Just depends on how you stack them and what you are trying to do. I would arrange them with as little dirt as possible so the Stiltgrass wouldn't be able to survive, but Sedums and Echevarias would.
I put just a little dirt on rocks and stumps and Sedums do just fine....with no weeds!
Mike
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Here's another one.
Most people are surprised that Sedums grow just fine with moss. They might not bloom as much though if they were growing in full sun.
Mike
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here's some Sedum growing on a stump with only half a shovel of topsoil.
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