The use of ornamental shrubs for sustainable urban greening: the case of Spiraea × vanhouttei L. grown on a constructed technosol

In the last years, research interest has shifted toward the sustainable usage of green practices and nature-based solutions for sustainable urban greening. Among these solutions, technogenic soils (technosols) are one of the most promising for urban requalification projects. In the present work, we evaluated the effect of a designed technosol compared to public soil (control soil) on the growth and healthy status of Spiraea × vanhouttei L. during 2021 and 2022 in the City of Turin (Italy). The technosol was a blend composed by earth materials, municipal compost, zeolites and mycorrhizae. No watering or fertilization was performed during the trial. After 12 and 17 months (in May and October 2022) biometric traits, mortality rate, SPAD and vegetation indices (VI) (i.e., normalized difference vegetation index, NDVI; green difference vegetation index, GDVI; photochemical reflectance index, PRI) were measured. Results indicated that plants cultivated in the technosol compared to control soil resulted in significantly superior growth (+112%) and GDVI (5.693 and 3.576, respectively). At the end of the summer 2022, one of the hottest and driest ever in Turin, all the plants present in the control soil have dried up. While, in the technosol the mortality rate was equal to 40%. At this time point, the survived plants resulted in higher value for each VI than after 12 months of cultivation. Overall, the obtained results demonstrated the importance of using NBS in urban green projects by mitigating the effect of climate change and maintaining high ornamental qualities in woody plants such as S. × vanhouttei.