The Emerging Turf War Over Anti-Soil Compaction Systems

When Nina Bassuk was a kid growing up in Brooklyn, her favorite block to ride bikes was one where a tree root had heaved the sidewalk up into a ramp. “It used to give me a nice little lift,” Bassuk says. Now as head of the Urban Horticulture Institute at Cornell University, she sees those familiar chunks of buckled sidewalk as evidence of an urban environment that values its concrete more than its jungle.

Lately, though, the jungle has made a comeback as cities have begun investing in more ways to improve street trees and their soil. That’s partly thanks to growing enthusiasm for green infrastructure and landscape projects as economic development engines. It's also due to federal regulations that require cities to draft and implement formal plans to keep storm water run-off from spreading pollutants and overburdening sewer systems. If rainwater can get back into the ground through by filtering through street tree soil, there’s less of it for the city to manage.

Ten years ago, Bassuk developed CU-Structural Soil to help street trees survive in soil that’s been compacted by traffic and construction. When dirt gets packed too tight, air and water can’t flow down to the roots. Often that means the roots head off on their own expeditions for nutrition and room, tearing up sidewalk on the way, or the tree dies out and the city spends time and money replacing it. CU-Structural Soil is a mix of dirt and rocks designed to bear the load of pavement and whatever rolls across it, while leaving enough open space underground for tree roots, air and water. It’s since been licensed and sold to builders and landscapers across the U.S. and Canada.

Landscape architects around the country have been working on competing soil compaction solutions, too. As serene as the profession of landscape architecture may sound, disagreements over the relative merit of various systems have actually spawned some turf battles. The argument has even prompted The American Society of Landscape Architects to host a “Great Soil Debate” for opponents to duke it out at its annual conference, for two years running.

“If it wasn’t huge amounts of people spending huge amounts of precious dollars, it’d just be a little cat fight between scientists,” says James Urban, a landscape architect recognized for his soils research and designer of SilvaCells, a popular alternative to structural soil.

Though hard numbers are difficult to come by, says Leda Marritz, a spokeswoman for DeepRoot, a San Francisco-based company that sells SilvaCells, there’s no question it’s a growing market. Marritz's business is steadily seeing more demand, and more competitors, she says. The mandatory use of structural soils is being written into city building guidelines. Boston’s new Complete Streets planning guide specifically encourages their use, says Vineet Gupta, planning director for Boston’s transportation department.

Will one system or company emerge as the top soil solution? Let’s meet the contenders:

De-compactor: SilvaCell

How it works: Milkcrate-like plastic boxes are buried underground and filled with dirt. The plastic cages hold up the sidewalk and leave hollow spaces for roots to grow and air and water to pass through.
Famous addresses: Lincoln Center, New York City; the Olympic Village in Vancouver; Martin Luther King, Jr. National Memorial, Washington, D.C.
Where it came from: Landscape architect James Urban developed it, and now DeepRoot, a green building firm, holds the license.

De-compactor: CU-Structural Soil

How it works: A mixture of 20-percent soil and 80-percent rocks (by weight) gets packed around a layer of soil surrounding the tree’s roots. The stones are all the same size and the ratio ensures that each stone touches another. When concrete or heavy traffic press down on the soil, the stones create a rigid skeleton that bears the load while the soil itself stays loose.
Famous addresses: World Trade Center, New York City; San Francisco Zoo Hippo Exhibit, San Francisco, CA; Ritz Carlton, Washington, D.C.
Where it came from: Cornell University professor Nina Bassuk led the development and Amereq now holds the license.

De-compactor: Sand-based Structural Soil

How it works: A top layer of crushed rock allows air, water and minerals to get through to surface roots. The rest is filled in with sand that presses together under pressure, but leaves microspaces and remains loose enough for roots, water and air to move through. Compost gets mixed in to help hold on to water and nutrients.
Famous addresses: Alcohol, Tobacco and Firearms Building, Washington, D.C.; Federal Reserve Bank, Boston, Mass.; coming soon to the Brooklyn Bridge and Brooklyn Botanical Garden, New York City.
Where it came from: Landscape architect Robert Pine helped develop it, but it remains an open-source strategy and is not for sale as a product.
 

The soil compaction market is still fairly young, and while quantifiable data on the industry hasn’t been collected, there’s been plenty of time to develop human criticisms.

Urban, for instance, worries that the CU-Structural Soil hasn’t undergone rigorous enough scrutiny for its water retention capacities, while Bassuk insists it has and sighs that Urban “feels that the way to promote his system is to denigrate ours.” Pine says he has concerns that the hydrogel added to the CU-Structural Soil mix won’t last more than a decade.

Meanwhile Bassuk says Pine’s sand-based structural soil won’t compact enough, and Urban says Pine is in “denial” about the poor results it produces.

Pine and Bassuk agree that Urban’s SilvaCells are an innovative product, but argue it is by far the most expensive option. As for his criticisms of their soils, Pine says, “Well, Jim is selling a product, and none of the systems is perfect, but they do very well. Are they doing as well as if they were planted in a field? Maybe not.”

As passionate as each designer is for their chosen method, all three caution against cities and homeowners being blinded by the niftiness of any particular anti-soil compaction option. Urban says what’s really needed is better strategies for identifying places where trees will do well in cities on their own. There are places where “you spit on the ground and drop a seed and you’ll grow something,” he says.

Maybe spit gardening will take off next.