mulch volcano - too much mulch at the base of a tree

Incorrect placement of mulch (mulch volcano). Photo: Fredric Miller Miller, Joliet Junior College,

Updated: March 14, 2023

What is mulch used for?

  • Mulch is used around plants to conserve soil moisture, moderate soil temperatures, protect roots, reduce weeds, and help prevent trunk damage from lawnmowers and weed trimmers. 
  • Many gardeners, as a routine garden task, add liberal amounts of mulch to trees and shrubs believing it is the “correct” thing to do.
  • Over time, mulch can accumulate to depths of five, six, or more inches causing plant problems.

Plant problems caused by too much mulch

  • Excessive mulch reduces soil oxygen for roots, suffocating them and causing them to die.
  • Roots in search of oxygen and water sometimes grow into excess mulch. During dry periods the mulch dries out and the roots in the mulch die. 
  • Mulch piled up against the trunks of trees and shrubs keeps the bark moist underneath the mulch causing decay and a possible entry point for diseases and insects. Pull back the mulch about 3 inches so it does not touch the bark. 
  • Light watering may only wet the mulch layer while the soil remains dry.
  • Too much mulch may delay the onset of dormancy in the fall by keeping root zone temperatures warm. In contrast, excessive mulch may prevent the breaking of dormancy in the spring due to cooler root zone temperatures.
  • Provides a hiding place for voles, a rodent that gnaws on bark and girdles trunks and the lower stems of shrubs. 

Mulch tips

  • Keep organic (bark and wood products, compost, and leaves or grass clippings) and synthetic (plastic, stones, or landscape fabric) mulches 3  inches away from the base of the trunk to prevent damaging the bark by keeping the area too moist. 
  • Organic mulches should not exceed three inches in depth. Although organic mulches break down over time they should only be replaced as needed to maintain their original depth.
  • Carefully monitor the watering of mulched plants to make sure the water percolates through the mulch layer into the soil. 
  • As old mulch decomposes or before applying fresh mulch it can be gently worked into the planting bed to improve the soil or removed and worked into other garden areas. 
  • Shallow-rooted plants like boxwood should not have more than one inch of mulch around their roots. Acid loving plants like azaleas can be mulched with pine bark or pine needles.
  • Additional mulch may be added occasionally to supplement and freshen-up the existing covering but do not let it exceed a depth of 3 inches. 
example of correctly mulched tree
Example of a correctly mulched tree with root flare visible.  

Mulching Trees and Shrubs

Additional resources 

Problems With Over-Mulching Trees and Shrubs | Rutgers University

What Is Growing in My Landscape Mulch? | Penn State Extension

Based on publication HG 803 Commercial Landscape Series: Mulch Problems; Authors David Clement and Mary Kay Malinoski (retired), Extension Specialists, University of Maryland Extension

Rev 2019