Mulching is a beneficial practice in landscape management, designed to mimic the natural floor of a forest by covering the soil around trees and shrubs. This layer of organic material helps conserve soil moisture, moderates temperature fluctuations near the roots, and suppresses weed growth. When applied correctly, mulch provides a healthier environment for the root system. However, a common mistake in landscaping, known as “volcano mulching,” turns this helpful practice into a significant threat to tree health.
Defining Volcano Mulching
Volcano mulching is the incorrect practice of piling a large amount of mulch directly against and high up the base of a tree’s trunk, forming a steep cone or mound shape. This mound often measures six to twelve inches high or more, completely covering the tree’s root flare where the trunk widens as it meets the ground. The mulch is typically stacked so that it is in continuous contact with the bark of the tree, sometimes even burying the lower portion of the trunk itself. This technique is often applied year after year, compounding the problem as new layers are added to the old, increasing the depth and density of the mulch pile. This high-piled application creates an environment that is biologically detrimental to the tree.
How Volcano Mulching Harms Tree Health
The primary harm in volcano mulching is the constant moisture retention against the trunk, which leads to crown rot. Tree bark is adapted to be exposed to air and dry out quickly, but the deep, moist mulch keeps the trunk tissue perpetually wet. This saturated condition softens the bark, making it susceptible to infection by decay organisms, such as fungi and bacteria. Once the disease takes hold, it can girdle the tree, interrupting the flow of water and nutrients between the roots and the canopy.
The excessive depth of the mulch also creates an anaerobic environment for the main root system beneath the pile, restricting the oxygen exchange necessary for healthy root function. In an effort to find oxygen and moisture, the tree often responds by growing shallow, weak roots into the mulch layer itself. These roots, known as adventitious roots, begin to grow horizontally around the trunk, eventually encircling and constricting the trunk or major support roots. This girdling action effectively strangles the tree, limiting the transport of essential resources and leading to a slow decline in tree vigor, stunted growth, and premature limb dieback.
The deep, warm, and protected layer of mulch serves as an ideal habitat for pests and rodents, such as voles and mice. These small mammals use the mulch for shelter during the winter and feed on the moist, soft bark at the base of the trunk. Their chewing can remove the phloem layer beneath the bark, which transports sugars produced during photosynthesis. When this layer is damaged all the way around the trunk, the tree is girdled and will eventually starve.
The Proper Way to Mulch
The correct method for applying mulch avoids contact with the tree’s trunk, creating a shape often described as a “mulch donut” or “saucer.” The first step is to ensure the root flare is fully exposed, meaning the base of the trunk should widen visibly at the soil line before the roots radiate out. Mulch should be spread in a broad, flat layer, generally extending out from the trunk as far as the canopy’s drip line, or at least two to three feet for smaller trees. The depth of the mulch layer should be maintained at a thickness of two to four inches, which allows for effective moisture retention and weed suppression.
A gap of several inches, typically two to six inches, must be kept completely clear between the mulch and the trunk to prevent moisture-related damage and decay. Appropriate organic materials for mulching include shredded bark, wood chips, or composted leaves, which break down slowly to improve soil structure and fertility over time.