By Mark Arena, Clemson Extension Agent and Barbara Smith, Home and Garden Information Center
Recently, I was driving along a two-lane rural country road and noticed a tractor spreading topsoil across a front yard. The owner claimed that he was putting down the topsoil to level the land. This required approximately 12 inches of soil to accomplish this task. I explained to him that 12 inches of additional topsoil would have a detrimental impact on the existing trees and shrubs.
Tree and shrubs that naturally occur in the landscape establish themselves at a specific elevation. Like the above-ground portion of the tree, roots require an exchange of gases, such as carbon dioxide (CO2) and oxygen, to freely exchange with the air. Depending on the physical properties of the soil, the roots will establish themselves at the appropriate depth in order for the carbon dioxide and oxygen exchange to occur. Typically, soils that have good drainage will result in roots being deeper in the soil. Depending on drainage, root establishment depth may vary from 4 inches to possibly 36 inches. The better the drainage after a heavy rain, the more likely the roots will establish at a deeper depth. Soils that have poor drainage may result in roots growing at slightly above grade to depths of 12 to 18 inches. Poorly drained soils that stay saturated prevent the proper exchange of gases. Therefore, the roots typically grow closer to the grade of the land. Poor exchange of gases results in low oxygen and high CO2 environment in the root zone area that may stress the tree. Over time, this condition may cause the tree to slowly decline.
In this situation, the trees have already established their preferred depth and by placing additional soil on top of their roots will create a greater distance for the root’s gases to exchange. Another way to look at this is to think of the roots as a snorkel in the water. If additional water is placed over the height of the snorkel, it is impossible for the diver to receive oxygen and breathe in a normal manner. Basically, that is what will occur with the additional topsoil being placed over the roots. The added soil will prevent the gases from escaping and exchanging with the environment. This will stress the tree and cause a slow and steady decline in tree vigor.
In the picture below, a vast number of roots are growing above soil grade. There are two reasons for this. First, roots grow in diameter, and over time, they will increase in size. If the roots are just below the soil surface, they will grow in diameter until they will eventually break the surface and become visible. Secondly, the area probably has poor drainage. In order for the roots to be able to breathe and freely exchange gases, their placement in the soil must be at the appropriate elevation.
The question that arises from this situation is how to manage the roots and the hazard that this situation creates. The best recommendation is to add mulch to cover the roots by approximately 1 inch at the most in order to eliminate the tripping hazard. Grinding or cutting the roots that are above ground is not a viable option. This would result in damaging the roots; therefore, increasing the odds that the tree will be infected by a disease.
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