Two inches of mulch is sufficient for regular mulching for the growing season, up to four inches can be added for winter mulching, but must be removed in the spring. Mulching up to the stem of a plant, especially in the “volcano” like fashion, is equivalent to burying your plant that deep in the soil. Imagine a plant that is supposed to be planted four inches deep, and is planted eight inches. The plant would rot at the stem. Therefore when mulching, keep the mulch three to four inches away from the base of the plant.
Adding dry mulch to dry soil can cause many problems. The first is obvious. It will be hard to moisten the soil to begin with. The other problem can lead to plant death. Have you ever noticed mold growing on mulch? Dry mulch harbors a natural fungus, which spreads when the mulch is dry. Wet mulch produces a bacterium that combats this fungus. Therefore, when mulching, always thoroughly water the mulch, this will increase the bacteria that helps prevent the molds that may cause plant death.