Conflicting uses and the desire for more greenery: prospects for the market square and city center

On hot summer days, trees provide shade and cooling. However, the densely built-up city does not offer space for this everywhere. "The city regularly receives requests for more greenery in the city centre and the need for this increases with every new heatwave," emphasizes Paul-Gerhard Weiß, Head of Planning and Construction. The focus is particularly on the market square, which is used by tens of thousands of people every day as a central transfer point for buses and trains. When the market square was redesigned, the city's planning department had already examined all options for creating more greenery, Paul-Gerhard Weiß recalls the discussion at the time and explains: "Unfortunately, the desire for more trees could not be realized precisely where the development is very dense. The market square is not only a traffic junction for people, but also an underground hub for all the lines in the city center."

Countless supply lines for electricity, gas, district heating, drinking water, waste water and telecommunications run beneath the paving. "When we renovated the market square, it was a huge challenge to find any space at all for new cable routes," explains Weiß. "Using the Stockholm model, we were able to plant climatically suitable trees in the ground to replace those that had to make way for the redevelopment and ensure their irrigation. However, this was only possible on a very limited area. We would have liked more greenery from the politicians, but we had to deal with the actual conditions on site."

In addition, there are always the same conflicts of use: Which areas can be greened? Which areas are required as walkways and for people waiting at bus stops? Where can parking be located? Where is there space for outdoor catering, seating and lighting? In addition, many areas in the city center are underbuilt with underground parking garages - there is no room for the roots of trees. "All uses have their justification and it is therefore important to provide the appropriate space for as many of these uses as possible." Where there is no other option, permanently installed concrete tubs, such as on Hugenottenplatz, are one way of providing trees with a habitat above the underground car park.

The redesign of the square in front of the S-Bahn station offers the prospect of more greenery around the market square in the future: "So far, there is no quality of stay there. This square, which is no longer part of the market square itself, is awaiting a design that we will definitely tackle in the next few years," says Weiß. Until then, interim solutions are needed - including the recently installed mobile tree tubs, which can be used in different places as required in future. "The tubs are of course no substitute for a properly landscaped green space," says Weiß and promises: "In future, it will also be greener there. That will be a key aim of the planning for this square." By then at the latest, the mobile tree tubs will make way for trees to be planted directly in the ground.

The young trees in the mobile planters are currently still being maintained by the gardening and landscaping company. The signs of deficiency in the meantime have been closely monitored there and by the city. According to the nursery, the visible wilting was a not uncommon plant shock after transplanting. This was exacerbated because the delivery of the containers had been delayed for production reasons and the trees had already fully sprouted during this delay. After being protected in the nursery's warehouse, they then had to cope with a very early hot summer in the city center.

The planting shock was particularly noticeable on two trees. Healthy trees can compensate for this relocation stress with the so-called St. John's shoot, which is usually formed at the end of June. The trees have formed this summer shoot, are now considered vital and have overcome the planting shock.

"There is criticism of the cost of these tubs. However, this criticism does not take into account the fact that the city has purchased them so that they can be used permanently in different locations. This is not money thrown away. Instead, it would have been money thrown away if a permanent green area had already been set up now, which would then have had to be removed again for the redesign of the square or, as is now the case, for the long period of time during which the railroad construction site is being used for the tunnel renovation, or would have had to be expensively renovated afterwards," emphasizes Weiß, Head of Planning and Construction. "So we were flexible and were able to move the buckets for the duration of the work on the tunnel. This shows, with all due understanding, that many people are not quick enough: fixtures only make sense once the area is permanently free, the subsoil has been carefully examined and future walkways, lighting, areas for outdoor catering and seating have been planned."

Especially as ground-based planting, as planned for the future, is also associated with considerable costs. Costs of up to 17,000 euros can be expected for each individual tree location in order to create sufficient root space and ensure irrigation using a special substrate. Construction methods using skeleton soil and biochar based on the Stockholm construction method would be the best choice for climatic reasons, but are even more expensive. As the preservation of trees and new plantings is an important goal for the city, the city councillors have allocated funds to develop ideas and ways to create more greenery in the city center that can withstand the climatic changes. The planning office is to develop a concept for a "green belt" that will stretch from Platz der Deutschen Einheit via Stadthof to Hugenottenplatz. It will serve as the basis for specific redesigns and individual measures. The municipal Office for Planning and Building has also been commissioned to involve citizens, who can contribute their wishes and ideas.