Mulch cushions the impact of the raindrop against the soil surface and allows the water to gently flow into the soil below and reduces soil erosion. Applying mulch too deeply, and Mounding mulch up against tree and shrub trunks. These are not only costly mistakes because they use far too much mulch, but they put the plants in danger of root suffocation and bark decay. Most mulches should only be 2 to 4 inches deep. Use the low end of the scale on clay soils and the higher range for sandy soils.
Excess mulch smothers the roots by reducing the gas exchange between the soil and air.
Deep layers of mulch may absorb all water applied from light rains and hold it well above the root system. If you are in the habit of adding a new layer of mulch each year to retain color and texture, be sure to take some of the old mulch away and put in the vegetable garden of other location. Robert Nuss is a horticulturist at Penn State. He coordinates all extension horticulture programs. He has bachelor's, master's, and doctoral degrees in ornamental horticulture and has been on the Penn State faculty since 1966.