However, as a barrier to the development of weeds, a mulch is only affective if a deep enough layer is used, and particularly pernicious perennial weeds are eliminated before applying the mulch layer. A typical depth for effective weed control is about 5 to 8 cm. In addition to weed control, mulches have other benefits. These are in terms of improving soil environment for plant growth. Such benefits include conservation of soil water, avoiding the extremes of soil temperatures, supplying nutrients to the plants, and lastly, mulches can improve the physical condition of the soil.
Mulches can aid water conservation in a garden.
Covering the soil with a layer of mulch will reduce the 'waste' of water through direct evaporation from the soil surface, as the main loss of soil water occurs through solar energy evaporating water from the soil if the soil surface is wet. Evaporation occurs directly from the soil surface or indirectly as transpiration from the leaves of plants when the water lost is replaced from the soil via the roots. It has been calculated that on a hot sunny summer's day 1 m-2 of bare wet land surface will lose 4 litres of water through evaporation or 1 gallon of water per square yard. Soils differ widely in the extent to which they are self-mulching. These microscopic pores behave as a network of minute channels through which water can more upwards easily to replenish water lost from the surface through evaporation.