In a world of online design competitions and social image sharing, many architects have taken to crafting ever more extreme models and renderings for public consumption. Some have even started covering their rendered buildings, from groundscrapers to high-rises, with gorgeous-looking trees. The effect can be breathtaking, but are these designs truly green or simply a fresh form of greenwashing?
The architectural motivations behind this trend are myriad. Vertical greenery gives a structure the appearance of sustainability. Greened towers suggest better air and a greener view both for building residents and the city. The brightly-colored renderings appeal to intrigued investors as well as sales-oriented developers.
Accordingly, representations of future skyscrapers with unlikely greenery are on the rise. The trend started with rooftops, but has grown to encompass all kinds of horizontal building surfaces.
Despite their visual appeal, many of these “treescrapers” will never get off the drawing board, let alone the ground.
For starters, the construction hurdles are daunting. Extra concrete and steel reinforcement are required to handle added weight. Irrigation systems are needed to water the plants. Additional wind load complexity has to be taken into account.
After installation, trees are also subject to high winds at altitude (but you never see them bent in renderings). The wind can also interrupt photosynthetic processes, while the heat and cold wreak havoc with many species of tree (especially tall, lush and lovely-looking ones).
Consider, too, that buildings have sides: the idea of putting the same trees all around, irrespective of wind and sun conditions, makes as little sense as finding trees equally on all faces of a mountain in nature.
And, of course, someone has to trim, maintain, replant, fertilize and clean up after all these living things as well.
A recently completed pair of buildings in Milan, Italy, called Vertical Forest, is a rare example of a finished project in the realm of largely-unbuilt treescraper concepts. Designed by Boeri Studio, the project consists of two residential towers, each a few hundred feet high and supporting an impressive array of plants. The plans called for nearly 1,000 trees, 5,000 shrubs and 10,000 additional smaller species.
This greenery was added in part to help filter air, reduce noise pollution and provide shade but also to support habitats for various species of birds and insects. The structural specifications had to account for root systems, plant weights and a central system for irrigation.
The towers have been considered a success in many regards. The project was granted LEED Gold, in part for its solar panels, won the Highrise Aware in 2014, and was named the 2015 Best Tall Building Worldwide by Council on Tall Buildings and Urban Habitat (CTBUH).
“The Bosco Verticale is a new idea of skyscraper, where trees and humans coexist,” says architect Stefano Boeri. “It is the first example in the world of a tower that enriches biodiversity of plant and wildlife in the city.” In short: the world’s first real vertical forest.
Critics, however, are quick to point out that the embedded energy involved in structurally supporting the trees (and hoisting them into place by crane) significantly offsets sustainability gains. Other reported problems include: higher than expected costs to tenants, structural issues with some specific trees and general construction delays.
Then there is the contrast between the drawings and actual structures. While the finished works, viewed independently, are impressive in their own right, they do not exactly look like the lush forested facades seen in the proposed designs. Perhaps there is a seasonal element, or the trees need time to grow and fill the space. Still, even on the website of the architects, a rendering remains the lead image, not a photo.
In some photos of the completed project, a plant in the foreground creeps into view. One starts to suspect the close-up tree’s role is to visually reinforce the presence of its sparser cousins above. The project still looks good in the end, but it looks significantly different than the visualizations, especially when it comes to the greenery.
This divergence of rendering and reality is not a one-off phenomenon; it is inherent to any tall-building project involving trees, and especially ones that propose much taller tree-topped towers.
We all know intuitively that a building will never look just like a drawing (architects often omit things like railings on balconies for a sleeker facade). Still, this particular trend may be getting out of hand, creating unrealistic expectations and leading to unsustainable solutions.
In the real world, between groundbreaking and ribbon-cutting ceremonies, most buildings will get at most a single tree during a “topping out” celebration. This Scandanavian tradition has ancient roots (possibly a way to appease displaced tree gods) and involves temporarily adorning the tallest beam of a structure with a conifer.
Beyond that token tree, however, extensive green coverage (a thinner layer supporting mosses, succulents, herbs and grasses) is often much more practical than intensive (roofs or balconies with shrubs and trees).
For extensive projects, the demands are lower for water, nutrients and ongoing management than they are for intensive ones. Structural requirements are also reduced: engineers need to account for just 15 to 50 pounds additional per square foot (for extensive) versus 50 to 150 or more (for intensive). For context: a normal balcony might be required by code to support 40 to 100 pounds per square foot, so an intensive green roof could more than double the engineering demands.
Yet even the thinnest green coverage requires a growing medium, filter fabric, drainage layer, insulation, waterproof membrane and more. Naturally, too, these flatter green surfaces tend to make for less attractive drawings, particularly wide-angle views at a distance. So for developers trying to sell units with impressive renderings, an extensive approach may seem less marketable even if it’s more sustainable.
There is a bigger question at the heart of this treescaping trend as well: what role should green space serve in cities?
A controversial recent design concept to wrap Central Park in a glass “sidescraper” was broadly criticized in part because it partially cuts the park off from the city, creating semi-permeable barriers between landscapes and citizens that need not exist.
A treescraper approach potentially suffers the same problem, but magnified: it lifts trees out of shared public spaces entirely, putting them up where they can be seen by many but enjoyed by few. Thus removed, they become more like window dressing, green ornaments rather than social activators. Civic greenery is a great asset to urban environments, but perhaps best when grounded in reality.