Yes, mulch can kill trees, but only when it is applied incorrectly. Mulch is defined as material spread over the soil surface, and it is a recommended practice by arborists for improving tree health. However, misapplication transforms this beneficial practice into a destructive one, causing decline and eventual death.
The Critical Role of Proper Mulching
When applied correctly, mulch creates a protective layer that mimics the natural environment of a forest floor. As organic mulches, such as wood chips or shredded bark, slowly break down, they incorporate valuable organic matter into the soil, enhancing structure and providing nutrients.
Proper mulching provides several key benefits:
- It significantly reduces the rate of water evaporation from the soil, helping to conserve moisture and reduce the need for frequent watering.
- It functions as an insulating blanket, moderating soil temperature by keeping roots cooler in summer and retaining warmth in winter.
- It suppresses weed growth by blocking sunlight, eliminating competition for water and essential nutrients.
- It enhances the soil structure, improving drainage and aeration.
- It protects the base of the tree trunk from mechanical damage caused by lawn mowers and string trimmers.
The Danger: Understanding Volcano Mulching
The destructive practice known as “volcano mulching” occurs when a thick mound of mulch is piled high against the tree trunk. This incorrect application interferes with the tree’s natural biology and creates conditions favorable to disease and pests.
Bark Decay and Disease
The primary issue is that volcano mulching keeps the tree’s bark constantly saturated with moisture. Tree bark is designed to be exposed to air and remain dry, acting as the tree’s protective layer. When the bark is damp from the mulch pile, it begins to soften and decay, creating entry points for fungal pathogens and bacterial infections. This decay weakens the tree’s defense system and leaves it vulnerable to infestation by insects, such as borers, which are attracted to stressed wood.
Root Suffocation
A significant threat from volcano mulching is root suffocation. A thick, deep layer of mulch, typically exceeding four inches, smothers the soil and restricts the necessary exchange of oxygen and carbon dioxide. Healthy roots require oxygen to function, and this deprivation weakens the entire root system, making the tree unable to absorb the water and nutrients it needs.
Girdling Roots
The high mulch mound encourages the tree to grow new, shallow roots into the mulch layer itself, rather than down and out into the soil. These roots are weak, poorly anchored, and often grow back toward the trunk. As they thicken, these circling roots can eventually wrap entirely around the trunk or existing structural roots, a condition known as girdling. Girdling physically constricts the tree’s vascular system, blocking the flow of water and nutrients between the roots and the canopy.
Essential Guidelines for Healthy Tree Mulching
To ensure the health benefits of mulch, the application must follow specific guidelines that promote a “donut” shape rather than a “volcano.” The most important step is to keep the mulch pulled back several inches from the tree’s trunk, exposing the root flare. The root flare is the area where the trunk widens as it meets the ground, and it must remain dry and visible to prevent bark decay.
The ideal depth for the mulch layer is between two and four inches. Applying a layer thicker than four inches can lead to the suffocation of feeder roots and the development of girdling roots. For areas with poor drainage, a thinner layer of two inches is recommended to prevent excessive moisture retention.
The mulch ring should spread out over the soil in a wide, flat circle, ideally extending to the tree’s drip line (the outer edge of the canopy). If extending to the drip line is impractical, a minimum diameter of three to four feet is recommended for young trees to maximize the benefits of temperature and moisture regulation. Organic mulches like coarse wood chips, shredded bark, or composted leaves are preferred because they enrich the soil as they break down and allow for better air and water penetration.