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What to do when a builder has placed fill soil over tree roots

The 12 inches of fill soil and the sprinkler spray hitting the trunk caused decay.
Adventitious roots have formed near the soil surface above the fill-covered root crown.

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.

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 in only a year or two but often decline becomes apparent in three to five years following construction and sometimes longer. 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), differences in soil type between the native soil and fill soil, 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).

The photo below shows the exposed circling and girdling roots at the base of the Maltby Oak trunk where the fill soil was. These roots developed from adventitious buds. The fill soil caused about one third of the 14.5 foot circumference of the trunk to decay.

Woody roots have an outer bark that contains suberin, which “waterproofs” the tissues. Consequently, roots can withstand constant contact with damp soil. But the above-ground outer parts of a tree trunk — the bark and outer layer of vascular tissue, cambium and sapwood — should not be constantly damp or decay damage will occur.

It was also discovered that a sprinkler was hitting the trunk of the Maltby Oak making the decay worse under the added fill soil. Irrigation spray should never be hitting the trunk of any tree for an extended period of time. In spite of the extensive damage to the trunk, the removal of the fill soil around the base of the tree allowed the lower trunk to dry on the healthy tissue and the decay progression has slowed on the decayed areas. The irrigation spray head has been redirected. The decayed area of the trunk will not become whole again. And the internal decay will continue slowly inside the trunk. The fungal conk, Ganoderma applanatum, is an indication of internal decay.

The tree, if healthy, will continue to add new solid, strong wood outside the decayed area. And tests with the Resistance Drill indicate the decayed areas at the base of the trunk are mostly near the surface and the interior heartwood is still very solid.

Before you add fill soil or infrastructure over roots

In those cases where you know in advance that fill soil or infrastructure must be placed over existing tree roots, there are techniques where roots can be protected and fill soil can be added above roots. Contact an arborist knowledgeable in the use of geogrid horizontal root protection barriers. Unfortunately many contractors are unfamiliar with these relatively new techniques of root protection.

One root proetection technique utilizes a geogrid horizontal barrier 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-stone layers also make 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 Vertical Mulching and Radial Trenching sections in this website.

Damage Caused by Fill Soil at the Base of the trunk

The Maltby Oak on the courthouse grounds in Palatka, Florida is a good example of what fill soil left against the trunk can do over time. Around 1985 someone in maintenance or administration decided there should be turf underneath the canopy of the Maltby Oak. About 12 inches of fill soil was added and sod placed over the fill right up to the trunk. Irrigation was added for the turf. Then 20 years later in January 2005 the Maltby Oak was condemned and scheduled for removal by the County Commission because large branches were dying and endangering the public who passed by the tree every day on their way to the courthouse. (For more details of the Maltby Oak history and treatment, click here.)

When a tree’s root flare is not visible, that means the root flare has been covered by fill soil. This can cause a series of problems for the tree.
When done properly, excess fill is removed and the tree and exposed root crown should look like this. Notice the dark area where the fill soil had been.