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Replace organic mulch every one to two years, depending on the variety. Wood chips and straw, for example, will last a couple of years, while grass clippings and pine needles need replacing every several months. Inorganic mulch—such as rock and rubber chips—can stay in place for up to four years, depending on erosion and general garden upkeep. 

A 40-pound bag of topsoil covers 0.76 cubic feet, enough for 4.5 square feet with a 2-inch depth or 36 square feet with a .25-inch depth, such as topdressing a standard garden bed. If you want 2 inches of fresh topsoil for your 36-square-foot garden bed, you’ll need eight 40-pound bags.

Pea gravel can retain some heat, but it generally stays cooler than materials like concrete, asphalt, or stone pavers. Lighter-colored pea gravel reflects more sunlight and tends to stay cooler, while darker-colored gravel can absorb heat and become warmer to the touch.

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.