More reasons to stop putting trees on skyscrapers

Bosco Verticale

Robert Krulwich, disagreeing with me:

Two residential towers, dense with trees, will have their official opening later this year in downtown Milan, Italy, near the Porta Garibaldi railroad station. (The image is not a photograph, but an architect’s rendering. The towers are built and the trees are going in right now.) I love this. I think these towers are gorgeous. Milan is a very polluted town; these trees will cleanse the air, pumping out oxygen and greening the cityscape. I think cities one day could look like mountain vistas; I’m enthralled.

But I am not Tim De Chant, tree lover, blogger, critic, who says this won’t work. All these trees, he thinks, are about to be dead. He recently posted an essay on his Per Square Mile blog, aimed at architects. He called it, “Can we please stop drawing trees on top of skyscrapers?” He thinks builders know squat about trees. I hope he’s wrong.

I know I seem like Buzz Killington to a lot of architects—and non-architects, Krulwich included—but that wasn’t my point…entirely. To me, trees atop buildings have become an architectural crutch, a way to make your building feel sustainable without necessarily being so. And that’s a charitable assessment. Here’s how I really feel—trees on skyscrapers are a distraction from rampant development and deforestation. They’re trees for the rich and no one else. They’re the soma in architecture’s brave new world of “sustainable” development.

In reality, trees on skyscrapers will likely be anything but sustainable. Structures built to support trees need to be over-engineered compared with their abiotic equivalents—trees are heavy, so is dirt (multiply so when wet), and so are watering systems required to keep them alive. If those trees are to have a chance on these windy precipices, their planters had better be deep, which further compounds problems raised in the previous sentence. A skyscraper that’s built to support trees will require more concrete, more steel, more of anything structural. That’s a lot of carbon, not to mention other resources, spent simply hoisting vegetation dozens of stories up, probably more than will ever be recouped in the trees’ lifetimes.

Bosco Verticale, the oft, and often only, cited example of a tower to be built with trees on top, is expected to cost $85 million. Stefano Boeri, the architect, estimates adding trees to the design pushed up construction costs about 5 percent. (No word on maintenance costs.) Whether that’s true or not, we’ll have to take his word for it. If we do, that means they will spend $4.25 million to put 2.5 acres—one hectare—of forest onto the side of a building.

Now, let’s say we take that money and resuscitate the region’s natural habitat.¹ Average costs run about $500 per acre for reforestation in U.S. national forests, with a top end of about $2,000 per acre. Let’s assume the worst. That means that with $4.25 million, you could restore 2,125 acres, or about 860 hectares, of forest. That’s 860 times more forest than is plastered on the side of Bosco Verticale. At the least. If restoration costs come in at the low end, about $200 per acre, it could be as high as 8,600 times more.

Then there’s the ecological value of each. Bosco Verticale will be home to a few birds (most of which will live in the city regardless) and some invertebrates, but not much else. It’ll also require massive human inputs—water, fertilizer, tending, and replacement. I covered the first of those three in my previous essay, so I’ll just elaborate on the last point here, replacement. Let’s say trees on a skyscraper will live for an average of 20 years—a generous assumption given that more than 50 percent of street trees, which are exposed to more benign conditions, die after just 10 years—what will we have gained? A skyscraper that needs an overhaul every 20 years.

A real forest, on the other hand, can replace itself. It can also support hundreds, even thousands of species, even in the middle of the city. A survey of the 315-hectare Central Park, for example, found over 800 species. Near Milan, at Parco Regionale di Montevecchia e della Valle del Curone, there nearly 1,000 known species on it’s 2,350 hectares. Biodiversity is just one measure. These forests also purify water, maintain nutrient cycles, and don’t require much in the way of maintenance (if any).

Here’s an alternate plan: Instead of planting trees on buildings, let’s focus on preserving and restoring places that already have, or desperately need, trees. Boeri and I agree on the importance of the latter. Bosco Verticale is the first stage of Boeri’s larger plan, one that includes preservation and restoration of existing land.² Bravo. It’s clear that Boeri understands the big picture, that to make a truly sustainable city, you have to incorporate ecosystem function on a broad scale.

We still disagree on the value of trees on skyscrapers: he, and Krulwich, see them as an inspiration; I see them as a distraction and potential liability—what if the Bosco Verticale becomes a brown eyesore, turning people off to his larger vision? I’d love it if Bosco Verticale and other proposed arboreal skyscrapers were sustainable and successful.³ Who wouldn’t want to live in a city full of tree towers? But I just can’t make a case for it. Plant physiology tells me that the trees, if they do survive, will require constant and costly maintenance throughout their short, brutal lives. Finance tells me that the money required to afforest a building would be more effectively used for restoration and preservation. And my gut tells me there are more equitable ways to give people trees, not just to those who can afford it.


  1. You could also plant street trees or reserve more land for parks, both laudable and equitable uses.
  2. Among the proposals is a greenbelt around the city. They’ll be great parks, but won’t do much to contain the city.
  3. Really, Robert, I do!

Sources:

Roman, Lara. 2006. Trends in street tree survival, Philadelphia, PA. Master’s thesis.

Gorte, Ross W. 2009. U.S. Tree Planting for Carbon Sequestration. Congressional Research Service R40562.

Illustration of Bosco Verticale.

Related posts:

Can we please stop drawing trees on top of skyscrapers?

Urban trees reveal income inequality

Income inequality, as seen from space

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  1. Thanks for the very good article! It has always been difficult to me to understand why people are so eager to make urban structures appear sustainable by putting trees or some vegetables on them. The distraction from the real issues through such projects is a crucial point – so many research, projects, writing, conferences and workshops about ideas that have no or only so little impact whereas the real problems remain untackled, such as urban sprawl, high comsume of energy in cities etc. I guess transit-energy oriented urban planning and energy-efficient-building-standards are just not as posh as putting a tree on a skyscraper. It simply appears to look cooler and even journalists find sustainable urban planning suddenly interesting!

  2. Hi. Everything you’re saying is correct, but I think it might be important to note that in the mean time, the neoliberal urban sprawl development-at-all-costs guys seem to win, while critical left-progressives who focus on issues like deforestation seem to lose. So without loss of sympathy for your argument, which I think is ultimately correct, it may be tempting to peel off little victories in terms of living walls, roof gardens, etc. I don’t think it’s a distraction as much as a concession.

    From a diversity of tactics perspective, I’m glad there’s guys like you “throwing bombs” at urbanization writ large, but I’m also glad there’s moderate eco-hippies selling out and growing trees on skyscraper eyesores.

  3. As an architect who does this, I find that this article (and the previous one) miss the point completely.
    Its not about dressing up a building, in fact I find it really depressing that Tim thinks of architects as so shallow.

    Its generally about the occupants of the building, its about the clients who ask for this.

    For anyone who has lived in a hi-rise or works in one all day, it can get really dehumanizing and depressing. It would be so much nicer to have trees and birds outside my window, even on the 30′th floor. Typically you have to get up, go into the corridor, wait for the elevator, avoid eye contact with strangers, walk out and down the street to the closest park to sit under a tree. To eat lunch on a terrace under a tree, or even to sit at a desk, and see a tree outside is what its all about.

    By the way it the clients who ask for all those trees anyways, not the architects. Some trees can handle the wind, gray water (sink water) can be harvested to water them. I am currently designing 12 hi-rises with multiple balconies, each balcony has full-sized trees on them. We are not using green roofs (too expensive), rather large stainless steel planters, 4m x 4m x 1.5m, that are externally bolted onto the concrete structure. Also Stating that a particular tree doesn’t work in a windy area doesn’t mean all trees don’t work there. I have seen many cliff-top trees and they look awesome. As someone who grew up in Arizona I have seen trees growing atop cliffs in very dry and windy conditions with only 2″ of topsoil to work with and a yearly rain storm. If one designs a safe and maintanance free system with carefully chosen plants to go with it, the sysstem can be almost maintenance free. Just go to Detroit and see the trees growing atop the abandoned buildings there.
    Also check out the new wholefoods in Brooklyn that will have a huge greenhouse on its roof. there are so many variations of this concept we have overlooked or shot down.

    http://www.fastcoexist.com/1681738/this-super-local-brooklyn-whole-foods-will-have-a-20000-square-foot-rooftop-greenhouse

    Again, lets not miss the point of why most architects now put trees onto hi-rises, its for the occupants. It is for the connection with nature within the urban environment, without having to go into an elevator, down the street, and battle the elements. I do also agree fervently that the natural resources around our cities need protection, and mostly from Suburban encroachment.
    I would take a green skyscraper over 100 acres of Mc Mansions any day.

    -Ben

  4. What is your take on smaller scale roof-gardens utilizing things like moss, shrubs, etc. to reduce the maintenance and infrastructure costs?

  5. It never ceases to amaze me to find that laudable goals are diluted by popular fads and shallow logic. (if there ever was any practical logic) This is an exercise in bragging rights, marrying a genuine cause with the rewards of self edification.

    Would it not be useful for a city council to at least vote on such projects and deem them to be slightly off the beam of reality and not practical. (assuming the council would also have practical logic… which doesn’t seem they do)

    At the very least their idea could be made more feasible if they would aim to plant only brush or other plants that would not grow to such great heights or have large girths. This would allow easier per floor maintenance. By having shorter brush-type vegetation the wind would not pose as high a threat and the density would support natural privacy as a benefit. The planter containers would not require as much depth or volume either.

    I believe if the additional costs of adding actual trees to create the proposed forests is only 5% more compared to a building that does not sport such “jewelry”, I will gladly declare the supposed 5% additional cost is COMPLETELY INSANE.

    Depending on the type of tree and it’s fully grown height, the roots of “trees” is usually at minimum as deep and wide as the tree tops. This means if you build a suitable plant container, the container will be 8-10ft high (minimum if I were to believe the drawings…) and have a diameter similar in size to the tree top fully grown.

    Where in the world will you have people live??? maybe IN THE TREES… the forest “eco-system” will take up a fair amount of space around the edges of each floor.

    Also note, this design will put the EXCESS WEIGHT on the potentially WEAKER part of the building, the edge of the floors (where designers usually allow open views to the outside…). Unless of course you will place additionaly support beams in between trees which would help with weight support per floor. Then your residents would be living possibly in a semi-fenced in greenhouse type structure with support posts between trees… how charming. But yes, when the trees become mature they may hide the building structures… maybe.

    As you pointed out, the very costs of additional support materials is WAY MORE than 5%: re-bar cement for floors AND containers, support beams on edge of floors, plumbing for watering, actual water in pipes on peak loads, planter box drainage pipes (excess water needs to go somewhere…), and sewer system (outside planter boxes) in case a tree watering pipe bursts, are all a per floor excess cost. Then there’s the basic watering pumping/filtering system that’s required. This would require a monitoring system on each floor to check and make sure the water did reach the trees, unless you count on each floor dweller to manually check this and tell, whom …?, that there’s no water on floor X. (maintenance issues abound…)

    My rant is both, the lack of practical logic and our collective lack of diligence in reviewing such plans at the city/region level.

    Just because we have an idea does not mean we must act on it.

    … especially when it seems the table the design plans were drawn upon was missing 3 of the 4 legs!!!

    Apologies for the length…