In hot, dry weather, mulch protects the roots and prevents evaporation. In cold, frosty weather it protects the roots from the cold.It also helps to suppress weed growth as prevents soil erosion. The layer of mulch should be at least 7 cm thick.Choose compost, bark chips or grass cuttings which will eventually be dragged down into the soil by the earthworms. This will improve the soil structure as well as providing nutrients and drainage. One could also use pebbles or peanut shells.