mulch, any material, usually organic, that is spread on the ground to protect the soil and the roots of plants from the effects of soil crusting, erosion, or freezing; it is also used to retard the growth of weeds. A mulch may be made of materials such as straw, sawdust, grass clippings, peat moss, leaves, or paper. For large areas under cultivation a tilled layer of soil serves the purpose of a mulch.