On top of every flat roof in Basel – new and old – you’ll soon find a thriving wildflower garden. For the past decade or so, the inner city has made it a requirement that all houses, offices and other buildings cover empty roof space with biodiverse greenery. All that natural insulation means lower energy bills, and the scheme has also made rare birds a more frequent sight throughout the city.
The scale of the project is unbelievable, with the green-roof mandate playing a large part in there now being five square metres of green space per inhabitant – the highest amount in any city, anywhere in the world. And while the gardens are all undeniably beautiful to look at (and make for a thoroughly pleasant place to live), there are plenty of other perks.
The environmental benefits of the scheme are huge. All that rooftop shrubbery stores a heck of a lot of carbon, slashing the city’s net emissions. Biodiversity – extremely difficult to maintain in a modern city – also appears to be flourishing. Almost 80 species of beetle have been spotted, along with 40 types of spider (many of them endangered). The gardens also provide refuge for 175 plant species, including nine kinds of orchid. And, as flash floods increasingly threaten the lives of city-dwellers, the soil can reduce water run-off by up to 20 percent. So the gardens aren’t just an attractive architectural feature – they’re a vital part of the city’s climate policy.
Locals are also big fans, partly because energy bills are massively reduced. During winter the leafy layer acts as insulation, trapping the heat inside, and in summer the temperature can be around five degrees lower. These benefits are obviously attractive, but it took legal action for the gardens to really take off across the city. Regulations dictate that new buildings have to have the roof forests installed, and whenever structural changes and checks take place on older properties, owners have to retrofit them with gardens too.
There’s a historical precedent in Basel for these green roofs. In the 1970s a city hospital built a rooftop garden to improve experience for patients. And way back in 1914, a water plant in nearby Zurich experimented with planted roofs to cool down the building. Researchers drew on these early ideas for their own plans, scaling them up for the whole of Basel.
They were led by Stephen Brenneisen, head of research at the urban ecology unit of Zurich University. He is keen to emphasise that cities can’t just grow a garden on a roof and think they’re making an impact. The substrate needs to be a certain depth – at least 12cm, and ideally 15cm – and the greenery should be native to the area. In Basel, regulations require a particular Swiss wildflower seed mix, which is sometimes even specified to a certain region.
Interestingly, the main problem scientists are now facing when it comes to green roofs isn’t disgruntled city officials or disinterested residents. It’s from their sustainable rooftop sibling, the solar panel. ‘At first, we basically had a good solution for unused space,’ says Brenneisen. No one was using the rooftops, so the installation of a garden wasn’t too disruptive. ‘Nowadays, though, the solar discussion isn’t easy to solve.’ The rooftops could be used for something else that is arguably even better for the environment. Brenneisen says his team is figuring out a way to allow the two climate solutions to coexist.
Despite this, it seems the green-roof revolution is here to stay, and Basel might not even be the most impressive example of roof-planting in Europe. Stuttgart is vying for the title of the continent's green-roof capital, having created more than 300,000 square metres of rooftop coverage, while fellow German city Linz isn’t far behind Basel, with three square metres of green space per inhabitant. Meanwhile, Milan is trying to introduce a rooftop garden scheme of a similar scale (as well as plant a whopping three million new trees).
Brenneisen is confident that an environmental revolution within cities is possible. ‘Societies are willing to change,’ he says. And while it took a hefty law change to drive through his scheme, perhaps other cities will see Basel’s roofs and think, simply: we could do with a bitta that.
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