The sweet chestnut (Castanea sativa) has rarely been used as an urban tree to date. In the course of climate change, it is likely to be a ‘winner’ on the northern side of the Alps (Zimmermann et al. 2014). It also has various characteristics that make it interesting as an urban tree species. Due to its fruits (chestnuts), it is a widely known and popular tree species and can therefore contribute to attractive ‘edible’ green spaces and neighbourhoods. The chestnuts can be collected with little harvesting effort, the fruit is protected from dirt by its shell and there are many different processing options. According to the tree register, there were around 60 sweet chestnuts in the city of Zurich in 2021. However, it was unclear how vital they are and how they are used. The ‘City of Zurich Chestnuts’ project was launched to enable the population to use chestnut trees more effectively and to explore the potential of this tree species in an urban context.

In the two project years 2022/23, the existing chestnut trees were examined, new chestnut trees were planted and the chestnut culture was publicised in the city of Zurich. The interest among the population and the added value proved to be very high and was also reflected in a political initiative by Zurich's municipal council (postulate 2022/496)1, which envisages examining the planting of 50 more chestnut trees in Zurich. As the soil in many places in Zurich is characterised by calcareous parent material (moraine of the Linth glacier) and many urban soils contain calcareous debris, the suitability of the sweet chestnut as an urban tree is not readily apparent. However, as there are some positive examples, the question arises as to the conditions under which the sweet chestnut can nevertheless thrive and what can be done to improve the growth and utilisation of the sweet chestnut as an urban tree in Zurich.

In collaboration with the ‘City of Zurich Chestnuts’ project and Grün Stadt Zürich, sweet chestnuts will first be tested in various substrate conditions as part of a three-year trial setting to answer the research questions. The plan is for 40 young plants from core growths to be scientifically monitored in four substrate mixtures (acidic, calcareous, nutrient-rich/humus and control variant) and their development documented. Once the chestnut trees have reached the desired size and quality, they will be planted at a location in Zurich for the practical test and monitored for a further three years.

The aim of the project is to develop the scientific basis for planting chestnut trees in urban areas. This will enable the essential knowledge gap in the use of sweet chestnuts to be closed and the great potential of this tree species to be utilised better and in a more targeted manner.