Lots of the wall tops around here get covered in red valerian which I think look nice. Here is a clip showing some in my own garden including one that has white flowers instead of the usual red. Some Sedum varieties might also be worth trying and I also have some euphorbia doing well on the top of some gate pillars I put together from some leftover stone from the construction of the house where I live.
I have fuchsia magellanica here, in the ground, and in recent years I've had to water it every day thru summer heatwaves yet it it is in a sheltered position. On the other hand, I see sedums and valerian growing out of dry, stony soils and walls and thriving with no attention.
Here is white and pink valerian last May. Looks great with the stone too.
Vendée - 20kms from Atlantic coast.
"The price good men (and women) pay for indifference to public affairs is to be ruled by evil men (and women)."
Some of the stronger-growing rockery plants might work if you want something trailing over the edges - things like arabis and aubretia. Lithodora is lovely but something in the back of my mind is saying it prefers acid soil, and if the wall is built with mortar there might be some lime leaching in over time. Valerian will love that though.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
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Here is white and pink valerian last May. Looks great with the stone too.