A soil-health program helped newly planted London Plane trees at Manhattan’s Grand Army Plaza recover after construction delays left them stressed in a plant compound. The work combined liquid biological amendments, Sorghum cover-crop seed and a vesicular-arbuscular mycorrhizal (VAM) spore drench, while staff managed moisture by hand rather than with permanent irrigation. James Sottilo’s account for ASLA The Field (published May 9, 2017) reports stronger soil-biology readings and denser tree growth over six months, but it is a practitioner’s before-and-after account—not a controlled experiment that can prove which treatment caused the change.
What happened at Grand Army Plaza
Grand Army Plaza occupies the southeast gateway to Central Park, where Fifth Avenue meets 59th Street. The plaza’s northern section, including the General Sherman statue, underwent a major restoration completed by the Central Park Conservancy in September 2015. The work rebuilt paving, stonework, benches and lighting and replaced electric, drainage and irrigation infrastructure.
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A double row of London Plane trees replaced trees lost in an October 2011 snowstorm. Because the trees would eventually need substantial rooting volume beneath the hardscape, the project installed CU-Structural Soil below pavements. That system provides a soil volume in an otherwise paved setting so roots can mature without sacrificing the surface needed for public use and circulation.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Construction delays meant the nursery-dug trees remained above ground in the Conservancy’s plant compound longer than planned. Staff cared for them, but the trees showed stress before planting. Once installed, they faced the additional pressures typical of a busy urban plaza: restricted access, changing construction conditions and the need to keep traffic moving while the restoration reached reopening.
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Sottilo described the objective as a soil and tree-health program that could operate without obstructing ongoing traffic or extending the project schedule. The approach treated the planting soil as a living system rather than relying on a single input or on the cover crop alone.
What the baseline diagnosis showed
Before treatment, the account recorded soil microbial biomass, estimated microbial carbon and nitrogen, and the percentage of roots colonized by mycorrhizal fungi. Sottilo characterized the microbial readings as low and the colonization result as weak. These are project-specific measurements, not universal pass/fail thresholds for every urban tree pit.
| Measure | Baseline reading | Reading at six months | How to interpret the comparison |
|---|---|---|---|
| Soil microbial biomass | 282 micrograms per gram | 473 micrograms per gram | The account reports a higher biomass reading after the program. |
| Estimated microbial carbon | 590 pounds per 1,000 square feet | 1,090 pounds per 1,000 square feet | The account reports a higher estimated carbon value after six months. |
| Estimated microbial nitrogen | 1.28 pounds per 1,000 square feet | 2.15 pounds per 1,000 square feet, labeled “microbial carbon” in the article | The labels do not match, so this is not a clean like-for-like nitrogen comparison. |
| Average root mycorrhizal colonization | 6 percent | 14 percent | The account reports a higher share of roots colonized by mycorrhizal fungi. |
All figures in the table come from Sottilo’s 2017 project account. The article gives no control planting, independent replication or causal analysis. The safest reading is therefore that the measured conditions changed after a bundled intervention at this site; the figures do not isolate the effect of Sorghum, the liquid amendments, the VAM agent, watering or weather.
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How the intervention was assembled
Liquid biological amendments
The team injected liquid biological amendments 2–4 inches below the surface around each tree pit. The account does not identify a commercial brand, formulation, dose per pit or application volume. Those omissions matter: an urban soil program cannot reproduce a result simply by ordering a generic “microbial” product without knowing the existing soil, root condition, moisture regime and construction constraints.
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Sorghum as a temporary cover crop
Sorghum seed was planted in the tree-pit soil. Sottilo said the choice was informed by scientific literature describing Sorghum’s use in commercial mycorrhizal spore production and germination, although the article does not identify that literature. The cover crop supplied living roots in a soil environment that had been disturbed and compacted by urban construction.
“Cover crops allow the break up of soil compaction by developing their own strong root systems while also releasing sugars and carbon for soil bacteria and fungi to break down into organic matter.”
—James Sottilo, Lead Consultant and Founder of Ecological Landscape Management
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That explanation describes two intended functions: physical exploration of compacted soil by roots and a flow of carbon compounds that supports soil organisms. It does not mean a cover crop can substitute for adequate soil volume, drainage, protection from construction traffic or appropriate watering.
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Vesicular-arbuscular mycorrhizal inoculation
The planting was drenched with a vesicular-arbuscular mycorrhizal, or VAM, spore agent. Mycorrhizal fungi form associations with roots; the project measured the percentage of roots showing colonization before and after treatment. The source does not name the organism mix, spore concentration or application rate, so those details would need to come from a qualified specification for another site.
Why the package matters
The program combined all three measures. Sorghum was not tested by itself, and the article does not compare the treatment with untreated pits or with a different amendment. Sottilo summarized the philosophy as “a program where nature could amend nature”: introduce living roots and biological inputs, then manage the conditions that allow them to function.
What happened after planting
- Within 10 days: the account reports visible Sorghum germination.
- After two weeks: Sorghum roots were reported at 4–6 inches deep.
- At six months: the follow-up soil and root measurements were higher for the biomass, estimated carbon and mycorrhizal-colonization measures shown in the table.
- During the 2016 seasons: the author observed dense leaf coverage.
- Approaching the third growing season: the account describes a healthy bud set.
The last two observations are period-specific field observations in the 2017 article. They should not be read as a current assessment of the trees’ condition, nor as a guarantee that another planting will follow the same trajectory.
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Water management was part of the treatment
The tree pits were not irrigated through a permanent system. Conservancy staff hand-watered them, and the program emphasized soil-moisture management. That detail is easy to overlook, but biological amendments and cover crops still need a workable moisture regime. A project that adds seed or inoculant while allowing a newly planted root zone to dry repeatedly is not implementing the same approach.
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Hand watering also reflects the site’s operational constraints. The plaza had active construction and traffic considerations, so the team needed a maintenance method that did not require new trenching or interfere with the reopening plan. On another project, the practical choice could differ; the essential point is to assign responsibility for checking moisture and delivering water before planting begins.
How to adapt the lesson to another urban construction project
The Midtown account offers a decision sequence rather than a universal recipe. A design or construction team considering a similar program can use the following workflow.
- Establish a baseline before choosing an amendment. Sample the planting soil and document compaction, moisture, drainage, available soil volume and root condition. If microbial biomass, carbon or nitrogen, or mycorrhizal colonization will be used as indicators, record the sampling depth, locations and laboratory method so later readings are comparable.
- Map the non-negotiable site constraints. Identify pavements, utilities, traffic lanes, work zones and the date the public space must reopen. Mark where equipment, injection tools and watering crews can work without damaging new paving or compacting the root zone again.
- Protect the designed root volume. CU-Structural Soil provided the Grand Army Plaza trees with soil volume beneath pavement. Any biological program should complement, not replace, the physical root-space design. Keep construction loads and storage out of the pits and maintain the intended separation between planting soil and traffic-bearing surfaces.
- Specify the biological package as a package. The source program used shallow liquid amendments, Sorghum seed and a VAM spore drench. A project specification should state the exact products or materials, rates, application depth, timing and quality controls; those details are not supplied by the Midtown account and should not be guessed.
- Plan the cover crop around the tree and the schedule. Confirm that the selected cover crop can establish in the available season and will not create an access or maintenance conflict. Decide how it will be terminated or managed as the tree canopy develops, and keep the seedbed free of foot and vehicle traffic while roots are establishing.
- Assign moisture responsibility. Name the person or crew who checks the pits, records conditions and hand-waters or operates irrigation when needed. Coordinate watering with weather, drainage and the construction calendar rather than treating it as an occasional emergency task.
- Monitor on a defined timeline. Short-term observations such as emergence and root depth can show whether the cover crop established. A later soil and root test can show whether the chosen indicators changed. Use the same sampling approach at each date and record management events so a change is interpretable.
Questions to ask before selecting a different approach
The source does not rank alternative treatments. If a project is considering compost, different biological inputs, mechanical decompaction or no cover crop, compare the options against the conditions that made Midtown difficult rather than assuming one method is best.
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|---|---|
| Site disturbance | Can the method be installed without reopening paving, damaging roots or adding heavy equipment to finished areas? |
| Traffic and access | Can crews reach every pit while pedestrians, vehicles and emergency routes remain operational? |
| Compaction | Does the method address the measured compaction, or will it merely add material above a restrictive layer? |
| Root needs | Will the treatment preserve the tree’s available soil volume and avoid cutting or smothering roots? |
| Water management | Who will maintain moisture during establishment, and can the drainage design handle the proposed inputs? |
| Diagnosis | What baseline measurement will show whether the intervention worked at this site? |
These questions prevent a common category error: treating a cover crop or inoculant as a standalone cure for a structural problem. If the limiting factor is inadequate soil volume, a blocked drainage path or repeated vehicle loading, biological additions alone cannot remove that constraint.
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What the reported numbers can—and cannot—tell you
The direction of change is encouraging within this one planting: the account reports more microbial biomass, a larger estimated microbial-carbon value and greater root colonization after six months. The visible establishment of Sorghum and later observations of foliage and bud set provide practical context for the measurements.
However, several uncertainties remain. There was no untreated comparison, so weather, nursery recovery, normal root establishment, hand watering or other restoration activities may have contributed. The project used a bundle of inputs, so no result can be assigned to the seed, amendment or VAM agent separately. The article does not provide laboratory methods, sampling replication, statistical analysis, product rates or a long-term follow-up series.
The reported 2.15-pound figure requires particular care. At baseline, the comparable value is identified as estimated microbial nitrogen; at six months, the article labels 2.15 pounds per 1,000 square feet as microbial carbon. Because the labels differ, readers should preserve the wording rather than calculate a nitrogen increase from those two numbers.
Why this example matters for construction teams
Grand Army Plaza shows how soil biology can be incorporated into a restoration schedule when the project cannot simply excavate again or close a busy public space. The intervention was shallow, distributed around existing tree pits and paired with an explicit maintenance plan. It also began with measurements, which gave the team a way to describe the starting condition and the changes observed later.
For contractors, landscape architects and owners, the durable lesson is procedural: diagnose the root zone, design around access and soil-volume constraints, install a coordinated biological program, protect moisture and document outcomes. A cover crop can be one tool in that sequence, but the Midtown account supports neither a universal prescription nor a promise of identical results. Its value is as a detailed example of fitting living-soil management into a demanding urban construction project.
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