Trees and Home Construction

Minimizing the impact of construction activity on trees

Bulletin 870-99

Excavating and Trenching

Excavating and trenching are common activities on construction sites for a variety of purposes including the creation of basements, crawl spaces, other foundations, footers for walls, and trenches for utilities and irrigation systems. Such digging, if not properly planned and exe-cuted, can damage or destroy tree root systems, restrict tree root growth, and alter subsurface soil water movement. In terms of tree health, a reduced or restricted root system can result in less water and nutrient absorption and less food storage, all of which can result in the weakening or death of the tree. Roots also provide anchorage and stability. A loss of a portion of a tree's root system or the restriction of root growth can produce an unstable tree that could ultimately fall, resulting in the loss of the tree, the expense of cleanup, and possibly loss, damage, or injury to other property or individuals. Filling deeper excavations or trenches with building materials that are relatively impervious to water (e.g., cement or concrete block) or with soil material that has a permeability markedly different from the natural soil can interfere with subsurface soil water movement, producing essentially a damming effect. In Ohio this most commonly results in an excessively wet area that could limit the species of trees that will grow.

Any tree located near a deep excavation, such as a basement or crawl space, where there will be an unavoidable loss of a substantial portion of the tree's root system, should be carefully evaluated for its potential to survive and grow and its potential as a hazard. No generalizations can be made concerning when such a tree should be removed, as many variables must be considered. The species of tree, the character of its root system, the tree's condition including any decay, the shape of its crown and any lean, and the type of soil will affect the tree's ability to survive. Certainly, however, any tree that will lose a quarter or more of its root system should be closely examined.

When trenching is necessary, avoid trenching within a tree's dripline. Minimize the number of trenches by placing all utilities in one or a minimum number of service trenches. When the utilities must pass beneath a valuable tree, consider trenching near the center of the tree's root system and "pushing" the utilities in a tunnel under the trunk (Figure 8). Provided no major roots are cut beneath the trunk, this system minimizes the amount of feeder and anchorage roots that are disturbed. Obviously, when trenching and tunneling near trees, efforts should be made to minimize equipment traffic and excavated soil should be piled outside the tree protection area to reduce compaction. If a portion of a tree's roots must be severed with a trench, excavate on the paved side of the tree if one exists. Few roots will grow beneath an impervious surface such as a roadway (Figure 9).

Figure 8. Tunneling can protect valuable roots and allow a tree to be retained.
Figure 9. Tunnel on the roadway side of a tree. Remember that tree roots need oxygen to grow just as people do.

The construction of footers (e.g., wall footers) can damage or destroy much of a tree's lateral root system as a result of the cutting of roots to place the footer below grade. Soil compaction is another consequence of footer construction. These impacts can sometimes be minimized by using a discontinuous footer or piers and a grade-level lintel. The lintel is a load-bearing beam (e.g., steel plate) that spans the void in the footer or the distance between the piers (Figure 10). A lintel can also be used to span low areas between footer segments or piers, avoiding root or soil disturbance and allowing natural water drainage to occur.

Figure 10. Soil compaction close to trees can be minimized by using discontinuous footers and a grade level load-bearing lintel.

The impact on the tree of any root reduction that must occur during excavating or trenching can be reduced by properly pruning any roots that must be cut that are larger than one inch in diameter. Allowing excavating and common trenching equipment to simply cut back the roots they encounter will result in shattered and frayed roots. This type of damage heals much more slowly than cleanly pruned roots and can substantially reduce the health and vigor of the tree.

Finally, trees that have had their root systems reduced during the excavation can benefit from timely watering and fertilization. In times of drought, watering may be essential to the survival of trees that have lost substantial portions of their roots. Trees so affected need time to regenerate an adequate root system, and that may take several years.