Nina Bassuk of Cornell University’s Urban Horticulture Institute understands that trees in cities tend to live “short, brutal lives,” she says. Thirteen years ago, Bassuk and co-creator Jason Grabosky, now of Rutgers University, developed C.U. soil, which has proven to be a turn-around development for urban landscapes.

Everything from roads to buildings relies on compaction, the idea that the foundation of things must never move and that weight and pressure will combine to squeeze movement from the soil.

However, the proof that plant life and concrete are compatible is in the air—or what seems to be in the air—on a bridge across Cascadilla Creek in Ithaca, N.Y.

With the sidewalk of Linn Street blended in with the small bridge’s own design, the suspended tree does not stand out at first. Ithaca City Forester Andy Hillman is quick to point out the unusual location.

“Look under the tree. What do you see? There’s water running under there,” Hillman says.

The tree, a young elm, is growing on a suspension bridge and is expected to grow to a full height. It looks healthy. Its limbs are flush with leaves that are light green. The trunk is straight and strong, the branches hearty and plentiful. The city engineers, Hillman says, have accounted for the expected increased tonnage and the height of the elm, which might grow to 100 feet or find itself waving precariously in the wind during a storm. The growing roots and the possibility of ice heaving have also been accounted for in the design of the soil.

Nutrients have been accounted for because the tree is compatible with the pH of the stones commonly found in central New York and the clay-based topsoil used in the mix.

Still, the elm has been planted in between two layers of construction material. Above and to the side is a two-lane, asphalt road. Underneath is steel and reinforced concrete. At root level, the tree sits in what looks to be an averaged sized pit, the soil ringed with a patio area made of un-grouted cobblestones, which means the cracks in the pattern are porous. The tree will have water and drainage, and the roots are expected to dig deep, eliminating worry over a heaving sidewalk or street. In providing enough soil, which is inside the bridge, the roots will stretch horizontally, so there will be enough strength provided for a windy day.

Underneath the tree is about 10 feet of air and below that a fast-running creek famous for runs of lake trout and salmon. It is one of the city’s signature landscapes. The tree installation, however, is a man-made achievement.

We drive to a parking lot that, for its small size, happens to draw visitors from out of town. A parking lot with room for eight cars that has become, if not exactly a tourist attraction, a must-see for urban landscapers, who will call Hillman on occasion to give notice they are on the way. “We want to see your parking lot,” they say.

The bridge on Linn St.
An inlet is very close to the parking lot and the porous asphalt and C.U. structural soil is all the infrastructure provided. Natural filtering of the rainwater through the soil will be enough to keep any water that gets to the lake clean.

The parking lot, near the Cayuga Inlet, is slightly darker than the road and noticeably craggier. Hillman walks down to the inlet with a jar and brings back a quart of water. Hillman pours the water onto the asphalt and it percolates through and is gone by the time he puts the jar back in his truck.

Just ahead of a new federal initiative to control storm water and runoff to keep the nation’s streams and rivers clean, the top, sealing level of asphalt was omitted, and the parking lot rests on 24 inches of C.U. structural soil. No runoff is expected for as much as a 100-year rain event, according to Hillman.

The water, which will run through the parking lot and through the soil, will be soil-filtered clean before it reaches the inlet of one of the state’s more beautiful lakes, which is less than 25 feet away. A system that works “without catch basins or storm water pipes or any additional infrastructure,” Hillman says.

On top of that, the parking lot, where six or seven young elms have been planted, is expected to have 80 percent canopy coverage within seven years. The most stringent requirement for canopy coverage in any city Hillman knows of is in Portland, Ore., which requires 40 percent coverage in 15 years, he says.

Hillman took a demotion and a cut in pay to move to his job in Ithaca from Oswego, N.Y., where he was the assistant commissioner of public works, in order to get back to urban tree landscaping and to work in a city where there was a special mandate to research new advances in the field. Ithaca, home of Cornell University, provided that, and arborists throughout the country recognized his enthusiasm, electing him the president of the Society of Municipal Arborists.

Bassuk, an author and internationally recognized expert in tree installation, has been called on to consult all over the country and at two Olympic construction sites: Beijing, China, and Sydney, Australia. She is also a Cornell professor and chair of the city of Ithaca’s Shade Tree Committee. “We’re partners in crime,” Hillman says.

The trick for C.U. soil is in the stones, which creates “a matrix or a lattice-work,” Hillman says, and provide enough strength to appease city engineers.

The stones used in C.U. soil are of uniform size and make up 80 percent of the medium by weight. If compacted, the stones hold their shape and enough touch points among themselves to not give way. The soil, 20 percent by weight, is not crushed, as it rests in the open spaces between the stones, and the plant roots meander between the stones. Research shows that the roots in C.U. soil will adapt, flattening out where they need to and correcting to a round shape where they can. They will envelope the stone, and they will be drawn to deeper soil, which allows the roots to grow in size without damaging the surface infrastructure. The addition of hydrogel as a stabilizing agent lends water retention to the medium without creating a waterlogged condition, which could suffocate roots.

“The stones have to touch, so when you compact it, stone touches stone and it creates a rigid lattice,” Bassuk says. “So, all the weight is being borne stone to stone to stone all the way through, and the pores in between are filled with soil and the soil is not being compressed, because the stone is bearing the load.”

The medium, she says, is not applicable for tree pits. It is simply not enough to dig a hole as big as a young sapling’s root ball and fill the hole with C.U. soil. The roots need much more territory than that. They need, essentially, an infrastructure of C.U. soil that provides for depth and horizontal growth. West State Street in Ithaca has 3 feet of C.U. soil under the sidewalks from the edge of the street to the building foundations.

A reasonable depth of C.U. soil—3 feet is recommended—also provides drainage, which helps eliminate sidewalks heaving from freezing water under the surface.

The right mixture (including hydrogel at .03 percent of the weight) is available online at The structural integrity of the soil depends on the stones being a uniform size (between .75 and 1.5 inches) and angular with no fines (smaller stones) that will clog the pores, which are also essential. Sand, for the same reason, is not used.

The author is a freelance contributor based in Dryden, N.Y.