Experts and BHG readers answer.
Black bottoms on tomatoes in pots
Sounds as if your tomatoes are getting blossom end rot. This is a problem often caused by irregular watering. It's especially difficult to keep the soil moist at all times, when the plants are growing in patio containers. It's best to provide an organic mulch such as compost which helps to keep the soil damp and be sure to keep them evenly watered. The size of your tomatoes will depend upon the variety that you are growing.