Posted: 08/03/2016 at 17:15
You'd need to access them sam, which means some sort of solid footing - so you might be best with Tetley's suggestion.
Peonies are beautiful in that kind of situation,underplanted with bulbs for spring, and some smaller structural planting with other perennials or annuals to follow on. Strong colour but all unified, rather than a bit of this and that. That gives a more formal look.
If you want a more informal look, that would work well too, but again, you need to get in amongst the planting for deadheading etc. Some structure will help set off the blousier stuff round it