Mulches prevent loss of water from the soil by evaporation. reduce the growth of weeds, when the mulch material itself is weed-free and applied deeply enough to prevent weed germination or to smother existing weeds. Mulches keep the soil cooler in the summer and warmer in the winter, thus maintaining a more even soil temperature. Mulches can improve the soil structure. As the mulch decays, the material becomes topsoil. Decaying mulch also adds nutrients to the soil.