What are the best perrenials/small bushes on top of a rock wall?

I have a rock wall (boulders) and I laid down landscape fabric at the top to prevent weeds, grass from growing. I have various perennials planted but I'm losing the dirt between the rocks. Are there other plants I should put up there or plants in between the rocks to hold the dirt? I'm having to replace dirt that has washed out. I live in Wisconsin so we need hardy plants. most of the wall is in the sun but one end has part/shade. Thanks!!

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