What To Do When A Builder Has Placed Fill Soil Over Tree Roots
Most homeowners aren't responsible for putting fill soil over their tree roots. Sometimes a builder will do it during construction or maybe the fill was added when the house was being built years before the current homeowner bought the property. Sometimes people try to save a tree during construction but the tree is just too close to a street, driveway a large amount of fill soil must be brought onto the property to raise it to a specified level before the structure is built.
Roots require oxygen for respiration. When fill soil is placed over the roots within the dripline of a tree, the oxygen content of the soil where the roots are located is reduced. This depletion of soil oxygen often causes the small feeder roots to die. In severe cases, the tree can show signs of decline after a year or two or sometimes longer. And there are occasional cases where the tree appears to have no ill-effects of the fill soil. The results depend upon the species of tree, the soil type (sandy vs. clay), soil compaction (is the fill loose soil or has it been compacted) and the depth of fill soil (3 inches of fill or 3 feet of fill).
For techniques on mitigating the negative effects of fill soil over roots, go to the Root Rejuvenation section of this web site. Or read the section below: "How to determine if your tree's roots have been covered by fill soil."
In those cases where you know in advance that fill soil must be placed over existing tree roots, there are techniques where roots can be protected and fill soil can be added above roots.
One technique utilizes a geogrid plastic placed directly on the natural soil grade. The geogrid is then covered with a 6-inch thick layer (minimum) of clean aggregate stones that are larger than the geogrid openings. Then the layer of stone is covered with a geotextile fabric that prevents fine soil particles from sifting downward into the layer of stones. The layer of stones provides an air layer where air can penetrate into the soil below where the roots are. The geogrid also makes it possible for the builder to compact the fill soil placed above the geogrid-stone-geotextile layer. The compaction can be 90 to 95 percent above the geogrid-stone-geotextile layer.
How to determine if your tree's roots have been covered with fill soil?
The easiest way to determine if your tree's roots have been covered by fill soil is to look for the root flare at the base of the trunk. The root flare is the wide area where the roots are connected to the trunk. This area should be visible and not covered by soil or mulch. If you are able, try to dig around the base until you reach the root flare. If there is only a small amount of soil piled over the root flares, then you can move the soil away from the flares gradually sloping the edge the hole back about 3 to 5 feet from the trunk. If the amount of soil covering the root flare is greater than 12 inches deep, you may want to consider digging out a tree well with a short retaining wall to keep the fill soil back away from the trunk and root flare.
When soil is piled over the root flare and against the bark on the trunk, these areas stay constantly moist. Over time, sometimes as long as 10 to 20 years, these moist areas can become decayed which can destroy the cambium and conductive tissue beneath the bark and weaken the root flares that connect the trunk to the roots and support the tree.
If the tree appears to be doing fine in spite of the fill soil, uncovering the root flares may be all remediation the tree needs. Often farther out from the trunk the fine absorbing roots will grow upward into the fill. If the tree has sparse foliage and branch dieback, vertical mulching and other root rejuvenation techniques may be recommended. See the Root Rejuvenation section of this web site.
