Many people are under the impression that lots of mulch is good for plants so they pile it up around their perennials, trees and shrubs year after year. After 5 years of adding 1 to 3 inches annually, you could have as mulch as 5 to 10 inches of mulch built up around the stems of your shrubs and trunks of your trees. This is actually detrimental to your plants and most commonly done by commercial landscape companies.
Have you ever noticed the mounds of mulch around the trees in parking lot plantings in many plazas and shopping malls? In our trade we call that 'volcano mulching'. Excessive mulch smothers the roots of plants and sheds water away from the roots. It will cause the decline of plants over time.
The optimum thickness of mulch should be maintained at about two inches. This is enough to suppress weeds and aid in retaining moisture for the plants. Fine Gardeners recommends adding only half an inch to one inch as needed to maintain a two inch thickness. Only use a dusting near the crowns of perennials, shrubs and tree trunks. Your plants will thank you if you follow these simple guidelines for garden maintenance and you'll save money by using less mulch.