Adding organic mulch, such as wood chips, on top of the soil around trees is one of the kindest things you can do for your trees. Even old, well-established trees will benefit from mulch. A mulch layer, 5-10 cm in thickness over the root zone of your trees, shrubs and even perennials helps to retain moisture, replenishes soil and insulates against temperature extremes. The mulch layer should be replenished on an annual basis, since wood chips breakdown over time adding their nutrients and organic matter to the soil. Only a thin layer needs to be added each year to keep the layer 5 - 10 cm in thickness.