Sounds like the story of my gardening life. Try to plant a pair of anything and something goes wrong with one of them. My guess would be too wet. Is this container shaded more than the other. Amaranthus like dry and heat. The only time I lose LLB is when it was buried in a bed or border and the roots stay too wet.

Thank you, taitai for your response. Too wet sounds like a good possibility. It seems that each time one of the plants suddenly keeled over, it was after watering. Both containers are in full sun. As I think about it, though, the LLB is on the outside of each pot and I tend to water those containers left to right. The keeled over LLBs were in the left pot getting watered first and the other LLBs safely on the outside right were probably getting less of a direct watering. Blah, blah, blah...I'm babbling.

Anyway, I think you are probably right and I'll try watering less. They are at the entrance to my potager which gets watered all the time because of the vegetable, so I'll have to resist the temptation to give the LLBs a drink every time. I think I'll go ahead and transplant one of the surviving LLBs from the right pot to the left.

Thanks very much!

Lisa