Why aren't there more vertical forest buildings?

Why vegetated buildings aren't mainstream and why that's not a bad thing.

We’ve all seen them. Two residential towers cloaked in vibrant vegetation.

Surrounded by concrete and glass, they make a statement that says: this could be the future.

The Bosco Verticale (‘Vertical Forest’) buildings in Milan have become emblematic of the city’s regeneration and – for some people – sustainable architecture in general.

They are undeniably beautiful.

They seem unquestioningly ‘sustainable’ (we’ll get to that).

So, why aren’t there vertical forest buildings everywhere? Why not just make every new building along these lines? Wouldn’t cities be much better places if we did?

In this article I explain why the vertical forest hasn’t become a standard building approach and look a little deeper at its sustainability credentials.

Reason 1: It’s a landmark building, not everyday urbanism

Most cities are made up mostly of standard building typologies - courtyard blocks, terraces, suburban semi-detached homes - you get the idea.

Peppered throughout these standard typologies are landmark buildings that highlight important locations or showcase economic investment.

Bosco Verticale is a luxury residential development as part of the wider Porta Nuova regeneration area. A quick check online shows an 82sqm one-bedroom apartment in the building on sale for €1.3 million. A 4-bed family unit will set you back €5.5 million.

These are not homes for the general population. It’s not a replicable model for housing people in lush, biophilic surroundings.

And, to be honest, it really needs to be a luxury development because it’s so expensive to design and maintain. The 800 trees and 20,000 plants that adorn the facade are in common ownership and looked after by the building’s management company. Specialist ‘flying gardeners’ abseil down from the roof to maintain the greenery, keeping everything safe and looking good.

That means one hell of an annual maintenance fee for the apartment owners. Not something the average person would be willing to pay.

The fact that later iterations of the concept – like this one in Tirana – are much less ambitious with the level of greening perhaps tells us something about the expense and complexity of the original Bosco Verticale.

Reason 2: Architects don’t like to copy each other (at least not so obviously)

Stefano Boeri Architects is the Italian studio behind Bosco Verticale and, according to their website, it has become their signature style.

Even if they’ve created a brilliant new building typology, most architects are simply not interested in copying someone else’s signature style.

Plus, as the first of it’s kind, the technical knowledge to deliver something like the vertical forest would be very difficult for another architect to copy. A lot of R&D went into this building and the learnings are being applied in subsequent similar buildings by Stefano Boeri Architects.

No doubt that it’s an inspiration to others and may be responsible for kickstarting the ‘trees on buildings’ aesthetic found from London to Shanghai. But realised projects as ambitious as Bosco Verticale remain few and far between.

Reason 3: It’s not actually that sustainable anyway

Sustainable design is multi-faceted and technical. It’s often more about carbon accounting and engineering choices than grand architectural gestures.

People see a building covered in trees and seem to instantly conclude that it’s the pinnacle of sustainable design.

But let’s look under the surface and see if that’s true.

For now let’s just agree that a ‘sustainable building’ should limit its environmental impact and offer positive benefits for communities and nature. That seems fair, right?

Carbon Emissions

The buildings were designed and built between 2007-2014. This was quite a few years before net zero carbon became such a mainstream concept. Sustainability has moved on a lot in the last 15+ years so it’s a bit unfair to compare it to today’s standards.

But there is an underlying carbon challenge with this building typology.

The embodied carbon figures aren’t available online but there seems to be a consensus (see for example the World Green Building Council) that mounting thousands of plants and many tonnes of soil on a building adds a lot of weight. That means the building needs extra structural strength in the form of reinforced concrete – a highly carbon intensive material.

That means these kinds of buildings have ‘extra’ embodied carbon than they would need.

FYI: ‘Embodied carbon’ is the term used to describe greenhouse gas emissions that result from the manufacturing of building materials and the construction and maintenance of buildings. Check out the LETI Embodied Carbon Primer for more on this subject.

As I was writing this article, Dark Matter Labs released a great exploration of regenerative design and used Bosco Verticale as an example of greenwashing. They write that the building structure is twice as thick as a conventional building, and therefore has twice the embodied carbon.

If this is true, there really can’t be any argument that it’s a sustainable building. It’s literally twice as polluting as conventional buildings!

According to the architect, the building’s greenery absorbs 30 tons of CO2 per year. To be honest, considering most buildings don’t sequester any CO2, this is pretty impressive!

But seeing as the embodied carbon of the whole building likely reaches into the thousands of tonnes (the balconies alone may contain 990 tonnes of embodied carbon according to UGREEN_US), 30 tonnes a year is pretty much nothing.

Overall, sticking trees on carbon-intensive buildings is not an efficient way of achieving the dramatic carbon reductions we know is necessary to mitigate climate change.

On a positive note, the design may provide some energy savings in use. The building’s greenery ‘regulate microclimate conditions, reducing humidity levels and lowering surface temperatures by up to 30 degrees. This significantly reduces the energy requirement for indoor air-conditioning, bringing down indoor temperatures by 2 to 3°C.’ (source).

It’s well understood that nearby trees help to reduce overheating and air conditioner use. But this is a challenge for highrise buildings, which can’t benefit from street trees above the first few storeys. But it’s not clear if doubling the embodied carbon to support the greenery can be offset by savings through energy efficiency. I’m doubtful.

Ecological Benefits

The number of plants on Bosco Verticale is truly impressive. 20,000 plants from 100 different species, including 800 trees. The vegetation study for the planting plan took two years to complete. It’s all very well thought through and has stood the test of time (for at least the last 11 years).

But is it actually benefiting wildlife?

A study in 2018 in the Journal of Urban Ecology conducted bird surveys at three vegetated buildings in Milan, including Bosco Verticale, and compared the results to surrounding areas and to nearby conventional tall buildings.

The vegetated buildings did attract a higher number and species of birds to land on them compared to conventional buildings. However! The number of species seen landing on the vegetated buildings was lower than those found in the surrounding areas.

In other words: some birds will use vertical forest buildings if they’re there. But alone they’re not enough to make a meaningful difference in urban bird populations.

The researchers say: ‘This is likely due to there being an insufficient quantity of greenery on the building to provide core bird habitat, limiting their role to stepping-stones within the wider vegetation network.’

This gets to the heart of the issues with green buildings that claim to be mimicking natural habitats.

A vertical forest is not the same as a real forest.

A vertical forest building is like one of those annoying deconstructed desserts. Yes, the ingredients are there but they’re not adding up to form a whole.

A forest (or any ecosystem) is a complex web of interactions between soil, plants, mycellium, bacteria, invertebrates and all the animals you see above the surface.

Trees are not ‘where birds live’. Trees and birds are part of a complex system that we call a forest.

If you deconstruct the system and rearrange it on a concrete structure, you don’t have an ecosystem anymore. You just have a bunch of isolated plants that can’t support significant biodiversity.

What to do instead?

At this point you might think I hate Bosco Verticale and similar vegetated buildings. But I don’t. I’m genuinely happy that they exist. They spark wonder and bring nature to the forefront in spectacular ways. They make us smile and invite ideas of a greener future. And that’s worth something.

These architects are attempting to answer the right question: how do we design cities in response to the climate and biodiversity crises?

But they are responding at the wrong scale: the scale of individual building projects. I know that is literally their job so no shame in being innovative. But it is up to us to employ critical thinking and not get stupefied by flashy aesthetics.

Architecture is not the whole story. Cities aren’t just a collection of buildings. Any meaningful response to the climate and biodiversity crises will be interdisciplinary: at the intersection of many disciplines including urban planning, ecology, engineering, architecture, sociology and more.

I don’t have all the answers (or any really).

But one thing I have become convinced by is that nature needs is more space for nature.

To unfold in surprising and wonderful ways.

To escape the mowers, clippers and chemicals that we use to control it.

To become more wild.

It’s in wild spaces that nature flourishes and we build real climate resilience, not in one-off pieces of elite architecture.

That’s why I founded Urban Wilding Hub, a new nonprofit working to mainstream rewilding in cities. Learn more at the website: https://urbanwildinghub.com/