The best mulches are loose materials that allow most rain or irrigation water to run through to the soil. They must also allow free access of air to soil and roots. The depth of the mulch should vary with its coarseness, from shallow for the finest to thicker for the coarsest (2–6 cm). Garden compost, leaf mould, dry lawn clippings, well-rotted animal manure, straw, tan and pine bark, lucerne hay and sawdust are common mulching materials. Do not dig mulches into the soil (especially bark materials and sawdust). All will eventually decompose naturally and become integrated with the topsoil. Mineral substances, stones, pebbles, gravel and sand are used as permanent mulches in gardens planted with trees and shrubs. Woven weed mats, too, are available by the metre from garden suppliers. These effectively suppress weeds while still allowing oxygen and moisture to penetrate the soil. In ornamental situations they need to be anchored in place and disguised by a more attractive layer of organic mulch.
Over-watering is wasteful because it washes out plant nutrients. A common mistake is to over-water in winter when plant growth rate is reduced. (In the case of deciduous shrubs and trees, the plants are dormant.) Over-watering in winter often happens with summer-growing lawns like couch grass. If a plant is not growing actively, little or no water is needed.
Wet soil may also favour the spread of some fungal diseases, so it is best to keep mulch a few centimetres away from the stems of plants or the trunks of shrubs and trees. Such a moist area may encourage root rots to attack the plant at or near soil level.
This information is from the Yates Garden Guide: fully revised & updated 44th edition, HarperCollins, $39.99. You can have this information and so much more at your fingertips by purchasing the Yates Garden Guide, available at all leading bookstores and Bunnings stores.