Fast-growing plants need plenty of water during warm weather to keep them growing and producing crops. Adding a thick layer of mulch after watering or rainfall helps to stop the water evaporating from the soil surface in the heat of the sun, and keeps the moisture close to the roots, where it is needed.
Simply add a thick 5cm (2in) layer of garden compost, well-rotted manure or bark chippings around the base of plants such as tomatoes, courgettes, and beans on a regular basis. Lay it on the soil surface and do not dig it in.
TIP Mulching helps to cut down on emerging weeds by smothering them and covering the seeds so they don't have enough light to germinate.

references: BBC 2011 gardeners book