Answer: I'd think in terms of an evergreen groundcover for a permanent solution to slowing or stopping soil erosion. If you simply want to mulch the area, you can use a heavy material such as 1-2" river rocks or bark nuggets. After installing the mulch, tamp it down with a lawn roller to make a dense layer that is somewhat interlocked. If you don't walk on the area, the mulch should stay in place.
Good luck with your landscape.
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