Ideally, mulch should be applied beneath the entire canopy, but smaller mulched areas are acceptable. Depth should not exceed four inches, and two inches is adequate on shallow rooted shrubs and perennials.
Mulch beds do not have to be round or symmetrical. Shrubs and perennials can be planted within the mulch areas, but solid masses of ground covers should be avoided where optimum tree growth is desired. The biggest mistake people make when mulching is piling it against stems of trees and shrubs. Mulch “mountains” and “mulch volcanoes” are so common in some regions that homeowners and some professionals think this is acceptable and even desirable. Stem tissues are not intended to remain constantly moist from mulch accumulation. Insect and disease infestations, as well as stem girdling roots, often develop on trees with excessive mulching.
Read more about mulching from Bartlett Tree Experts: