Black bottoms = end rot

Q. My tomatoes growing in containers have black bottoms on the fruit. What is causing this and what can I do about it?

Peggy from Poland

A. This has been quite the wet year, so we have seen very little of this disorder on tomatoes. Yes, it is a disorder – not a disease.

The issue at hand is the uptake and use of calcium by the plant, but other factors as well during fruit production.

If you aren’t taking a soil test to know what to apply or if you apply too much nitrogen, you will have a problem.

Some of the components of fertilizers compete with calcium when getting taken up by the plant.

If certain ones are in excess, they will win the battle and get into the plant.

This leaves calcium levels lower than is necessary for the cell walls in the tomato fruit.

Usually, water is the main issue when it comes to reducing the potential for blossom end rot to become a problem.

In a wet year such as this, water should not be an issue.

In a dry year – or a dry period yet this year – watering at least 1 inch per week will do the trick.

With larger fruit, up to 2 inches per week is best for fruit production.

But, in Peggy’s case, not even water will fix her problem. So, why not?

Peggy is growing her tomatoes in a container. Like most of us, Peggy uses potting mix in containers.

There is a difference between potting mix and potting soil.

Potting mix is a soilless mix. Usually, the components are peat, vermiculite, bark, and fiber. There is no soil.

They have good water holding capacity, but very little nutrition for the plant unless it is added.

Thus, the use of a soluble fertilizer with micronutrients is required on a regular basis throughout the growing season. There are even vegetable fertilizers that contain calcium.

If you use potting soil or amended garden soil, you won’t have the water- holding capacity of potting mix, but you will hold on to nutrients longer and thus reduce the chances of having blossom end rot.

Regular fertilization is still required for tomatoes grown in containers – no matter the soil or mix you are using.

To learn more about blossom end rot on tomatoes, visit

To learn more about potting mixes vs. potting soils, visit

Eric Barrett is OSU Extension educator for agriculture and natural resources in Mahoning County. Call the office hotline at 330-533-5538 to submit your questions. Regular clinic hours are 9am to noon Mondays and Thursdays.

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