In 1972, the streets of Milan, Italy, were a hotbed of politics and protests, and 16-year-old Stefano Boeri was at the center of the action. But as Boeri marched for economic justice, an eccentric street artist named Friedensreich Hundertwasser was preaching a different ideal, one that would inspire the future architect’s vision of sustainable design.

“I remember him holding up a small oak tree in one hand, and he was talking about the trees being considered as houses for tenants,” says Boeri, now 60. “I wasn’t especially interested in ecology, but Hundertwasser was one of the first to observe the relationship between humans and trees from a different perspective, and that was very, very influential.”

Hundertwasser eventually helped design one of Vienna’s most popular tourist attractions, the Hundertwasserhaus, a mixed-use complex with 250 trees and bushes growing inside residences and across the building’s facade.

Boeri went on to study architecture in Milan and earned his PhD from the Istituto Universitario di Architettura in Venice. Today, his Milan-based company, Stefano Boeri Architetti, is one of Italy’s most renowned sustainable design firms. It’s also a pioneer in “Vertical Forests”—buildings that feature hundreds of trees that sprout from balconies, creating an ecosystem of flora and fauna that stretch across the structures’ exteriors. The architect’s leaf-covered creations can be seen in cities across the world, from Switzerland to the Netherlands to China.

Boeri’s flagship Vertical Forest is a $60 million, geothermally heated, Leadership in Energy and Environmental Design (LEED) Gold building in Milan completed in 2014. The solar-paneled residential towers—27 and 19 stories tall—feature 900 trees, each 10 to 30 feet in height. On flat land, that’s equivalent to nearly five acres, or 215,000 square feet of forest. Along with 20,000 shrubs and flowering plants, the greenery creates what Boeri and his team call a microclimate, producing humidity, filtering dust and pollution particulates, absorbing carbon dioxide, and producing oxygen.

In summer months, the Vertical Forest can reduce the building’s surface temperature by 54 degrees Fahrenheit, cooling the inside by as much as 5.4 degrees. And rainwater that would otherwise flow unused to the sewers is recaptured by the building’s vegetation and an advanced irrigation system.

Individually, Boeri’s Vertical Forests make measurable impacts on energy use and the buildings’ immediate surrounds. The Milan towers remove 11 tons of carbon dioxide from the air each year, according to Boeri’s firm.
But the designer’s next project will do much more than build a Vertical Forest on a city block. It will transform Shijiazhuang, China, one of the most polluted urban centers in the world, into a “Forest City.”

A greywater filtration system (used with water that has gone down the sink or shower) ensures the trees are adequately watered.

“Lung busting” pollution

On December 19, 2016, a thick, noxious smog hung over Shijiazhuang, China, shutting down schools and businesses for the city’s 10.7 million residents. The South China Morning Post described the air as “lung busting.” Even inside their homes, citizens could not escape the choking smog from burning coal and car emissions. And it was not an unusual day in Hebei province.

The World Health Organization ranks Shijiazhuang’s air quality 14th among the world’s most polluted cities. On that chilly winter afternoon, the level of PM2.5 and PM10, two of the most toxic airborne particulates for human health, were reportedly more than 1,000 micrograms per cubic meter. The WHO recommends exposure be kept under 25 and 50 micrograms, respectively.

Which is why Stefano Boeri’s plan for Shijiazhuang seems both brilliant and, well, crazy. The Italian architect plans to turn a 556-acre section of the metropolis into a “Forest City,” with dozens of Vertical Forests stretching across five multi-use districts. The idea is to create a borough-sized air purification system for 100,000 city residents. Based on the data from Boeri’s one-off Vertical Forests in Milan and elsewhere, the Forest City could have a massive impact, removing tons of carbon dioxide from Shijiazhuang’s air and reducing the levels of toxic particulates to safe, habitable levels.

“Every year in China, 14 million people abandon the countryside for the city,” says Boeri, “and Shijiazhuang is the result. It’s one of many megalopolises where public policies are creating a nightmare in terms of environmental impact and quality of life, and [the Chinese government is] now seriously considering the consequences of their planning.”

Boeri’s team is currently exploring locations but has not announced a construction date or budget for the project, branded ForestCityShijiazhuang. He says it will likely strive for LEED certification.

The development will be built on just under one square mile of Shijiazhuang and focus on vertical density. Shijiazhuang proper spans 825 square miles and is the largest city in northern China’s Hebei province—almost three times the size of New York City. That urban sprawl has multiplied the impact of development and pollution across the landscape.

The master plan calls for five petal-shaped districts, each housing 20,000 residents in mid- and high-rise buildings. The petals surround a central park with a hospital, school, and cultural activities, but each multi-use district will provide nearly every amenity to residents: commercial, retail, public spaces, and gardens. The result is a reduction in both urban sprawl and the pollution that comes from transportation.

The centerpieces in Boeri’s master plan, though—and what distinguishes it from other multi-use urban developments—are the Vertical Forests themselves. Based on each Vertical Forest’s green cover and the design’s scale, the project could create millions of square feet of forest, all within one square mile of city space. Boeri’s hope is to use ForestCityShijiazhuang as a prototype and replicate the design in polluted cities around the world.

The Forest City in Shijiazhuang will be a new city for 100 thousand inhabitants. A city of a newgeneration, capable of becoming a model of sustainable growth in a large country seeing, each year, 14 million farmers migrating to the cities.

Every square mile of a Vertical Forest façade absorbs approximately 0.4 kg of CO2 a year.

Growing an ecosystem

Planning a Forest City’s greenery is not a task for the average weekend gardener, so Boeri tapped Studio Laura Gatti, an award-winning landscape consulting firm in Milan, to develop Forest City Shijiazhuang’s horticultural strategy. Gatti and her colleagues worked with Boeri on several Vertical Forests—including the designer’s first in Milan—and they’re now studying the best approach to realizing a living, breathing Forest City in Shijiazhuang.
“These are more than residential buildings,” says Gatti, an agronomist who also teaches at the University of Milan. “They’re an intervention of trees—a way to regenerate the whole environment.”

That regeneration begins with an exploration of the region’s climate, soil ecology, and native tree species. From there, Gatti and her team will reach out to local nurseries to see what trees are available, selecting each to fit the buildings’ design, balcony dimensions and weight limits, outside environment, and the trees and vegetation that will surround them. Only “safe trees”—those free of defects and impervious to strong winds—will make the final cut. And because each tree is planted in multi-membrane, concrete containers, trees with aggressive root systems are also out.

Once the trees are chosen, Gatti will work with nurseries throughout the growing process. And from day one, the team must coordinate and communicate its choices with the project’s architects, engineers, construction crew, gardeners, and maintenance personnel.

After the Vertical Forests are completed, maintaining the trees requires a highly sophisticated irrigation system. Israel-based Netafim, which pioneered high-efficiency drip irrigation technology on Israel’s desert farms, designed the Milan Vertical Forest system. The company’s technology uses a combination of collected rainwater, graywater from residents’ apartments, and underground water sources to drip irrigate the building’s thousands of trees and plants. Each planting container has a chip, sending soil moisture information to central computers that command the pumps.

Trimming the trees also requires trained professionals—horticulturists who are unafraid of heights. To clip the outer reaches of the Milan Vertical Forest’s nearly 1,000 trees, a crew of three rappels down the building’s sides, snipping and shaping the trees to maintain adequate light and shade for residents.

Focusing on tree planting and care, Laura Gatti advises cities and developers on innovations of mixed-use design, urban regeneration, urban soil conservation, vertical greening, urban farming, and green building with environmentally sustainable principles.

Gatti’s horticultural process is exhaustive, and it takes time to create and monitor a working ecosystem in an ever-changing climate. She and her team entered the Milan project in 2008 and planted the building’s vegetation in 2012.

While ForestCityShijiazhuang’s scale will be a challenge—and its impact will be just part of a holistic solution to pollution in cities around the world—Gatti is optimistic that Vertical Forests and Forest Cities can lead to more significant change in how humans view the natural environment. Since opening the Milan Vertical Forest, she says, residents report improved well-being from seeing flowering trees and plants—and the birds, bees, and butterflies they bring—just outside their windows and balconies.

“Vertical Forests are places where people can recognize our relationship with nature,” says Gatti, “not just green design. They can be a [sign] that things can be different. That cities can be different.”

Boeri agrees. And while his Vertical Forests and vision for ForestCityShijiazhuang might seem as eccentric as his street artist muse, the architect aims to do more than turn heads with his designs.

“Art is best when it opens new perspectives,” he says. “When it anticipates new environments and ways of thinking that are not yet real.”

From polluted metropolis to forest city