Inorganic mulches, such as rock, can also hold in moisture. Mulches also improve water penetration and minimize soil crusting. Some people prefer to use spun or woven landscape fabric under wood chips or gravel. Unlike solid sheet plastic, which is no longer recommended, these materials promote air and water exchange within the soil. Mulches also control temperatures around plants.
Organic mulches are cooler than rock mulches in the sun.
Rocks absorb and store heat for long periods, reradiating heat to plants. Organic mulches are often used to alter temperatures in other ways. Applying mulches at planting keeps soil temperatures above freezing, which protects fall transplants and allows a longer period for root growth before winter. To protect plants from winter drying and frost heaving, apply mulches after the ground is frozen. This protects small bulbs by delaying spring growth until weather is warmer and more settled.