Mulch is a protective layer applied to the soil surface around plants, consisting of organic materials like wood chips or inorganic materials like gravel. When used correctly, mulch is highly beneficial, helping to retain soil moisture, suppress weed growth, and moderate soil temperature fluctuations. The perception that mulch is harmful stems not from the material itself, but from a common, incorrect application technique known as over-mulching. Improper use introduces significant risks to a tree’s health and longevity.
The Problem with Mulch Volcanoes
The most critical and damaging application error is creating a “mulch volcano,” a thick, high cone of material piled directly against the tree trunk. This practice is often done for aesthetic reasons but fundamentally misunderstands a tree’s structure. The trunk is adapted to remain above ground in drier air, while the roots are in the soil. Piling mulch against the trunk covers the root flare, the area where the trunk expands and meets the roots. This simulates planting the tree too deeply, which is unnatural and detrimental to the tree’s physiology and encourages biological damage.
Biological and Physical Damage to Trees
Piling mulch against the trunk compromises the protective bark layer by trapping continuous moisture against it. Bark is dead, dry tissue that shields the living cambium layer from the environment. When perpetually damp, the bark becomes susceptible to decay, fungal infections, and pathogens, leading to crown or trunk rot. This decay weakens the tree’s base, making it structurally unsound and vulnerable to failure.
Thick, wet mulch also creates an ideal, sheltered habitat for pests and rodents. Voles and insects, such as borers, live within the mulch layer and feed on the softened bark or lower stem tissue. This feeding can girdle the tree by chewing a ring around the circumference of the trunk. Girdling restricts the flow of water and nutrients, slowly starving the tree.
Excessive mulch depth, especially over four inches, also suffocates the roots by limiting gas exchange with the soil. Roots require oxygen for respiration, and a deep layer of mulch creates an anaerobic environment, causing root stress. In search of oxygen, roots may grow upward into the mulch layer, eventually circling back to press against the trunk. These “stem girdling roots” restrict the flow of water and nutrients as they thicken, effectively strangling the tree.
Essential Rules for Safe Mulching
To ensure mulch provides its intended benefits, it must be applied using the “donut” method, focusing on proper placement and depth. The goal is to create a flat, wide ring of mulch that resembles a donut, leaving the center open and clear around the trunk. This application keeps the mulch from touching the root flare, allowing air to circulate and the bark to remain dry.
The mulch layer should be spread no deeper than two to four inches across the entire mulched area. Finer mulch materials or sites with poorly drained soil should use the lower end of this depth range. The most critical technique is leaving a clear, dry buffer zone of three to six inches between the mulch and the tree trunk.
The diameter of the mulched area should extend outward as far as possible, ideally to the tree’s drip line, to maximize benefits. For material selection, organic, coarse mulches like shredded bark or wood chips are recommended. These materials slowly decompose, adding beneficial organic matter and nutrients to the soil. Regularly check the depth of the mulch and replenish it only to maintain the proper two-to-four-inch thickness, avoiding the tendency to pile new material on top of old.