We just added a retaining wall to our front flower bed. We have azaleas in the rear of the flower bed just in front of a porch. About 1 foot in front of the azaleas, the ground is lower and the retaining wall is about 16" high. If I place my hosta in the existing dirt, it is way too low and you can't even see it over the retaining wall from the road. It would take a dump truck full of dirt to fill in the entire flower bed. Is there any way to put the hosta in a container possibly raised up on brick to make the plant higher?
Are your Azaleas sitting any deeper than they were before the wall? What kind of wall did you install? These plants are acid loving plants. Concrete causes the pH to be raised. Mortar?
You need to send a picture or two. Azaleas are very shallow rooted and putting any more soil over their roots might just kill them. Same with mulch. Hostas do well in pots. Until the winter.
Where do you live? If you have plants in pots out of doors and you have winters your potted plants have roots exposed to the cold temperatures which could kill them. Otherwise, hostas will do far better planted in the larger body of garden soil before winter sets in.
Where do you live? Please send a picture of your wall. How was it installed? A foundation hopefully or concrete or compacted gravel and drain pipe behind and at the foot of this wall behind this wall to drain the water away from behind the wall? How wide is this bed behind this 16" wall? (perfect to add 2 more inches of a concrete 'cap' for your wall to sit on and sit pots on and use as a table for dinner ware)...