The Path of Least Resistance: ESCS Arboriculture Applications for Tree Root Health

by Chuck Friedrich, Horticulturist, PLA, ASLA

Joyce Kilmer Memorial Forest (Robbinsville, NC)

In Joyce Kilmers’ famous poem “Trees”, he wrote: “I think that I shall never see a poem lovely as a tree.” So beloved was this poem, a 3,800 acre tract of publicly owned virgin forest in western North Carolina was named Joyce Kilmer Memorial Forest.

As urban areas grow, revisions to this poem might be “I think I shall never see these trees again!” Construction of roads, buildings and infrastructure alters soil structure and chemistry, creating a challenge for trees to thrive.

Tree roots will follow the path of least resistance. That pathway involves three key elements for that growth: air, water, and particulate matter (soil) for support. The balance for all this to work in nature is 25% air, 25% water, and 50% solids. In the natural wooded environment, the breakdown of organic matter, mostly duff from decomposing leaves on the forest floor, is the key. As the organic matter breaks down it produces a slime which “glues” soil particles together creating soil clots. Tree roots push the clots apart and grow in the voids deeper into the soil profile. The deeper the roots the easier it is for the tree to get moisture and produce more roots for support. Now, let us leave the forest and examine what happens to tree roots in the urban environment. The determent to tree root systems in urban sites are compaction and lack of space. Soil compaction occurs from foot and vehicular traffic, rain, gravity, and vibration from various activities. Compaction changes the soil structure by squeezing the air out of the soil and changing the particle shape and distribution. Once the air is gone the roots can no longer maintain function and have to retreat or die. Damaged roots also makes the tree susceptible to disease which in turn weakens the tree and it becomes a liability.

Structural Soil
Trees growing in confined areas like a six foot by six foot wide tree pit, usually within a grate in the middle of a sidewalk, are not likely to survive to maturity. Those that do live have found a way for the roots to escape, usually following an air gap along foundations and utilities or by heaving pavements. Neither are good scenarios for reaching maturity. New trees planted in three feet of ESCS engineered structural soil that extends under the pavement have proven over decades that those trees can reach maturity. Providing a volume of structural soil of two cubic feet for every square foot of canopy crown projection is the rule of thumb.



911 Memorial Structural Soil and Installed Trees (NYC)

Vertical Aeration and Radial Trenching
Trees need additional root space as they mature. Soil compaction within the drip zone limits root growth and support for the tree. Weakened tree structure can cause rot or breakage. Shallow roots tend to make a tree vulnerable to blowing over from high winds or saturated soil during heavy rains. In the situation where help is needed to reduce compaction and improve aeration, drilling holes in the ground (vertical mulching) or cutting narrow trenches in a spoke pattern (radial trenching) within the drip line is the best solution.

Directions for Utilizing ESCS in Vertical Mulching
1. Remove all materials, equipment and debris from the soil surface of these areas.
2. Identify any subsurface irrigation lines, utility lines or other obstacles within 24 inches of the soil surface and plan to avoid these during the procedure.
3. From the perspective of a top view looking down on the tree and its available root zone from drip-line to drip-line, plan to locate holes in a grid pattern over the entire available root zone area from the edge of the drip-line in to within 1-2 feet of the trunk.
4. Using the pattern described above, using a powered drill with an auger, prepare holes at least two inches in diameter, 16-18 inches in depth and 18-24 inches apart.
5. After creating the holes in the root zone areas as described above, fill them completely to the top with 3/8” – 1/4” ESCS aggregates.
6. There is no need to top dress the root zone area. Distribution of a layer of organic mulch not to exceed 6-8 inches over top of the holes and the root zone area is advisable.

Radial Trenching
1. In my opinion this method is the most effective and quickest method for long term aeration and root paths. As with the vertical drilling, remove all materials, equipment and debris from the soil surface of these areas and identify any subsurface irrigation lines, utility lines or other obstacles.
2. From the perspective of a top view looking down on the tree and its available root zone from drip-line to drip-line, plan to locate the trenches in a wheel spoke pattern, starting from 1-2 feet outside of the edge of the drip-line and working toward the trunk of the tree.
3. Using the pattern described above, allowing 8-10 feet between the start of each trench at the edge of the drip-line for large trees (30 inch DBH or larger) and less distance between the start of the trenches for smaller trees (less than 30 inch DBH) proportionate to their trunk diameter down to no less than 2-3 feet between the start of the trenches. Prepare trenches at least 6-8 inches in width and 16-18 inches in depth. The ideal and preferred tool recommended for this procedure is an air spade to avoid severance of and minimize injury to roots; the procedure could also be performed with a trenching machine, shovel or mattock.
4. Fill the bottom one half of the trenches with 3/4 inch to 1/4 inch ESCS aggregates and lightly tamp to enable the particles to lock together to create pore spaces in between.
5. Repeat the above procedure for the upper one half of the trenches and allow 3-4 inches beneath the soil surface for a layer of mulch or a well drainable soil for turf seeding or sodding. Ideally, mulch the entire area under the drip line for the best tree health.

Root Bridging
Existing tree roots need protection from compaction. During construction with grade changes, chances are the roots will be damaged from compaction or will be buried. Just eight to ten inches of soil graded above the root collar can kill a tree by cutting off the air supply to the roots or cause root rot. How many times have contractor’s unloaded materials or parked vehicles and equipment under trees that were meant to be saved. This has led to having the tree police level stiff fines on the offending parties who cross the tree protection fence during the construction phase of projects. To provide support, air space and drainage to the root systems, utilizing ESCS as a lightweight fill over tree roots is a proven method for protecting tree roots under various development scenarios. Where sidewalks, parking, roads, or plazas are planned for expansion over existing tree roots, or simply in an area of high foot traffic, root bridging with ESCS lightweight aggregates has been successful for many decades protecting tree roots and extending the life of trees. ESCS can be placed just a few inches or five feet over root systems. Installation is easy: remove loose organic matter, and spread the ESCS aggregates at the depth needed to meet the new grade. Mulch, decorative stone, or a layer of well-draining soil for turf or planting can be placed on top of the aggregates. Where greater depths are required, a tree well will have to be installed to protect the trunk and root collar. It is not recommended to place materials against the trunk of the tree, including ESCS. When paving is involved, ESCS can be compacted to meet specifications for supporting concrete, asphalt or paver units. When paving must be installed over tree roots, the installation method requires additional operations. All organic matter and mucky soil must be removed from the surface first. A geotextile fabric is then laid down and the ESCS aggregates are placed in twelve inch lifts. The only compaction equipment that should be used is a portable vibratory plate; two passes are enough to seat the ESCS material for each twelve inch lift up to the final grade. After the material is placed the paving can be installed. Concrete can be placed directly on the aggregates. For asphalt, a four inch layer of ABC stone is required to run the equipment on over the ESCS. Pavers will need a layer of filter fabric for the sand laying bed.

The most notable root bridging project using ESCS is at the University of Virginia. A very prized ash tree graces the Peabody Library building. However, during the construction of the underground UVA Special Collections Library, the grade had to be changed for the new pavement to meet ADA requirements. A mixture of ESCS and soil was placed over the tree roots as deep as four feet on one side of the tree. That was in 2005 and today the tree is still healthy and thriving according to Todd Romanac, Landscape Projects Senior Supervisor, University of Virginia.

University of Virginia

”I think that I shall never see a poem lovely as a tree,” take care of those, alive and healthy, all for us to see.
Chuck Friedrich, PLA, ASLA, GRP