral deficiency (if its foliage was abnormally yellow, red or stunted, etc.), I certainly wouldn’t choose Epsom salts as my treatment of choice, but rather a complete biological fertilizer, like seaweed fertilizer or fish emulsion, one that contain all the major and trace elements. After all, it may not be magnesium or sulfur the plant is lacking, but another trace element: zinc, iron, boron, molybdenum or whatever. When not cover all the bases rather and give the full range of minerals rather than only two? You can even treat mineral deficiencies with compost, another product that almost always contains the whole range of trace elements, although compost takes longer to give results.

If you apply sulfur or magnesium is applied to a soil that doesn’t need it, especially repeatedly, both products will simply work their way into the water table or lakes and rivers: yet another pollutant! How unfortunate!

Specific Cases

Blossom End Rot

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Blossom end rot on tomatoes. Photo: Scot Nelson, Flckr

A shot of Epsom salts is often recommended to prevent or cure blossom end rot in tomatoes. In this deficiency disease, the end of the fruit opposite the stem blackens and starts to rot.

So, Epsom salts to the rescue! Dilute them in water, water the plant and the fruits that follow will be cured of the disease. It seems to work!

But it didn’t.

The blossom end rot is caused by a calcium deficiency, a not magnesium or sulfur one. Usually it is not so much that calcium is lacking in the soil (it’s very common in most soils), but that the soil was too dry and the roots of the plant therefore couldn’t absorb it properly. So the basic treatment is simply … to water the plant regularly. Avoid irregular watering that leaves the plant drought stressed half the time and soaking in water the other half. Instead, learn to keep the soil evenly moist and blossom end rot will disappear as if by magic. You’d have had the same result whether you added Epsom salts to the water or not, because it’s the water that makes the difference, not the minerals that are dissolved in it.

Big Tomatoes

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If you want giant tomatoes, plant varieties known for their gigantic fruit, like ‘Big Zac’. Photo: zra’s Organics

To grow the biggest possible tomatoes, fertilize your tomatoes correctly with compost or a complete fertilizer (one that contains the full range of minerals, including all the trace elements), water regularly to keep the soil evenly moist, offer good growing conditions (full sun, protection from drying or cold wind, etc.) and you will have beautiful tomatoes as big as that variety ever gets. Applying Epsom salts won’t make the fruits any bigger. If you want really giant tomatoes, grow a variety known for its huge fruits, such as ‘Big Zac’, and care for it well.

All-Purpose Fertilizer

Some sites recommend Epsom salts as a sort of all-purpose fertilizer for any plant you grow, claiming that it will give extraordinary results, especially keeping leaves green and stimulating root growth. And it will help give nice green leaves and good root growth… but only if the soil is lacking in magnesium or sulfur. And most soils already contain both elements in sufficient quantities to maintain healthy plant growth.

If, for some reason, you suspect your soil is an exception to the rule (very sandy, acid soils do quite often lack magnesium), have it properly analyzed. Send in a soil sample to a specialized laboratory and if, when the results come back, they indicate a lack of magnesium or sulfur, yes, you could apply Epsom salts.

It’s far more likely that the analysis of naturally poor soil will show other deficiencies as well. If so, the most logical treatment would be to apply a complete fertilizer (one that offers the full range of trace elements) or compost (if you’re not in a hurry) rather than Epsom salts.

Rose Fertilizer

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For beautiful roses, a good complete fertilizer will give better results than Epsom salts.

From the 1930s through the 1960s, several well-known rosarians recommended Epsom salts applications to stimulate beautiful growth and abundant flowering in roses … but that’s not so today. Although each expert still has their preferred fertilizer regime—some prefer complete fertilizers, others mixtures of products, etc. —. few today still recommend Epsom salts, widely considered outdated.

There’s no use looking for studies showing the efficacy of Epsom salts on roses: they simply don’t exist.

Disease Prevention

No reliable study shows that treating with Epsom salts prevents or cures infectious fungal, viral or bacterial plant diseases. On the other hand, of course, if you consider mineral deficiencies as diseases,  Epsom salts can be used to treat magnesium or sulfur deficiencies .

Insect and Slug Repellent

There is no evidence that Epsom salts can repel harmful insects or mollusks. That one seems to really be only a garden myth.


In conclusion, Epsom salts are only really useful as a garden product in the rare situation of plants or soil suffering from magnesium or sulfur deficiency. Even then, an application of complete fertilizer or even compost would have given the same result without requiring the purchase of a special product. In the gardening world, Epsom salts are pretty much useless.

References

Miracle, myth … or marketing: Epsom salts

Epsom salts for Plants