Does Excess Mulch Depth Lead to Poor Tree Growth and Condition, Root Girdling, and Decay? A Systematic Literature Review

Mulch is placed around the base of trees to improve soil conditions, water conservation, and tree growth while decreasing weed competition, mower damage, and soil compaction. Current industry best practices and trade magazine articles recommend a mulch depth of 5 to 10 cm (2 to 4 inches) and caution against exceeding this depth, warning of issues affecting stem tissue like stem girdling roots and pathogens. To examine scientific support for this threshold, we conducted a systematic review of the peer-reviewed literature on excess mulch depth. We identified 11 studies that examined the effects of increasing depths of mulch on tree and soil physiology. All but two studies tested mulch depths exceeding the 5- to 10-cm (2- to 4-inch) range. The impact of deep mulch is unclear; methodological differences, including mulch type and examined variables, limit comparisons between studies. It is possible that fine mulch with low porosity results in deleterious effects similar to planting trees too deeply, explaining observations by practitioners. While further research should determine the effects of mulch depth beyond 10 cm (4 inches) on tree physiology, there are often negative side effects reported for exceeding 10 cm (4 inches) but few negative effects reported for mulch depths within 5 to 10 cm (2 to 4 inches).