Garden Advice: Blossom End Rot

blossom end rot - public domainYou’ve put in the effort and grown your own juicy tomatoes. But when you finally go to pick them, you realize that the end opposite the stem is sunken and oozing. Yuck—it’s disgusting. Who wants to eat a tomato that’s rotting on the vine?

What you have is a tomato with blossom end rot (BER). It’s just what the name suggests—the blossom end of the tomato (where the flower fell off long ago) is decomposing, ruining the fruit. (Yes, we all know that tomatoes are fruit.) The question isn’t identifying the problem, it’s solving it. How can we keep our tomatoes (and peppers, watermelon, cucumbers, and squash) from succumbing?

Long ago, it was believed that BER was caused by a calcium deficiency in the soil. While it’s true that in cases of BER, the developing tomato doesn’t have enough calcium, the issue lies with the plant’s inability to transport that calcium from its roots to its fruit.

It’s quite rare for soils to be deficient in this very common mineral. For example, here in Colorado the soil is largely made up of decomposed limestone, also called calcium carbonate (CaCO3). No shortage here!

Unless a soil test has indicated a shortage, adding more calcium to the soil won’t help—and may cause harm, as it raises the pH. Adding eggshells won’t help. Neither will burying a Tums under the roots, liming the soil, adding a dead fish to the planting hole (although you may attract a raccoon), side dressing with bone meal, or adding Epsom salts (MgSO4). As you can see, Epsom salts don’t contain calcium. There, we’ve just eliminated 99% of the advice you’ll find online.

So how can we get more calcium into that tomato fruit? Foliar sprays won’t work because calcium is only moved around the plant in the xylem, from the roots to the leaves, and not from the leaves to the fruit. (However, one study suggested that providing the leaves with extra calcium may leave more of the calcium from the roots available to the fruit. It needs further research.)

Why not simply spray the fruit with calcium? Calcium applied on the skin has no way to enter the fruit—the skin is thick and waxy, and there is no way through it. Scratch that idea.

Adding fertilizer can do more harm than good. An imbalance of fertilizers can interfere with calcium absorption at the root level. Also, fertilizers are high in salts, and high salt levels also interfere with the roots’ ability to absorb calcium. High nitrogen levels push the plant into growing more leaves, which compete with the fruit for what calcium is available.

So what can we do about BER?

We can encourage a healthy root system, so that the calcium available in the soil is taken up by the plant. Cold, wet soil isn’t conducive to root growth, so wait until the soil has warmed before planting.

Erratic soil moisture levels make BER worse. Careful irrigation and mulches may help. Be sure to check the soil regularly to see if more water is needed. Sadly, we can’t do much if heavy rains saturate the soil.

Choose varieties that are more resistant to BER. While no cultivar is BER-proof, Celebrity, Mountain Pride, and Pik Red appear less likely to have problems, while Big Boy, Whopper, and Fantastic are more susceptible. Read the catalogs carefully.

If the length of your growing season allows, the best approach may simply be patience. Typically, the first tomatoes of the season are the ones that succumb to BER. Pick off any early fruit that show signs of rot, as you can’t cure it once it starts. It’s likely that subsequent tomatoes will be fine.

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