The "500 trees for Rodgau" project has now been completed. At the town council meeting in September 2016, it was decided that a new tree would be planted for every tree felled. It was also decided to plant 500 additional trees in the built-up inner city area. The additional trees are intended to beautify the cityscape and improve the urban climate. This decision made the town of Rodgau a pioneer for climate adaptation in cities in the region. In 2016, in collaboration with ZORN Sachverständigenbüro Gartenbau from Usingen, an initial investigation was carried out to determine where trees had been felled in recent years and where there were other potential tree locations. Overall, it was a challenge for everyone involved, as the new tree locations had to be state of the art. After the planning phase, the first 100 trees were finally planted in 2018. Climate change also had a significant impact on the supply of the young trees. The usual 10 to 15 watering sessions were not sufficient in the first year. Instead, there were up to 40 waterings per tree in the dry months. The previous total duration of care also turned out to be out of date. Whereas in previous decades a tree was considered to have grown after three years of care, the duration of the care programme had to be increased to five years for the last plantings in the project. Both green spaces and sealed areas were planted as roadside greenery as part of the project. The costs per tree location were heavily dependent on the necessary construction measures and ranged from €1,000 to over €10,000. For example, in the case of a tree pit that is subsequently to be covered with paving, the road construction costs are added. In particular, the overbuilt tree locations must be created with a special planting substrate that can absorb the forces from traffic (e.g. parked cars) and yet is not too highly compacted so that the tree can still take root. The city of Stockholm has developed a method in which the forces of the traffic are diverted into the subsoil by installing large stones, and areas that can be rooted through are created by slurrying substrate into the gaps. This method was also implemented for the first time this year at four tree locations in Wilhelm-Busch-Straße. However, the decision to use the "Stockholm construction method" also has purely economic reasons due to rising transport and material costs. With this method, the existing soil, prepared with various additives, can be reused and reinstalled. This saves resources and also CO2 due to the reduced transport costs. The 1,200 potential tree locations analysed resulted in 500 new tree plantings, 230 replacements for felled trees and 278 additional locations for future plantings. 192 sites were not suitable. The improvement in the urban climate will only become noticeable in the coming years. The planted trees must first develop and can only then realise their full potential. Based on the long-term tree register of the city of Rodgau, average losses of around 2% per year are to be expected in reality. It is therefore ideal to plan for new tree planting in the future and thus give the present project a secure and permanent foundation. If the project is continued, future tree locations could be optimised through the use of digital sensors. These measure the soil moisture, for example, and report it digitally to the monitoring centre in the office. If it is too dry, an alarm is triggered so that irrigation can be initiated early enough. In this way, irrigation could always be scheduled at the optimum time and resources could be saved overall. Another optimisation point would be the increased use of tree locations as infiltration areas. This could be taken into account particularly in new development areas.
