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It's one of the world's most iconic examples of green design. To start, it's literally green — encased in hundreds of trees and thousands of shrubs that ascend on ladder-like balconies 26 storeys into the sky.
Milan's Bosco Verticale — "vertical forest" in Italian — opened in 2014 to wide acclaim from the design world. Its attractive vision of skyscrapers with leafy-green canopies has spawned dozens of imitations from France to Shanghai, including one underway in Toronto's Annex neighbourhood.
But critics say these buildings typically share some not-so-green traits: their construction relies on vast quantities of carbon-intensive concrete, and they are very expensive to own.
"I think it's completely missing the point of green design," said Lloyd Alter, who teaches sustainable design at Ryerson University in Toronto and is the author of Living the 1.5 Degree Lifestyle.
For Alter and other critics, Bosco Verticale represents an exclusive, and expensive, vision of a green future, where the benefits of living closer to nature are accrued to an enriched few, at an enormous carbon cost.
All of which helps explain why Stefano Boeri, the Bosco Verticale's celebrated architect, turned his attention to a different project: duplicating his iconic design as public housing in the Dutch city of Eindhoven.
"This is really the goal we had from the beginning of the vertical forest," Boeri told CBC News, "to show … that it's possible … to realize [a vertical forest] that is affordable for everybody, smart and sustainable."
That project, dubbed the Trudo Vertical Forest, officially opened last month. The pared-back recreation of the Bosco Verticale features some 125 trees and 5,000 shrubs over 19 storeys, filled with 540-square-foot starter apartments for young couples and emerging professionals.
It's Boeri's hope that this tower answers critics that vertical forests are greenwashing for an elite few. But even though he's succeeded in making his innovative design more affordable, there are reasons critics of the model like Alter remain unconvinced of its merits.
A forest in the sky, with sky-high costs
Boeri's vision for Bosco Verticale, developed in the early 2010s, was a version of architecture that did not centre around sheltering human beings from the environment. In his words, "living nature is not an ornamental presence" but a "basic component" of the building.
The pair of Milan towers — standing at 18 and 26 storeys, respectively — house a staggering total of 800 trees and 20,000 smaller plants, equivalent to three hectares of forest.
The trees eliminate the heat-magnifying effect of glass-fronted skyscrapers, absorb carbon dioxide from the air, and act as an urban oasis for dozens of species of birds and insects — and they're pretty nice to look at, too.
But all of that splendour comes at a cost. Carrying the weight of growing trees required enormous amounts of concrete, one of the most carbon-intensive building materials available.
"How many decades or centuries will it take for the tree to absorb the carbon dioxide that was emitted making the balcony and the planter that is holding [it]?" Alter said. "I didn't think it would ever make sense."
Then there's the question of who gets to enjoy this forest. A two-bedroom flat typically costs more than $5.7 million. Condo fees, used to pay for teams of specialized gardeners who rappel from the rooftop, run tens of thousands of dollars per month.
All to say, there is a reason the Bosco Verticale became known to Milanese as the "home of the elite."
Nowhere to go
In recent years, critics have been more vocal about this issue with the vertical forest philosophy — that it privatizes nature at a high environmental and financial cost.
In an article for Artribune, an Italian art criticism magazine, architecture critic Fabrizio Bellomo drew a contrast between the Bosco Verticale and a Milanese park notorious for drug dealing, known as the Rogoredo Grove.
"This grove," he wrote, "even with the facets of decay and marginalization connected to it — it still remains an environment to be lived in, a public space in all respects."
As for the Bosco Verticale, he said, not so much.
Kurt Kohlstedt, writing for the U.S.-based architecture and design blog 99 Percent Invisible, said of vertical forests: "This particular trend may be getting out of hand…. It lifts trees out of shared public spaces entirely, putting them up where they can be seen by many but enjoyed by few."
That is particularly hard to swallow in a city like Milan, which is struggling with an affordable housing crisis driven by skyrocketing rents.
Maria Chiara Cela, a spokesperson for the Milanese social housing co-operative DAR=CASA, said wait lists for geared-to-income housing in the city have topped 20,000 names, even as hundreds of flats sit empty awaiting public funds for renovation.
"Public housing has not been supported by public funds for a long time," she said.
Boeri, aware of these criticisms, says he always viewed the Bosco Verticale as a prototype for a theory of design rather than a completed project.
"Criticisms are always useful," he said. "You learn from your mistakes, and you … test new solutions."
So he was intrigued — grateful, even — when a Dutch social housing corporation, Sint Trudo, approached him in 2016 with the idea of replicating his design in Strijp-S, a former industrial district of Eindhoven.
Social housing in the Netherlands is a unique beast. Less than 15 per cent of the country's housing is private rentals. Instead, the vast majority of rentals are managed by public housing corporations like Sint Trudo that offer units at below market rents, geared to income.
But, similar to Italy, private rents have so rapidly outpaced incomes that wait lists for these public apartments are very long.
"On average in the country, [wait times are] somewhere between five and eight years," said Frans Schilder, a housing expert with PBL Netherlands, an environmental advisory group to the Dutch government. "In a city like Amsterdam, it can be up to 15 years."
That housing crunch hits young people particularly hard, as they tend to be stuck in the middle — with incomes too high to qualify for social housing but too low for private rentals.
"Our ambition was to create a centre … that was attractive to the young talents," said Jack Hock, the project lead on the Trudo tower. He described it as a place for that "missing middle" to call home.
So Trudo raised its income limit for applicants to $58,000 per year, and allocated its 125 apartments in a lottery this past summer based on "motivation letters" outlining how prospective tenants would contribute to a "self-starting" community.
'This can't be real'
The result is a vertical forest where tenants, most in their mid-to-late 20s, pay less than $1,000 a month to live in an iconic piece of modern architecture.
"When I first saw my apartment, I thought, 'No, this can't be real,'" said Roos Tullemans, a 29-year-old education assistant who is one of the building's "pioneers," tenants who run social committees and other groups.
"In Eindhoven, it is really, really hard to find housing, especially when you're my age," she said. "You either need to be on the list for a really long time, like 10 years, or you need a lot of money."
Rutger Rauws, a 26-year-old software engineer, said he got "instant goosebumps" the first time he visited his Trudo tower apartment.
"For some reason, I'm really enjoying the shadows that are cast on my windows," he said. "If I wake up at night, I look out the windows, and I still have the feeling I am dreaming."
Like Bosco Verticale, the Trudo tower is a prototype. While Hock said the costs were not much higher than for other social housing projects he's overseen, he acknowledges that even with its pared-back design, "it's not the most climate-friendly tower."
"It's a lot of concrete," he said. "If I had to do it again, I would take a lot more attention to the environment and the climate crisis."
But in this case, Hock says, the environmental benefits were not the point. Instead, the goal was to introduce greenery in a former brownfield industrial site, and create an exciting landmark for a new community of creative young professionals.
"That's a choice, and it is also a criticized choice we made," he said. "But it was the right choice to make."
Could Canada have a vertical forest?
Though Boeri and Hock both say the real work of green design is in less attractive solutions, such as cladding heat-leaking buildings and greening roofs, that lesson has not yet made it to the dozens of imitators who would try to recreate Bosco's design.
In Toronto's Annex, Brisbin Brook Beynon Architects is producing its own high-end version, a 22-storey tower housing 100 condos adorned with 400 trees.
Brian Brisbin, a partner with the firm, said the building was originally inspired by a desire to break the "heat island" created by downtown Toronto's alleys of glass-fronted skyscrapers, and a pledge by Toronto Mayor John Tory to increase the city's tree canopy.
But Brisbin is also honest about the environmental credentials of the design, which he says is only "green in the literal sense."
"We're basically using the money of the people who can afford to be here to work on a prototype," Brisbin said, which he hopes will eventually become accessible to other developers and institutions to develop their own less-expensive designs.
Boeri was not involved in the Toronto project. He says his own approach has evolved. In some of the dozens of vertical forest projects now underway around the world, he says he's using carbon-friendly timber framing and shorter balconies that minimize the use of concrete.
But he says those critics who see his vertical forests as an impediment to real green design are missing the point.
"What we've done is absolutely more radical," he said. "It's become a manifesto of a new modern conception of what architecture could do."