Scheduled for completion later this year, Bosco Verticale, by Boeri Studio, will be the world’s first vertical forest. The project’s inspired many supporters, but also many detractors. Speaking to its controversiality, Lloyd Alter, the architect, sustainable design enthusiast, and managing editor of Treehugger, called it “the rendering that launched a thousand blog posts.”

And perhaps no blogger caused more stir in the architecture community than Tim De Chant, who implored “can we please stop putting trees on skyscrapers”? De Chant’s article set off a maelstrom of comments from ArchDaily users, who vigorously debated both for and against the idea of putting trees on buildings.

To get to the bottom of this, we talked with Lloyd Alter himself about vertical forests and the real challenges and benefits they present. Lloyd is a regular contributor to Inhabitat, The Huffington Post and numerous other publications; he also teaches at Ryerson University School of Interior Design. Read on for Lloyd’s take on this controversial trend, after the break.

What are the biggest challenges in construction of vertical forests, and how, if at all, can these obstacles be overcome?

Trees and the soil that support them are heavy. The concrete planters that support them are heavy. I wonder how long it will take for the CO2 released in making the concrete that holds up the trees to have the equivalent amount of CO2 absorbed by the trees. I suspect never.

What are the pros and cons?

The biggest benefit is the shading. Vines and trees are the perfect device for this function because the leaves fall off, shading in summer and letting the sun through in winter. They obviously absorb CO2 and clean the air. There are few downsides.

But the one downside I can think of is the appropriateness of trees versus vines or other plants. If I was convinced that it would ever look like the rendering I would have no qualms at all.

Why do you think Bosco Verticale, a vertical forest design in Milan, is on the track to succeeding where so many others have failed?

I am not certain that it is on track to succeeding. Give it ten years. Personally, I love the idea. I just hope it works.

What do you think Bosco Verticale will look like ten years down the road?

I suspect not much different than it does one year down the road after all the trees are installed. They may not die, but they are not going to grow into the forest that is shown on the rendering. There just isn’t enough room for the roots. You see this in city trees all the time; they survive but they don’t seriously thrive and grow big, they don’t have the room.

Milan is one of the most polluted cities in the world, and part of the objective of Bosco Verticale is to use its trees to clean the air. Will this have a perceptible effect on air quality, and how will this affect the health of the trees?

I am certain that this will have a perceptible effect on the air quality of the people who live in the building; just sitting under a tree anywhere is lovely and cool because of the shade and transpiration of moisture. Trees are the perfect sunshades for a building; the leaves fall off and the sun shines through. In a lot of ways it is absolutely the most logical thing to do in a building, they are the perfect seasonal shading device. My biggest concern here is that the reality will never match the rendering, because, again, they are not in the ground but in a limited size planter.

Are there additional safety concerns that need to be taken into consideration when building vertical forests?

Weight, and breaking limbs in big storms. This happens on the ground and the limbs kill people. what happens when it falls off the building?

You wrote that putting trees on skyscrapers is greenwrapping – can you elaborate on this?

By greenwrapping I mean using something green, like a roof or solar panels, to disguise or attempt to minimize the impact of a noxious use or something that just doesn’t fit. This is not greenwrapping; the trees are a big part of the building, its most important distinguishing feature.

Do you have any favorite vertical forest designs?

Look at the work of Edouard Francois, who started with the Flower Tower and has moved on to bigger projects. He doesn’t do green walls, but he does living facades, planted in the ground and growing to cover the buildings. The Building that Grows is my favorite, where the wall is designed like a natural cliff, a gabion of rocks that promote natural growth.

From Edouard Francois’ website:

“One morning, when handling rocks, chicken wire, and concrete, we invented a living skin. It had to grow; to sprout. We put bags of potting soil and plants behind the stones. We watered it with organic fertilizer. It was seeded by mountain climbers. Then we installed an automatic irrigation system on the façade.

The building grows. Slowly. Its skin has become a kind of mini-ecosystem. The water collects in the interstices, algae forms and then dies, mosses grow and herbs colonize the resulting compost. Scattered, physical traces reflect these transformations.”

Save this picture! The Building that Grows / Eduard Francois © Nicolas Borel

Are there any promising designs for vertical forests in the works?

I have seen a lot of them, but I cannot remember if they are Evolo wet dreams or reality.

Do you think vertical forests will ever be practical?

I have serious doubts.

How much of the problem is the trees? Would buildings covered in other forms of vegetation be more sensible? If so, what kind of vegetation?

That is part of the problem; trees evolved to go into the ground. There are all kinds of plants that evolved to climb and creep. Some of them grow out of the tiniest of crevices and cling to little bits of rock. I would think that there are many kinds of plant that one could design a building to promote and support.

What advice would you give to upcoming architects who are interested in working in this area?

I had lunch today with a lawyer who had a problem: his clients wanted to build houses on back lanes where there were no sewer and water services, and he wondered, were there alternatives? In fact there are many, from rainwater collection to composting toilets, you can go off pipe as well as off grid. Green roofs and vertical forests provide the option of increasing our independence and resilience; I would kill for an orange tree on my balcony that could make my breakfasts sweet. We also have to look at every way we can to make high-rise living as pleasant as living at grade, and that means green.

What advice would you give to people who live in cities and want to bring more greenery into their lives?

First of all, invest in your rooftop. It should be in the zoning bylaws that every roof should be built to be green, and I don’t just mean the thin green roof, but a real, habitable plantable roof that can support real trees or real agriculture.

Then we have to take back the streets. It is criminal that a person gets to occupy 200 square feet of space to park a car, when you can grow $1400 worth of food in that much space. Who gave it all to the cars? Perhaps we wouldn’t need to think about vertical forests and farms if we properly used the horizontal space on the roofs and in the street.

Living in Brooklyn, New York, Alex Levin is a writer specializing in sustainable design. For information on responsible tree care, check out the Sherrill Tree Learning Center.