Q: My wife and I are growing tomato plants in two locations. Mine are in the garden, hers are in containers on the back deck. Tomatoes on her plants are turning black on the bottom. All ten of mine are much healthier and have more tomatoes than hers, not that we’re having a contest or anything. Any ideas as to what can be causing the blackened tomatoes?
A: “Not that we’re having a contest or anything…..”
Blossom end rot is common on tomatoes growing in containers. The condition is caused by a lack of calcium in the fruit when it is young. A container usually doesn’t have enough soil to moderate moisture swings and heat accumulation during the day. This leads to poor calcium uptake and sap movement problems.
In case you do decide to have a contest in the future, I recommend your wife start with one tomato plant per half-barrel-sized container, no smaller. Use a planting soil mix specifically designed for outdoor containers. Avoid heavy fertilization.
The results will be a lot more in her favor.
I used to recommend adding lime or gypsum to the soil before planting tomatoes. It turns out that adequate soil moisture is the real thing to manage.
Here’s some interesting research. Garden Myths About Blossom End Rot