When decomposition results in a loss of mulch material, replace it to return the depth to the original two or three inches. All too often, gardeners and contractors apply a fresh two to three inch layer on top of what is there to keep the specific looks created by the fresh product. Over a period of years this mulch layer can accumulate and have the same affect as setting the plant too deep into the ground at planting time. Such conditions will smother the root system are result in gradual decline in vigor and plant quality. Most organic mulches, except the bark of coniferous trees, are high in cellulose and low in lignins.
Cellulose decomposes rapidly and leaves little residue on the soil surface.
On the other hand, the bark of coniferous trees like pine and spruce are low in cellulose and high in lignin. They are more resistant to decomposition and last longer on the soil surface. Under these conditions, the ground accumulates high levels of fine material as the original mulch decomposes. As additional layers are added each season to keep a fresh look, the fine layer below the visible top layer grows. The accumulation of bark fibers remains wet and seals the soil surface.