These two residential buildings:

Or this one here?

Both are located in the Porto Nuova district of Milan.

And from what I could tell when I walked by them yesterday, they’re pretty comparable. They have similarly deep balconies. And they even appear to have the exact same exterior cladding.

Of course, the big difference is that the former — the celebrated Bosco Verticale — has about 800 trees, 4,500 shrubs, and 15,000 plants sitting on its 3.3m cantilevered balcony slabs. It also has an elaborate irrigation system that services said greenery.

Okay, so which is more “sustainable”?

First impressions would suggest that it’s the former. Trees and green things are good for the environment. So putting trees on a tall residential building must also be good, right? Maybe.

The main counterargument is that it requires a lot of additional work to get trees, shrubs, and plants onto a tall building. You need more concrete, more structural reinforcing, an irrigation system (maybe not always?), and a way to maintain everything going forward.

In this case, all of the greenery is a common element, and so it’s maintained by the building and not by any of the individual residents. Among other things, this preserves a uniform aesthetic.

But all of these additional materials increase the building’s embodied carbon. And so there’s an important question to consider: Do the benefits of putting trees up in the sky outweigh the impacts of actually doing it?

This is one of the great debates surrounding this project, and it’s a good reminder that being more sustainable isn’t so simple. There’s a lot to balance, and there are countless details to figure out.

However, innovation does require iteration. And already there are new iterations of the Bosco Verticale, such as this one in Paris, that plan to swap concrete for mass timber construction.

So even more trees in the sky. That’s probably a good thing.