Greening our cities

The words 'concrete jungle' take on an entirely different meaning for Italian architect and urban planner Stefano Boeri. He is best known for his 'vertical forests': high-rise buildings integrated with trees and plants, which encapsulate his philosophy that places a premium on humans living in harmony with nature. The most striking (and famous) of these is the Bosco Verticale in Milan, a complex of two residential buildings that look like they have been overrun by vegetation. It symbolised the revival of an area that had been bombed during the Second World War and abandoned in derelict condition, and it has been replicated in many world cities. In this interview to Shaastra, Boeri outlines his vision for forested cities of the future. Cities, he points out, are the main contributors to climate change, and introducing plants into urban spaces is "like combating the problem at its source". Excerpts:

How did the ideas for vertical forests and then forested cities evolve in your mind?

My obsession with trees has been part of my life for a long time. I think trees are like us. The relationship between trees in a forest is not so different from the one of humans in a city. In 2007-2008, when I was teaching a course at Harvard, we went to Dubai to follow the expansion of this amazing city — all done with several glass facades and skyscrapers. I was thinking about this paradox of a glass city growing in the desert. There, Gerald Hines, a Texas-based developer, asked me to design a high-rise building for Milano and it got me thinking, "Why can't we do something completely different — a skyscraper with a biological facade with plants and branches and leaves everywhere?" Hines and Manfredi Catella, who was CEO of the real estate company Hines Italy, said it was impossible and crazy but they gave me three months to come back with the answers to some very simple but strong technical questions... I immediately gathered a bunch of friends/experts in different fields like botany and engineering to find answers and after three months I went back to Hines and Catella with answers. They said "Let's do this".

Can forested cities be implemented on a large scale?

Yeah, I think it's possible. We have developed many vertical forests all over the world. We are designing buildings tailored to the specific climate conditions of the location. The point, however, is not only to build new high-rise green buildings but to intervene in existing buildings. From that point of view, we are now developing different experiences. One in Brussels is super interesting because we are switching from an office building to a residential building, adding a new green facade which is completely autonomous from a structural point of view. We are also working a lot with social housing buildings that are affordable, by adding green facades. I also campaigned for the forestation of cities. With the Food and Agriculture Organization (FAO), we have promoted the first World Forum on Urban Forests, where we are creating a network of cities that are investing in urban forestry. The goal is to connect all the green and biodiversity hotspots we have in our cities. To do that, we need to substitute parking areas with plants with permeable organic lands. There are many challenges in this — the most important one is about traffic and parking areas. The main challenge is to gradually enhance public transport infrastructure while simultaneously introducing more plants to create biological corridors that connect biodiversity hotspots across cities.

How do you choose the trees for a particular location? Is there any chance that trees can damage buildings?

We work closely with botanists and experts who study the life of plants and trees, and we have learned a great deal from them. For example, we know which trees are more resistant to specific conditions. On the southern facades of buildings in central and southern Europe, which are exposed to intense sunlight in summer, we use trees that retain their leaves in winter. On northern facades, we do the opposite, selecting trees that lose their leaves in winter to allow more sunlight, which is scarce in northern Europe, to enter the buildings. For the southern facades, we choose Mediterranean trees like olives, while for the northern facades we select species better suited to maximising sunlight during periods of low light. We also account for factors like humidity, height, temperature and wind. Some plants have leaves capable of withstanding extreme wind, while others are less resilient. To ensure suitability, we test each new building, conducting wind tunnel tests to evaluate how plants endure windy conditions. This process has become an integral part of our work, as we treat trees as a strong and crucial material in our architectural approach.

What impact will urban forests have on the world's cities when implemented at scale?

There are two possible answers to this. From a symbolic point of view, we have built 15 vertical forests worldwide and are constructing another 15. We have designed over 60-70 buildings, and each time, the design evolves based on the specific climate conditions. Only by understanding these conditions can we select the right plants for the facade, as we treat trees as tenants. Each tree has specific needs depending on its stage of growth and the facade must cater to these requirements. This gives the buildings a unique iconic quality, as they stand apart from their surroundings. For example, in Brussels, Antwerp and Eindhoven, or cities in China like Huanggang and Nanjing, we create structures that are not only iconic but also serve as manifestos for what is achievable — making the seemingly impossible, possible.

The second answer is broader. These buildings represent a radical shift in how we envision the future of cities. Cities are the main contributors to climate change, producing approximately 75% of global CO₂ emissions. Introducing plants into urban spaces is crucial: it's like combating the problem at its source. Vertical forests offer several significant advantages in this context.

First, while we have solar panels, wind turbines and geothermal systems to mitigate emissions, only plants can absorb the CO₂ already in the atmosphere. Second, leaves can capture the dust from urban pollution — microparticles invisible to the eye but harmful to human lungs. Third, the shade provided by trees becomes increasingly vital as global warming intensifies.

Additionally, trees absorb and conserve water, support biodiversity by providing habitats for birds and other life forms, and offer psychological benefits. Numerous surveys confirm that green spaces improve mental well-being and even aid in recovery for the ill. We are applying this principle in projects at hospitals in China and Italy, where plants are integrated into every space, emphasising the importance of a close connection with greenery.

Are there design or technology advances that you expect will improve your ideas over the next decade or two?

I think it's always important for every architect to study how new technologies could improve the quality of their projects. We are working extensively with prefabrication systems because they offer a pragmatic way to reduce construction costs while significantly cutting construction times, enabling us to complete buildings in half the time required by traditional construction methods. Additionally, we are now incorporating wood and timber into our designs, which, alongside prefabrication, helps reduce carbon dioxide production. We are focusing heavily on this approach, to multiply the number of social housing vertical forests and affordable green buildings. Three years ago, we completed one in Eindhoven, Holland, designed exclusively for young couples and students on a rental basis, entirely built using prefabrication. We have also constructed two other buildings in Huanggang, China, and are now expanding this new typology of social housing – high-quality green buildings – on a broader scale.

I also believe that AI is a significant tool in this process. For example, we have designed a university city in South Mexico to host 130,000 people. In such projects, it is crucial not only to diffuse plants throughout the city but also to focus on managing public sanitation and water systems, as water management will become increasingly vital in the future. Smart networks can play a tremendous role in improving these systems, helping citizens engage in city maintenance and decision-making processes.

We use new technologies to make everyone more aware of their role in collective efforts to enhance the urban quality of life. However, there is a delicate issue: ensuring that we manage AI effectively without allowing it to govern us.

See also:

Cities of the future
The future of cities
Fantastic plastic!
Power to the people
Walking the green talk
Birds and the city
Cities' growth pains