A highway overpass might seem like an unlikely place for a garden, but if you’re growing algae, it’s ideal: All algae need to thrive are sunlight and CO2. The pollution from cars driving below is actually an asset.

In a prototype built above a busy Geneva highway earlier this year, architects from The Cloud Collective tested a system for growing algae in tubes on the wall of the overpass. Though the tubes could be used on any wall, the designers wanted to take advantage of the abundant CO2 from the highway–and the symbolism of the location.





“We proposed a ‘garden’ to be built on a viaduct as we thought these sorts of locations are quite emblematic for our cities of today,” explains Joris Lipsch, one of the architects in the collective. “They’re generic, aggressive, unattractive, but at the same time seemingly inevitable in our daily lives.”

Rather than trying to turn the overpass into a more conventional garden, the architects wanted to adapt a garden to the site. “This resulted in the idea of growing algae in a tubular system, using only sunlight and CO2,” Lipsch says. “By doing this, we showed the potential of these sort of sites for being transformed into productive and active locations while leaving valuable and fertile ground free for other uses.”

The algae can be harvested by draining the tubes, and filtering the green goo inside. The material can be used in food supplements, since it’s high in protein, or to make products like cosmetics. Algae can also be used to make biofuel or turned into green electricity; algae produce five times as much biomass as plants.

The test in Geneva was only up temporarily, as part of a local festival. But similar experiments are being built elsewhere on building facades, roofs, and bridges. The designers believe the gardens are a logical step for cities everywhere.

“As architects and urbanists, we think that this project shows the potential of reflecting differently on sites we consider being ‘wasteland’ or ‘industrial,'” Lipsch says. “If we consider the scarcity of fertile land and green open space, using every bit of potential of these sort of sites is crucial for the quality of our cities and landscapes.”