Fill Can Kill a Tree

The addition or removal of soil around the base of a tree may seriously disturb the delicate relationship between roots and soil and may considerably damage or kill the tree.

In many areas, fill is often added to a building site and no precautions are taken to minimize the shock of placing the roots in a new environment.

When fill is added over existing soils, air and water, which are essential for normal functioning of the roots, are partially excluded. As a result, the roots die and the symptoms soon become visible in the above ground parts. Symptoms may appear within a month, or may not appear for several years. The visual symptoms are small yellow leaves, presence of numerous suckers alone the main trunk and branches, many dead twigs and in some instances, large dead branches.

The extent of injury from fills varies with the kind, age and condition of the tree; the depth and type of fill; drainage and several other minor factors. Maple, oak and evergreens are most seriously injured, while elm, ash, willow, sycamore and locust are least affected.

Obviously, the deeper the fill, the more marked is the disturbance to the roots. Clay soil fills cause most injury because the fineness of the soil shuts out air and water most completely. The application of only an inch or two of clay soil may cause severe injury. Sandy fills may be added to a depth of four inches. Gravelly fills cause least damage, because both air and water penetrate them more readily. As a general rule, the application of a layer of several inches deep of gravelly soil, or even of the same type of soil in which the tree has been growing, will do no harm. The roots will eventually become accustomed to the new situation by producing additional roots near the surface.

Little can be done to save trees that have been suffering from grade fills over an extended period. However, much can be done to prevent damage to trees by excessive fills.

The initial cost of proper precautionary measures is high and replacement of the tree may be more economical, particularly if the tree is young, in poor condition, an undesirable species, or very susceptible to insect and disease pests.

Where fills are placed around a tree, four or six inch agricultural tiles are laid in a wheel-and- spoke design with the tree as the hub. The radical lines of the tile near the tree trunk should be at least one foot higher than the ends joining the circle of tile. An open-jointed stone or brick well is then constructed around the trunk up to the level of the new fill. The radical tiles extend into the well. The inner wall of the well should be two feet from the tree trunk. Six-inch bell tiles are placed above the junction of the two tile systems, the bell end reaching the planned grade level and coarse gravel or stone added to hold the bell tile in place. All tile are covered with small rocks or cobble stones, then a layer of gravel, to a level of 12 inches from the final grade. A thin layer of straw or hay should be placed over the gravel to prevent soil from sealing the air space. Then good topsoil should be spread over the entire area except in the tree well and the bell tile. To prevent clogging, gravel should be placed inside the dry well over the opening of the radical tile.