Many different materials make good garden mulches. For weed control and moisture retention, apply at least three inches of mulch. How often mulch needs to be replenished depends on the mulching material. Leaves and shredded wood mulches decompose fairly quickly and need to be replenished frequently. Mulches such as rock and gravel rarely need replenishing. Mulching your garden will reduce the amount of time you spend weeding, watering, and fighting pests.