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While you can put gravel directly on dirt, it won’t be very stable. It also won’t block out any weeds. Most experts recommend layering weed barrier and a layer of packed sand or other foundation material to prevent the gravel from sinking into the earth and being overtaken by weeds.

Sand is a common sublayer for hardscape features like driveways, paver patios, and more. Most projects that involve sand require a 1-inch layer to promote evenness, drainage, and weight distribution. For further drainage, most projects have a 6-inch minimum layer of gravel beneath the sand. 

The ideal amount of mulch is the square footage of your desired depth in inches divided by 324. Aim to keep your old and/or new mulch at a depth of no more than three or four inches.

The most important thing to put under a rock garden is the appropriate soil. The specific soil needed will depend on the type of plants you’re growing as well as the environment you live in. You can also put non-woven or woven fabric under the rocks to add to the garden's structural stability.