To ensure your garden drains properly, it’s important to layer rocks to their correct depth. The depth of your rock layer depends on the size of the rock you’re using.  --Stones smaller than 1 inch: Layer 2 inches deep.  --Stones 1-3 inches in size: Layer 1-3 inches deep. --Stones 3-8 inches in size: Layer 3-8 inches deep (So if stones are 5 inches large, you’ll want to layer 5 inches deep).