Can we please stop drawing trees on top of skyscrapers?

Editt Tower

Just a couple of years ago, if you wanted to make something look trendier, you put a bird on it. Birds were everywhere. I’m not sure if Twitter was what started all the flutter, but it got so bad that Portlandia performed a skit named, you guessed it, “Put a Bird On It“.¹

It turns out architects have been doing the same thing, just with trees. Want to make a skyscraper look trendy and sustainable? Put a tree on it. Or better yet, dozens. Many high-concept skyscraper proposals are festooned with trees. On the rooftop, on terraces, in nooks and crannies, on absurdly large balconies. Basically anywhere horizontal and high off the ground. Now, I should be saying architects are drawing dozens, because I have yet to see one of these “green” skyscrapers in real life. (There’s one notable exception—BioMilano, which isn’t quite done yet.) If—and it’s a big if—any of these buildings ever get built, odds are they’ll be stripped of their foliage quicker than a developer can say “return on investment”. It’s just not realistic. I get it why architects draw them on their buildings. Really, I do. But can we please stop?

There are plenty of scientific reasons why skyscrapers don’t—and probably won’t—have trees, at least not to the heights which many architects propose. Life sucks up there. For you, for me, for trees, and just about everything else except peregrine falcons. It’s hot, cold, windy, the rain lashes at you, and the snow and sleet pelt you at high velocity. Life for city trees is hard enough on the ground. I can’t imagine what it’s like at 500 feet, where nearly every climate variable is more extreme than at street level.

Wind is perhaps the most formidable force trees face at that elevation. Ever seen trees on the top of a mountain? Their trunks bow away from the prevailing winds. That may be the most visible effect, but it’s not the most challenging. Wind also interrupts the thin layer of air between a leaf and the atmosphere, known as the boundary layer. The boundary layer is tiny by human standards—it operates on a scale small enough that normally slippery gas particles behave like viscous fluids.

For plants, the boundary layer serves to control evapotranspiration, or the loss of gas and water through the tiny pores on a leaf’s underside, known as stomata. In calm conditions, a comfortably thick boundary layer can exist on a perfectly smooth leaf. But plants that live in hot or windy places often have adaptations to deal with the harsh conditions, including tiny hairs on their leaves which expand each leaf’s surface area and thus its boundary layer. Still, plants in these environments aren’t usually tall and graceful. In other words, not the tall trees we see in architectural drawings.

Next let’s add extreme heat and cold to the mix. Extreme cold, well, we all know what that does. It can kill a plant, turning the water inside its cells into lethal, crystalline knives. At the other end, hot conditions post a different set of challenges. To cool off, plants can “sweat” by opening their stomata to release water vapor, at least as long as there’s water available. But even then, plants reach a limit. At certain temperatures, which vary from plant to plant, the photosynthetic machinery inside a leaf starts to break down. Keep in mind these are temperatures on the surface of a leaf, not ambient air temperature. The surface of leaf—especially in direct sunlight, as on the unshaded side of a skyscraper—can be many degrees hotter than the air, up to 14˚ C in some species (nearly 26˚ F).

Then there are the logistical concerns. How are these trees going to be watered and fertilized? Pruned? How will they be replaced? How often will they need to be replaced? As someone who grows bonsai, I can tell you that stressed plants require constant attention. Daily monitoring, in fact, and sometimes even more frequently. It’s not easy. Growing simple green roofs is a chore, and those plants are chosen for their hardiness and low maintenance. Trees are generally not as well adapted to the wide range of conditions likely to be experienced on the side of a skyscraper.

All of this may sound a bit ridiculous coming from someone like me, an advocate for more trees in urban spaces. It probably comes from having seen one too many sketches of a verdant vertical oasis but too few of them actually built. Plus, having studied plant physiology, I know that it’s a pipe dream in many ways. Trees just weren’t made for such conditions. Now if someone want to gin up a tree that can survive on top of a skyscraper, go ahead, I guess. But I can think of far better things we should be putting our time and effort into, like preserving places that already have trees growing on them or planting more on streets that need them.

  1. “What a sad little tote bag. I know! I’ll put a bird on it.” Etc.

Illustration of Editt Tower, a proposed 26-story building in Singapore.

Related posts:

More reasons to stop putting trees on skyscrapers

How TED and The City 2.0 took the internet for a ride

Urban trees reveal income inequality

Income inequality, as seen from space

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  1. I tried to sign in with my name on slate about 12 times but finally was able to write this anonymously:

    You never used to see aerial views of buildings from architects; all that was up there was black tar and air conditioning equipment. Now all the renderings are from above, a verdant green landscape, that hides how big or awful the buildings are. They used to just cover the buildings in mirrored glass and pretend they disappear; now they put trees on it. It’s a trick to make it all look so warm and green and get approved, but nobody ever sees it that way, unless you are a bird.

    With Milanbio, I asked a landscape architect about it, looking at the section through the planters that is on a few websites. The trees are shown as being huge, but the planters aren’t big enough to support that kind of growth.

  2. Tim, I was glad to see you post this. I’ve been thinking about this exact fad a lot lately (and wrote a bit about it recently), and think it has to do with discounting the fact that plants and trees are part of an overall ecosystem, and are not just building adornments.

      1. Insect damage. So, nothing to do with being on top of a building. Also: comparing a tall building to a mountain? Really?

        1. Stressed trees are vastly more susceptible to insect damage than healthy trees. So yes, everything to do with being on top of a building.

  3. It’s more of a question really. They are just trying to figure out if there are better options than glass, glass and more glass.

    Nothing wrong with that.

    And I was at a lecture from Boeri last week. People asked him the same things you just did. And let’s just say that they yes, they did their homework.

  4. I enjoyed the piece.

    A 14 degree C difference is closer to a 25 degree F difference. 60 degree F takes into account the differing freezing points on the scales, but for this comparison, that is irrelevant.

  5. Just a couple of thoughts. If the architects were to draw alpine climate trees, would you feel better about it? I would like to think that a team of professionals including Landscape Architects, Arborists, Ecologists, etc., would be able to help the Architect come up with a viable solution to the issue. Hmm…maybe I should start a new business.

  6. Seeing these things drives me crazy. We should be focusing on the problems which have reachable solutions, like rethinking how we plant and manage street trees in our cities.

    If for a moment we thought about the maintenance of these things, the renderings would stop there. The costs of just window washing for high rise buildings is very high, think about regular pruning and tree maintenance, which would be extensive. How are we going to deal with leaf drop? What happens when limbs break on the 34th floor. Trees planted over structure need 3′ soil depth minimum (thats a minor engineering problem right there) to have a modest chance at survival, and will have to be closely watched and managed indefinitely.

    It doesn’t seem like the renderings you see of these buildings answer any of the major issues of how these things work, they just generate some quick publicity for an architect who probably spent about 2 minutes thinking about if it was a good idea or not.

  7. You’re probably right about skyscrapers, but if you look up in Manhattan, you would be surprised how often one sees greenery on terraces and balconies of so many buildings under 20 stories.

  8. I call hogwash. Plants are amazingly resilient. They live in some of the most climates on the planet: underwater, on mountains, in deserts, in the tundra, indoors, in the dark. As they grow, they learn and adapt to their environment – just like we do. Is it a challenge to grow plants on skyscrapers? Yes. Is it impossible? Absolutely not. Just like all the ignorant naysayers that said removing streets will increase traffic, we’re seeing ignorant naysayers from the peanut gallery saying plants won’t grow on top of buildings. You are WRONG. You will be proven wrong. This article will look stupid in a few years when these projects come to fruition. Will a lot of the trees die? Yes, but many will live. And that will make the fundamental premise of your article WRONG.

    And, by the way, they are BEAUTIFUL. And they help the environment. Get a clue before you publish ignorant opinions like this.

  9. while i agree that, in general, architects drawing renderings of buildings with abundant greenery can be problematic, you cannot generalize simply from looking at the renderings that architects haven’t put in the research necessary to determine whether such designs are possible. (and even if they are not – sometimes the work of an architectural rendering is to inspire new ways of thinking, new possibilities).

    also, rather than trees in cities being an either/or proposition, cannot they not be a both/and possibility? i.e. preserving land that naturally has trees, planting more street trees, AND including them in buildings?

    trees in buildings have been done successfully: check out the work of hundertwasser.

    1. I vote for both/and! BioMilano is not a cheap rental complex; its a very upscale condo building with prices starting a nearly 1 million dollars. Dr. Evil would stick his pinky in his mouth just to get in the front door. 😉 But let BioMilano be built for the early adopters and the price of tree-lined balconies will come down over time. They’ve done the research. The architect even put different species of tree in wind tunnels to test which species would do best on different sides of the building. Once Boeri gets the concept working and pays for the wind tunnel tests, others will just have to copy him to do it more cheaply.

  10. Good points. Thank you. From the green roofs that I have been it, it is apparent that not all vegetation thrives when on top.

    As to skyscraper aerial ecologies, I once had a corner office on the 41st floor of the Penobscot Building in Detroit. The major perk was a window that opened up onto a balcony that wrapped around both sides of the northwest corner of that building.

    On any given summer night, if I left a light on, the balcony would within hours completely overfill with white shrouds of spider webs that were quite difficult to penetrate; we’re talking balcony ledge high on the outside and taller than head high on the inside. Oh, and they were filled with hundreds or thousands of fat, black spiders, each with an abdomen about the size of a large thumb.


  11. What about something in between? Must it be trees to be green? Why not buildings engineered to grow other plants maybe even some that can produce fruit or veg? Smaller plants play a part in cleaning the air and providing O2. Shrubs? I don’t see why it need be all or nothing.

  12. There are some buildings with successful living trees in them. The one I think of, which may have really started the trend, is Foster’s bank high rise in Frankfurt. But that one has them enclosed, so he cleverly takes care of many of the issues you bring up.

    I feel like some of your information is quite good, but as others have pointed out, there are some successful planted buildings around the world. And, also, I think you bring up two subjects as one: A) Trees on high rises; and B) green roofs. The latter has shown to be successful in many places. But your main issues are the trees.

    My problem with these renderings we see all the time now are that the designers obviously have no idea how much soil it takes to successfully keep a tree alive. They often show sections of trees that are 20′ high growing in 3′ of total structural depth. When I see that, I know they don’t know what they are doing.

    I do believe plants and buildings should be brought together more often to help create better passive systems. The problem is, most designers make assumptions about what will work and don’t understand how to do them correctly. Or what climates they work in, etc. Foster, at least, did his homework.

    So- mostly I agree with you. And I like the Portlandia comparison. But I think you are being too one-sided on the issue. Still, it is time to call for an end to the “blind faith” jungle-high-rise.

  13. Very good points and information you have brought up. But no need to be pessimistic. It’s true that architects are ignorant (generally) of the issues you’ve brought up, but there’s no reason to believe they will stay that way. There are many ways I can see the issues you’ve brought up be rectified.

  14. Thank you for this article. It’s good to hear the words of an expert on such matters. I’m an environmental engineer myself, and I can’t tell you how many trendy “green” trends turn out to be neutral or even harmful to the environment.

    And for all y’all, there are differences between rooftop gardens and these massive productions. It’s the same difference between a flowerbed and an arboretum, scale. Yes, you can trim, water, and take care of a small tree or two in your spare time. However, you’d need an army of gardeners to handle the design in the page picture.