Was there drainage in the wheelbarrow?
Sounds like a classic case of root rot to me.
Linda C

Sounds like they were too wet. They don't like a ton of water.

Calabrochoa don't like our Southeast hot summers. They are like petunias. They fill the gap between early spring flowers and hot weather flowers, but give up the ghost when the really hot weather moves in. You experience is typical of mine. You don't say where you live in the southeast, but if your daytime temps are hitting in the nineties, I'd bet that is your problem.

yes, there is a drainage hole in the bottom of the wheelbarrow, but I don't think I overwatered, as I allowed the soil to get mostly dry, but not competely, before watering. And there was no sign of rot, anywhere, on the plant--the roots, stem, and leaves were all perfectly normal, except for a few little leaves, at the base, that had died and dried up.

I didn't know that calibrachoa don't do well down here. Why do they sell them, then, at the nurseries, like Lowe's and Home Depot?

Yet I still have one calibrachoa left, a double one, that is fine, except it isn't blooming much. The verbena next to it is fine. I think I need to fertilize, which I don't think I have done, since I put them in, back in late April.

Thanks for all the advice. I live a little south of Atlanta.

Why do they sell them, then, at the nurseries, like Lowe's and Home Depot? I am sorry, but it's a caveat emptor out there. I have seen Tropical Hibiscus at the local Wally World with a perennial tag on it. Sure it's perennial, in a tropical climate it will perform as one. In Ohio, come frost and if left outside it's a dead duck. But, it's still a perennial.