Answer: If you choose groundcover plants with fibrous root systems they will help knit the soil together and slow erosion. If you are installing large decorative rocks you'll want to excavate some of the soil and bury about half of each of the rocks to secure them and make them stable. Almost like "planting" them in the soil. Sedum Autumn Joy will eventually grow large enough to cover most of the rocks, or you could add a groundcover such as Vinca, Ajuga, Wintercreeper, Cornus canadensis (Bunch Berry), Bergenia, or Lamium maculatum to grow up to and partially over the decorative rocks. If you are installing gravel or other stones, the slope can't be too great or they could roll down to the bottom of the hill. Without seeing the property it is hard for me to assess so I can't really make any recommendations about whether or not landscape stones are a good idea. Sorry I can't help more. Good luck with your project!
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