I would really appreciate some planting advice, but have to embark on a long explanation first! Bear with me, I'm a novice.
My garden also serves as the roof of a Grade 1 listed grotto (the Shell Grotto in Margate). It is largely laid to lawn, under which sit French drains. This was deemed by English Heritage to offer the best protection from water ingress for the Grotto which is cut from the chalk bedrock (the apex of its ceiling is only a few feet below ground level).
The garden is on a slope and there is little topsoil above the drainage; what there is is alkaline.
We have a flint wall running along one boundary and I would like to create a border along it. This area is in almost full shade and the soil is pretty heavy. It's very close to the Grotto, although not directly over it. What plants to use? Obviously nothing with invasive roots! And ideally I would like to introduce plants that assist with keeping the Grotto dry. A visitor yesterday suggested that irises are great for sucking up damp - would they suffer the shade?
Ideally, I'd love a bank of hydrangeas, or rhododendrons or anything that my other half considers to be 'old lady plants'. But the Grotto might take priority over my wistful longings for a cottage garden!
I'd be grateful for any suggestions.