Mulching will help to maintain moisture around the root system of your plants thus reducing the number of times you need to water.As an added bonus it will significantly slow down weed growth.
For the best results you will need a minimum thickness of 5 centimetres although the recommended thickness is 10 centimetres.
Do not use black plastic as a weed barrier as it creates problems for the soil/root systems. The soil system is a life system in and of itself. Covering with plastic puts a halt on many natural events such as gas exchange between soil atmosphere and the air, water infiltration and percolation, breakdown of organic matter to feed the microorganisms, and more! Plant roots need oxygen just as much as they need water. Use Mulch or Woven Weed Mat which allow the soil to breath while keeping the weeds at bay.