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The Science of Shade

Three decades of re-imagining urban horticulture

periodiCALS, Vol. 7, Issue 1, 2017

Nina Bassuk
Nina Bassuk, program leader of the Urban Horticulture Institute and a professor of horticulture. Photo: Robyn Wishna

Pedestrians enjoying the shade of the honey locust trees in New York City’s Zuccotti Park may not know that the leafy canopy is rooted—literally—in a bit of Cornell ingenuity. The CU-Structural Soil underfoot, a patented mix of large gravel and soil, brokers a truce between the pavement and the trees. A rigid lattice of rock holds the weight of the sidewalks, with room for soil in between for roots to scout for nutrients and water, resulting in larger, healthier trees.

The product, developed in the 1990s by then graduate student Jason Grabosky, M.S. ’96, Ph.D. ’99, and professor Nina Bassuk, has been used in more than 2,500 projects in 46 states and abroad from Ireland to Israel and is credited with changing how people think about trees and cities. It grew out of one of the central challenges in Bassuk’s career: how to make trees grow in cities when they’re surrounded by pavement. The impulse emerged when she was a child in Brooklyn with a postage-stamp yard and windowsills for experimentation.

“It was sort of visceral,” she says. “I always wanted to try to figure out why things would or wouldn’t grow.”

In three decades as the program leader of the Urban Horticulture Institute and a professor of horticulture, she has taken that question into arguably one of the most difficult environments for plants.

“Disturbed landscapes are found anywhere there’s been a road, building, excavation or grading and, as a result, the soil has been really compacted. The soil structure that allows plants to grow has been destroyed,” she says. “We have to come up with a way to bring that back once the dust has settled. They say, ‘Make a landscape here.’ But it looks like the face of the moon.”

But green infrastructure—a complement to the ‘grey infrastructure’ of pipes and sewers—is becoming more and more important in urban planning. It not only contributes to well-being and aesthetics, but it also solves several other problems associated with city landscapes at once, including handling storm water, reducing the urban heat island and reducing pollution in waterways. Bassuk’s landscape installations can guide water from a parking lot through bowl-shaped plantings, where it infiltrates the ground and can be cleansed, rather than running directly into nearby waterways. It’s an elegant ecological solution that doesn’t require upgrading municipal pipes and sewers.

“When plants thrive in a site—not just survive but really do well—then we get all these ecosystem benefits in terms of energy conservation and storm water abatement,” Bassuk notes. “Dealing with stormwater is a big deal in every city around the world. It’s not just a Northeast issue; it’s international. The climate or plant selection may differ, but the problems of urban soil compaction are pretty universal.”

The first lesson you learn from Bassuk is that although plants and trees may seem to define a landscape, soil always has the upper hand. That reality drove the development of CU-Structural Soil, as well as other strategies she has tested over the years. Another example is the ‘scoop and dump’ method students in her Creating the Urban Eden course have used in restoration projects around campus. Heavy equipment is used to ‘scoop’ the top 18 inches of soil and incorporate a vein of compost, followed by planting and then mulching it every year. Research conducted by graduate student Miles Schwartz-Sax 12 years after the initial ‘scoop and dump’ revealed surprising progress in soil improvement.

“It was interesting to see soil that had been compacted was recovering not just in its organic matter content and biological activity, but we also started to get granulation of the soil—clumps that allow for root movement, water movement and water holding—what we call soil structure,” she says. “It’s really fascinating because over 12 years there were big effects. People think it takes a thousand years to make soil, but we can actually get a fix in a few years. So it’s worth doing.”

Solving the problem of compacted soil is just the start. What to plant? Bassuk is eager to dispel the notion of the ‘perfect’ street tree. The reliance on a small number of tree species puts the urban forest at risk for devastating epidemics like Dutch elm disease, which killed an estimated 75 percent of elm trees—more than 77 million in North America alone. As the leader of Ithaca’s shade tree committee for three decades, she has used the city as a testing ground for her ideas, resulting in quite a unique urban forest.

“Typically in cities, maybe 10 trees make up most urban plantings. And maples are basically 40 percent,” she notes. “People sometimes want to retain uniformity, but that’s not the way to do things because there will always be some insect or disease coming down the road that’s going to get you. So the idea is to be diverse. In Ithaca, we have 200 different species on our streets.”

Aware that the lure of uniformity is very powerful—perhaps hardwired by images of European allées and majestic tree-lined avenues—she has also been developing designs that meld diversity with visual harmony, using trees that look similar but are genetically diverse. In a few years, she hopes to see her new hybrid oaks commercialized, the culmination of decades of breeding and selection for tolerance of difficult soils, drought and disease. They are trees equipped to thrive in urban areas in the century to come. But is climate change a game-changer for future urban forests? Bassuk suspects that is unlikely.

“Urban environments always had a different climate because of all the reflected heat from the asphalt and buildings,” she says. “Some people talk about the urban heat island in cities being 2 to 4 degrees (Fahrenheit) warmer than the surrounding countryside, but at the street level, it already could be 10 degrees warmer. The real challenge isn’t likely to change: giving trees the greatest soil volume you can and using drought tolerant trees.”