Super green into the future with Europe's largest green façade

A superlative is the most fun when many people benefit from it: Kö-Bogen II has completed Düsseldorf's new center and, with Europe's largest green façade, is setting a widely visible exclamation mark in terms of climate architecture. When spring arrives on the façade of Kö-Bogen II, delicate green leaves sprout from the more than 30,000 hornbeams growing in 3,500 troughs. In early summer, the 1.30 meter tall hornbeam plants produce hanging yellow flowers alongside their dense foliage. Taken together, they form a hedge of around eight kilometers. Take a deep breath and fill up on green!

They wanted to see the seasons depicted on the façade and were looking for a plant that was typical of the location and as insensitive as possible, says Düsseldorf architect Christoph Ingenhoven*. In 2014, his office won the international competition to redesign the area at Gustaf-Gründgens-Platz. Central to the concept: Europe's largest green façade. For around three decades, Ingenhoven Architects have not only been pursuing their comprehensively sustainable approach - the term Supergreen® has been trademarked. Christoph Ingenhoven has been working on the redesign of Düsseldorf's new city center for just as long.

Today, it is architecturally defined in particular by the interplay of the iconic buildings Dreischeibenhaus (HPP, 1960) and Düsseldorfer Schauspielhaus (Bernhard Pfau, 1969) with the new landmarks Kö-Bogen I (Daniel Libeskind, 2013) and finally Kö-Bogen II, completed in 2020. It is hard to imagine that just over a decade ago, the "Tausendfüßler" with its prestressed concrete stilts cut a swathe through the urban landscape here - in the original Sixties style, but still. The elevated road had to make way in 2013. It was made redundant by the construction of the Kö-Bogen tunnel and the resulting banishment of traffic underground.

Green valley and new center - Kö-Bogen II

The Wehrhahn line with its six subway stations, which went into operation in 2016, further eased traffic congestion. The adjacent Schadowstraße was also massively upgraded by this major urban planning project. The shopping mile, which is one of the busiest in Germany and is reserved for pedestrians and cyclists, is lined with the glass fronts of Kö-Bogen II, or rather its main building - an office and commercial building towering up to a height of 27 meters with well-known fashion brands and other retailers. This is because Kö-Bogen II is in two parts: if you move from Schadowstrasse to Gustaf-Gründgens-Platz and thus back to the side of the building with the terraced green façade, the ten-metre-high annexe building, which stands opposite the main building, comes into view. The triangular pitched roof is designed as an accessible lawn. Lingering, relaxing, enjoying the sun - all expressly permitted!

Those who accept the invitation have a box seat with the best view - and not just of all those strolling through the Ingenhoven Valley, as the area is also known. From the lawned roof of the so-called market hall, which houses a burger store and a café, among other things, you can enjoy the green backdrop opposite. But the green façade is not just pleasing to the eye. In times of climate change, it is a valuable contribution to the city of the future. The jury of the German Sustainability Award described Kö-Bogen II as "pioneering work for future building projects with intensive façade and roof greening". It was shortlisted for the award in 2021. With the DGNB Platinum Seal, awarded by the German Sustainable Building Council, the project also received the highest rating for a newly built commercial building. It also received the DGNB Diamond award for its outstanding architectural and design quality.

Beech trees instead of oaks

Around 80 mature deciduous trees would have to be planted to achieve the ecological benefits of the hornbeams in Kö-Bogen II. However, there is often not enough space in the city for urban forestation - loosely based on Joseph Beuys, who once gave the Documenta city of Kassel 7,000 new oak trees. Planted roofs are just as much a solution as vertical forests and gardens. These green architectures are now being tried out all over the world, from Milan's Bosco Verticale to Sydney's One Central Park. In the case of Kö-Bogen II, the staircase of the building lent itself to creating the largest possible planting area and good growing conditions, inspired by terraced construction in mountainous regions. The stepped form and the sloping triangular roof of the market hall opposite also create the characteristic Ingenhoven valley, which opens up towards the towering Dreischeibenhaus and the pure white, organically curved Schauspielhaus. In this way, it sets the stage for the famous duo of post-war modernism.

The Kö-Bogen II - a major energy converter

Breaking up sealed surfaces: The living greenery of Kö-Bogen II not only binds CO₂ and fine dust, it also produces oxygen. It absorbs noise and promotes biodiversity. The green building skin also stores moisture and lowers the ambient temperature through evaporation. In summer, the hornbeam hedges act as a heat buffer by shading the building - this also has a positive effect on the microclimate, considering that facades can heat up to 70 degrees in strong sunlight. This heat then radiates into the surroundings. Heavy rainfall events can also be better countered. In line with the sponge city principle, the greenery provides a large infiltration area - an effect that is all the greater as the roof of Kö-Bogen II was not left out during planting.

The building is a great energy converter, says Prof. Karl-Heinz Strauch from Beuth University of Applied Sciences Berlin. As a phytotechnologist, he provided scientific support for the vegetation concept of the green façade. "Almost half of the solar energy is converted into water vapor. The surrounding air is humidified and not heated. Buildings like this close the typical urban gap in the natural water cycle. In addition to other effects, this is particularly important and effective for the urban climate." Christoph Ingenhoven speaks of the "expansion of a green space planned for continuity", "we need a green, continuous network".

Cooling Düsseldorf from the inside

Düsseldorf is also in the midst of climate change. According to data from the German Weather Service, the average annual temperature in the state capital has risen by 1.7 degrees between 1949 and 2024. There are now around 22 summer days (days of 25 degrees and above) and over 382 hours of sunshine per year more than in the middle of the 20th century.** By the end of the century, the temperatures in the state capital could be the same as those in Toulouse (Düsseldorf's so-called climate twin) today. We are moving towards conditions in the south of France. Densely built-up inner cities heat up particularly strongly. The causes are: sealed surfaces, heat-storing building materials, lack of air circulation. Düsseldorf must cool from the inside, according to a key finding of two analyses conducted in recent years, the "Climate Adaptation Concept for the State Capital Düsseldorf KAKDUS" (2017) and the "Climate Analysis 2020". As part of the latter, the Hofgarten, Germany's oldest public park, was also examined in more detail in terms of its climate - a particularly important green space given its inner-city location. Compared to the neighboring Schadowplatz, the Hofgarten is around three degrees cooler at night. During the day, the perceived temperature difference can be even greater - up to ten degrees.

Link to the Hofgarten green lung

Strengthening blue-green structures is important for the cooling effect and climate resilience. This is exactly what the almost 28-hectare Hofgarten offers. Thanks to the new center, Düsseldorf's green lung is moving back into the center of the city. Kö-Bogen II is a green link with radiance. And it is not the only one: Kö-Bogen I and the redesign of the former Jan-Wellem-Platz public transport hub will create another green arc - in the truest sense of the word. The arched course of the historic Hofgartenstraße along the Landskrone watercourse will be recreated. The Landskrone was also extended as part of the redesign of the area. Every March, Japanese cherry blossoms enchant visitors with their small pink buds on the Hofgarten terraces. The view is of the water, swans chatter. Everything is blue-green.

^{* Cf. interview with Christoph Ingenhoven, published in June 2020 in DBZ Deutsche Bauzeitschrift}

^{** Cf. climate impact monitoring of the state capital Düsseldorf 2024}

Text: Eva Westhoff
Photos: Visit Düsseldorf