I have felicia, simple white geraniums (pelargoniums), scented leaved geraniums with lighter green leaves mid pink flowers, and a very pretty bright blue creeping Veronica which I’ve previously only used in beds. I’ve also put in a compact phlox to see how it gets on, but it needs more water I think. I usually have more traditional bedding plants but last year only my verbena did well.
My troughs are on the ground, so nothing needs to trail much, otherwise I’d probably have gone for white ivy leaved geraniums.
I’m looking forward to see which plants others have found to do well in the heat.
I’ve also got coriander, rocket and basil in little self watering troughs and they’ve done brilliantly.