How deep should the mulch layer be around trees and shrubs?

FAQ
Question: 

How deep should the mulch layer be around trees and shrubs?

Answer: 

Mulches, such as wood chips and shredded bark, help to conserve soil moisture and control weeds.  Mulches also simplify maintenance activities around trees and shrubs and reduce the risk of mechanical damage to plant materials from errant lawnmowers and string-trimmers.  The optimum mulch depth depends upon the type of mulch, soil texture, plant type, and other factors.  A thin layer of mulch will not effectively control weeds or conserve soil moisture.  An excessively deep layer of mulch may result in water logging of heavy soils and may interfere with the movement of oxygen into the soil. 

A 3- to 4-inch-layer of wood chips or shredded bark would be appropriate for well-drained sites around trees and shrubs.  On heavy soils, a 2- to 3-inch-layer would be suitable.  Do not pile mulch against the trunks of trees.  Mulch piled up against tree trunks may create favorable conditions for fungal cankers, root rots, and rodents.  Keep the mulch at least 6 inches away from the trunks of trees.