Understanding Blossom End Rot on Tomatoes

Blossom end rot on beefsteak tomatoes (Solanum lycopersicum) caused by irregular watering
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What is Blossom End Rot?

If your tomatoes look black on the bottoms, they're probably suffering from blossom end rot.

Blossom end rot can be identified by a black or dark discoloration, usually with a water-soaked appearance. It can also appear on peppers and eggplant. The good news is that it's not a disease, but rather the result of a calcium deficiency when the fruit was forming.

Causes of Blossom End Rot

Particularly in container gardening, the calcium deficiency that causes blossom end rot is often caused by inconsistent watering.

If the soil gets too dry, the plant isn't getting the calcium it needs in order to produce healthy fruit. If the plant gets too much moisture, the same thing can happen.

Blossom end rot can also be the result of over-fertilization during early fruiting.

Blossom end rot sometimes occurs in Earthboxes, which have very consistent supplies of water and just the right amount of fertilizer and dolomite, which should provide all the calcium the plants need. In an Earthbox, blossom end rot might be the result of unusually rapid growth—the plant can't take in enough calcium quickly enough.

How to Stop Blossom End Rot

The good news is that if you growing indeterminate tomatoes (that set fruit all season) and you have a few tomatoes with blossom end rot, it doesn't mean that all your tomatoes will be affected. Even without treatment, some of your later season tomatoes may be fine. 

Here are some ways to prevent blossom end rot from happening in the first place:

  • Harden off plants slowly
  • Don't plant tomatoes in cold soil,
  • Don't over-fertilize, especially with high nitrogen fertilizer,
  • Don't under-fertilize. Tomatoes are heavy feeders and potting soil, unless it is pre-fertilized, doesn't provide the nutrients tomatoes need.
  • Use high-quality potting soil that drains well.
  • Use good watering practices. Don't let your tomato plants dry out. Keep the soil moist, not wet
  • Add dolomite or lime to potting soil when planting.
  • Some people say that adding crushed egg shells and watering with diluted milk or yogurt can add calcium and prevent blossom end rot.

Fend Off Blossom End Rot in a Grow Box

If you experience blossom end rot in a grow box, mix 1/4 cup of lime with one gallon of water and pour it into the reservoir. Only do this once. This should fix the problem.