Eight kilometres of green hedges, some 30,000 plants in total, cover the surface of the Kö-Bogen II in the heart of Düsseldorf. But how can green facades help in the battle against climate change? Architecture writer Jessica Holzhausen asks ingenhoven architects, the firm behind the project.
With the Kö-Bogen II, ingenhoven architects has set the record for Europe’s largest green facade. But the greenery is not just a striking aesthetic feature, it also constitutes an essential building feature and contributes significantly to the improvement of the surrounding urban climate. In fact, green facades like the one on the Kö-Bogen II solve an increasing problem in the concrete jungles that still mark so many inner-city spaces – concrete surfaces create heat traps.
Consequently, Kö-Bogen II’s facade hedges were chosen for their microclimate improving qualities: they protect the building against sun rays in summer, and by doing so reduce the overall urban heat in their surroundings. ”Conventional mineral and bitumen surfaces, especially in summer, store a part of the absorbed heat, emit it over time and make inner city spaces heat up even further.
Greenery, on the other hand, functions as an energy converter, they are always slightly cooler than the air temperature,” says lead architect and office founder Christoph Ingenhoven. “This way, they counteract the inner-city heat effect.”
Another widespread urban problem is that the sealed surfaces and lack of natural irrigation common in many cities, interrupt the natural water cycle and make places more prone to flooding in the extreme weather events that will become more common in a changing climate. This is a problem that can be helped by creating green roofs; they close a gap in the water cycle and naturally avoid overstressing the sewer system. Furthermore, the hedges bind carbon dioxide and dust, thus improving the air quality, and they store moisture and absorb noises.
Green spaces in any form also support a higher biodiversity by providing living and breeding space for insects and birds. According to ingenhoven architects, the ecological benefit of the Kö-Bogen II hedges equals that of “approximately 80 fully grown deciduous trees”. In short, an intelligent green concept can provide a valuable mitigative effect of the results of the climate crisis, while also creating a better environment for people to live and work in.
Finding the right plant
As a native hardwood species, the hornbeam plants used for the Kö-Bogen II facade do not lose their leaves in winter. This means the façade of the building remains in great shape all year round, even though the plants’ appearance change slightly with the seasons. “Hornbeam hedges radiate in a fresh bright green during spring, in strong dark green in summer and gold brown during autumn,” explains Christoph Ingenhoven.
Yet, choosing the right plant was not only about aesthetic; ingenhoven architects worked closely with Professor Dr Strauch from the Beuth University of Applied Sciences in Berlin to develop a phyto-technological concept for the building. (Phytotechnology is an emerging field that tries to find solutions for scientific and engineering problems by making use of existing properties of plants.)
One of the initial challenges of the ambitious planting project was to define the practical requirements of the hornbeam hedges. For instance, what kind of planting container was needed to support the 1.3-metre-high hedges, and how could the plants be supplied with nutrients and water? There were also maintenance aspects to be considered: for example, how to ensure that the hedges kept their shape and size. To make the hedged facade possible, the architects finally designed a separate support structure that is permanently fixed to the building.
The guiding principle through the different solutions was an improvement of the inner city’s climate and a sustainable upkeep of the hedges. “In winter months the plants need to keep their leaves, without being evergreens. The water use of plants without active foliage is reduced to a minimum in winter, which means the threat of them drying out is significantly reduced,” says Ingenhoven.
The chosen plants should of course not only be native to the region, but also non-toxic, easy in their upkeep and resistant to parasites and wind. Another important consideration was that the plants would not damage the façade with their roots. “Looking at all these factors already excluded a variety of plants – boxwood, ilex and cherry laurel because they are toxic, wine, hydrangea and field maple because they do not keep their leaves. Only the hornbeam fulfils all requirements,” says Ingenhoven.
To ensure that the plants could be delivered to the building site with fully formed roots, a tree nursery started cultivating the plants during the planning and construction phase about three years earlier.
A paradigm shift
The Kö-Bogen II’s main part, a business and office building, has five stories and a trapezoidal shape with a sloped facade; together with its three-storey counterpart, it creates a dynamic entrance to the Gustav-Gründgens-Platz in Düsseldorf’s city centre.
Included in the built environment around the structure are two icons of post-war modernism: the first, the Dreischeibenhaus, a 94-storey-high office and administration building erected between 1957 and 1960, and for many, a symbol for Germany’s post-war ‘Wirtschaftswunder’. The second, the Schauspielhaus building from 1970. “Kö-Bogen II is a contemporary response to these two historic landmarks, without competing with them,” the architects say.
As part of an extensive urban renewal project, the Kö-Bogen II, however, cements a paradigm shift in urban planning and development towards a more people-orientated approach. Since the beginning of the automotive area, cities in Germany have often focused on infrastructure for vehicles instead of other means of transport, creating parking spaces and elevated motorways instead of spaces for people to mingle in a healthier and more sustainable atmosphere.
In Düsseldorf, an elevated motorway dominated the space around the Gustaf-Gründgens-Platz, where the Kö-Bogen II is situated, and it is only since 2013 that the place has been reconnected with the Hofgarten, Düsseldorf’s large inner city green space and park. Ingenhoven, however, is convinced that urban green zones should be an integral part of architecture. “It has to be planned from the onset, and developing site-specific answers is essential,” he says.
Kö-Bogen II in Düsseldorf is not the only sustainable project ingenhoven architects has worked on. For decades, the architectural firm has made it its mission to give back as many green spaces as possible and has trademarked its supergreen® concept. Since 1992, the team around founder Christoph Ingenhoven has made the redesign of Düsseldorf’s centre one of its main goals, but the firm also works internationally on projects in Singapore, Japan and Australia.