Mulching in this manner is asking for trouble, because it holds moisture against the tree or shrub bark for months at a time. Bark simply is not designed to be constantly wet; if it is, opportunistic pathogens may infect the plant. When applying mulch, think “doughnut,” not “volcano.” Keep the mulch shallow and a few inches away from the base of the shrub or tree and then pile it a few inches deep from there outward. Never exceed a mulch depth of about 3 inches. Deeper applications of mulch may also attract rodents.