Forests in the Sky: How One Italian Architect Is Reshaping Urban Greenery

“You know when you have an idea, it’s assembled from memories, experiences, books—it’s always very complex the way an idea is born. For a long time I was trying to study how it could be possible to put together a natural city in a different way; how to move forests inside the city, to increase the number of trees, to create more parks, to imagine how plants could grow on a building. It was an obsession for me.”

So began Stefano Boeri’s winding path to creating what he’s dubbed vertical forests: buildings with thousands of trees, plants and shrubs growing along its facade. The Milan, Italy-based architect’s founding vertical forest, or bosco verticale, consists of two side-by-side residential towers in Milan. Standing tall at 250 feet and 360 feet, the towers are home to a combined 900 trees and over 20,000 plants, all situated along many expansive balconies.

“We grew thousands of trees in a nursery and were training them to grow in a certain way so that they’d work on the building,” Boeri recalls. “At the end of 2012, we started to move the trees one by one from the nursery to the construction site and up to the different balconies. Today, it’s completely full with tenants, not to mention dozens of species of birds that also live in the trees and plants. It’s an ecosystem. For every tenant, you have basically two trees, eight shrubs and 20 plants.”

While the building may be its own ecosystem—a microclimate within the larger city—the vertical forest cannot run without the help of a dedicated team. Thanks to a centralized maintenance system, dozens of botanists and technicians can monitor the condition and humidity of each and every plant along the facade. Twice a year, what Boeri calls “flying gardeners” tend to the plants to make sure they’re properly pruned and maintained.

With the Milan towers serving as a successful proof of concept, Boeri has started building towers outside of Italy. Vertical forests in Lausanne, Switzerland, Paris and Tirana, Albania, are well into the advanced design phase (the team will start construction on Lausanne’s building in just a few weeks). Now, Boeri is ready to tackle countries where carbon-absorbing and oxygen-producing buildings are much needed, and he’s turning his sights to China.

“We know that cities are producing around 70 percent of carbon dioxide in the atmosphere and forests are absorbing around 40 percent,” he says, making vertical forests a natural solution in the fight against China’s growing pollution.

Boeri’s buildings in Nanjing, the capital of China’s eastern Jiangsu province, are already underway and should be finished by the end of 2018. The two towers will be taller than Milan’s—one standing around 650 feet and the other close to 500 feet—and are expected to house offices, a museum and even a Hyatt hotel. Boeri claims they’ll absorb a combined 25 tons of carbon dioxide each year and will create around 130 pounds of oxygen per day.

“The criteria we follow to select the different species are related to humidity, the different heights of the plants, the different sunlight exposures of the building and the wind conditions,” Boeri says. “Nanjing’s buildings will have different plant species than Milan because Nanjing has different conditions and its own biodiversity, but the philosophy and approach for the buildings is the same.”

But individual vertical forest buildings are only the beginning. Boeri’s firm, Stefano Boeri Architetti, has been asked to design vertical forest cities—sustainable miniature urban environments where all the buildings will be covered in local vegetation. While the masterplanning process is already underway for cities in Liuzhou and Shijiazhuang, the projects would take years to complete.

“After all, it wouldn’t just be residential towers, it will be schools, hospitals, museums, everything that makes up a city,” he says. “It will be a vertical forest revolution.”