During tomato growing season we get regular queries about the problem of blossom end rot. Blossom end rot is sometimes found on tomato and eggplant. It is a disorder associated with a low concentration of calcium in the fruit. It begins as a small water-soaked area at the blossom end of the fruit while the fruit is green or during ripening. As the lesion/spot develops, it enlarges, becomes sunken and turns black and leathery. In severe cases, it may completely cover the lower half of the fruit, becoming flat or concave.
We recommend adding gypsum to your growing soil before planting tomatoes to help prevent blossom-end rot, especially if you grow them in containers (not Earthboxes). Gypsum is calcium and sulfur in a form that allows the calcium to be available to plants.
So, because we are in the desert and because our water here is hard – has lots of calcium – we called on our good pal grower Gregory Ware of Dos Arbolitos nursery to help answer the question of blossom-end rot on tomatoes that are grown in Earthboxes and other containers. Gregory has taught the Earthbox Tomato workshop here at Southwest Gardener many times.
Here’s Gregory’s response:
“Yes, BER (Blossom End Rot) is caused by irregular watering, and lack of calcium. I’m guessing that it’s not a watering problem if the Earthbox or container is being watered every day. Sometimes you may need to add water twice a day. Shading the Earthbox may help. You could also try adding the calcium hydroxide to the water, (pickling lime, Earthbox manufacturers suggest this). In my “guestimation,” that may only complicate things. When the boxes were planted we all added the Dolomitic lime, and that should be enough calcium to get them through.
“Here’s the kicker, we have hard water (lots of calcium). As one continues to add calcium rich water to the Earthbox, the soil becomes more alkaline. The calcium collects in the soil and becomes unavailable due to the alkalinity. So in actuality, there is a ton of calcium in that Earthbox. There are calcium foliar sprays that one can buy, which may help. I would recommend trying (I stress try) this: Dissolve 1/8 teaspoon citric acid in a gallon of water, then pouring the water into the Earthbox. Do not put the citric acid in the hole and then add water. Maybe do this twice a week, any more than twice a week could cause problems. Mixing it stronger could cause problems. I have been experimenting with this myself.
“I am not sure if this is the answer, but I want to find out if it will help. The thought is that if you add acid to the Earthbox, it will help counteract the alkalinity and make the calcium that is there available or at least less tied up. This fall, I am going to try using gypsum in the Earthbox instead of Dolomitic lime, to see if that makes any difference.
“Some things that may figure into this are the up and down temperatures and the wind. When it is hot and windy, the plant could be letting out more water than it can pull up out of the soil. This scenario mimics erratic watering, the reality of it is the plant is stressed for water even though it is standing in water.
“By the way, my tomatoes in the Earthbox are loaded. A few of them have BER, most are OK. The BER started with the windy days and now seems to have stopped.
“Sometimes there is no answer to blossom end rot.”