Exposed tree roots are common in mature landscapes, often creating tripping hazards or making lawn care difficult. While the impulse may be to cover them with a fresh layer of soil, this practice is generally harmful. Adding standard dirt over a tree’s root system can lead to root suffocation and serious long-term decline in health. Understanding the biological needs of the roots and the dangers of improper covering is essential for safe management.
Why Tree Roots Need Air
Tree roots require oxygen for cellular respiration to generate the energy needed to absorb water and nutrients. The vast network of fine, fibrous feeder roots responsible for absorption typically resides in the top 6 to 18 inches of soil, where oxygen levels are highest. These shallow roots exploit the air-filled pore spaces between soil particles to access necessary gases.
Burying this shallow system under a new layer of dense soil limits essential gas exchange, effectively suffocating the roots. The tree’s root flare, where the trunk widens and transitions into the root system, must remain exposed above ground. The tissue of the trunk and root flare cannot tolerate constant moisture and lack of air.
When the root flare is covered, the bark tissue softens, creating entry points for pests and fungal diseases. Covering this area also triggers the tree to grow new roots closer to the surface to reach oxygen. These new roots often grow in undesirable patterns that threaten the tree’s long-term structure.
The Detrimental Effects of Adding Soil
Applying a heavy layer of topsoil over exposed roots immediately compromises the tree’s ability to breathe, leading to suffocation. Standard garden soil is prone to compaction, which quickly reduces the air pockets within the soil. This dense environment prevents oxygen from penetrating the feeder roots, causing them to die back.
A thick layer of soil also creates poor drainage, trapping excess moisture around the roots. This waterlogged environment can lead to root rot and fungal pathogens that thrive in low-oxygen, high-moisture settings. The tree’s health declines as its ability to absorb water and nutrients is diminished.
A harmful long-term consequence of adding soil is the encouragement of girdling roots. These roots grow in a circular pattern around the trunk or other roots, often triggered when the root flare is buried. As the tree and the girdling root increase in diameter, the circling root chokes the trunk, restricting the flow of water and nutrients. This slow strangulation can take five to fifteen years to cause severe decline, manifesting as a thinning canopy, stunted growth, and early leaf drop.
Safe Strategies for Root Management
The safest method for managing exposed roots is the application of organic mulch. Mulch acts as a protective layer that insulates the roots, retains soil moisture, and prevents compaction without blocking air exchange. A layer of shredded wood chips or bark mulch should be spread 2 to 4 inches deep over the exposed area.
It is essential to use the “donut” method, keeping the mulch 3 to 6 inches away from the trunk itself. Piling mulch directly against the bark, known as a “mulch volcano,” traps moisture and encourages decay and the formation of girdling roots. Proper mulching mimics the natural forest floor, providing a safe, aesthetically pleasing covering that benefits the tree.
If roots must be covered for aesthetic reasons, a very thin layer of specialized, highly porous material can be used. This should be a well-draining mix, such as a root zone mix or a blend of compost and sand, applied no more than 1 to 2 inches thick. This thin layer allows for visual coverage while still permitting the necessary air and water flow.
Planting shallow-rooted groundcovers is another effective strategy that helps stabilize the soil and hides the exposed roots. Species like vinca, barrenwort, sweet woodruff, or creeping thyme work well because they require minimal soil depth and compete less with the tree for water and nutrients. When planting, use hand tools to carefully place the groundcover in pockets of soil between the tree roots to avoid damaging the existing root structure.