Wall flowers for a retaining wall

I have a natural stone retaining wall and I am thinking of adding some wall flowers or perennials that would droop over the wall and give some added colour. I don’t want them to take over as the wall is very nice so I am thinking they would droop over no more than one foot or so. Thank you for your time. We have recently buit a pergola and would like to plant wisteria with intention that it will cover it in time. However we have been told that the planters are too small to have wisteria they measure 36inchs x 14.5 inchs & 18.5inchs deep and built of brick. Would you think they are too small ? The pergola has 6 sets of planters and posts, if we do have the suitable size how many plants will i need to achieve complete coverage. i have sent you a photo. If its a case wisteria would not be suitable what could you recommend insteadCan you suggest any flowers that would work and also that would be perennial? Are there any that would flower for most of the summer?