Susanne Hablützel breaks up her work day by admiring a rooftop garden through her window. The view may not be extraordinary – a pile of dead wood adorns an untidy plot housing chicory, toadflax, thistle, and moss.
However, Hablützel, a biologist responsible for nature projects in Basel, is captivated by the plants and animals that have taken up residence on the roof. “Tree fungi have established themselves on the trunks, and they are a pleasure to witness – I love mushrooms. You can also spot birds now – that wasn’t the case before,” she remarks.
Overlooked by the bustling streets of Basel lies an environmental marvel: thousands of gardens nestled on rooftops that would otherwise go to waste. Because of policies implemented decades ago, the city now boasts some of the greenest rooftops in Europe – averaging more than five square meters (50 sq ft) per person in 2019, equivalent to a spacious balcony.
These rooftops range from small office buildings, where Hablützel can see blackbirds catching worms in their beaks, to expansive spaces topping shopping malls, warehouses, and hospitals. However, industry experts say that what sets Basel apart is its commitment to using native seeds and plants – this is not merely a gesture to fulfill regulatory requirements.
“Basel’s green roofs resemble industrial wastelands, full of rich wildflowers,” explains Dusty Gedge, president of the European Federation of Green Roofs and Walls, who brought inspiration from Basel back to London. He describes the city’s rooftops as akin to brownfield sites, closer to dry and biodiverse grasslands rather than nondescript green meadows.
“If you show this to most people, they would say, ‘I don’t want that on my roof,'” Gedge comments. “But that’s what we need for biodiversity.”
Basel’s foray into green roofs began in the early 1990s, when residents voted to allocate part of their bills to a fund for energy-saving initiatives, including green roofs. Though not the first city to embrace green roofs, Basel quickly introduced stringent requirements for all new and renovated buildings with slopes below 10 degrees.
The success recipe for Basel, Hablützel says, was the combination of legislative mandates, support subsidies, and collaboration between scientists and the city.
A key figure in Basel’s green roof movement is Stephan Brenneisen, who leads the city ecology research group at Zürich University of Applied Sciences. His academic presentation on green roofs 30 years ago piqued the interest of Basel’s authorities, leading to his development of guidelines for the city’s green roof designs.
Brenneisen and the city have worked together to establish standards for garden depth and seed varieties. As climate change has intensified, he notes, the city has increased the minimum substrate thickness to 15cm to accommodate extreme weather conditions.
Despite these adjustments, Brenneisen maintains that the basics of green roofs remain straightforward technology.
Supporters value green roofs as an affordable means for cities with limited space to create urban oases. Like parks, green roofs cool the air during heatwaves and store water during storms. They also provide noise insulation and a home for wildlife that residents can enjoy.
However, without policies to back green roofs or reflect their social benefits, property developers may be deterred by the costs and owners by the maintenance. The additional weight may also necessitate more concrete and steel, potentially increasing a building’s carbon footprint.
Despite potential hurdles, “it’s a technology for the times we live in,” asserts Brenneisen. “We are in the Anthropocene—there is almost no wild nature left, and green roofs can provide that.”
