Effective street tree and grass designs to cool European neighbourhoods

Street trees and grass are important elements for cooling cities, yet where and how to distribute them at the neighbourhood scale is still unclear. This study aims to identify effective street tree and grass design scenarios to maximise cooling in European neighbourhoods with temperate climate. 32 design scenarios were developed by combining urban design parameters of vegetation type, coverage, and spatial distribution in four neighbourhood typologies. The microclimate effects of these scenarios were then simulated using the ENVI-met model. To evaluate their cooling effects, three indices were applied: PET reduction, percentage change in thermal sensation class, and cooling efficiency. Results reveal that even fully covering streets with grass has a marginal thermal impact in reducing the mean Physiological Equivalent Temperature (PET) by up to 1.1 °C across the neighbourhood, while street trees lower PET by up to 8.7 °C. Neighbourhoods with wide radial streets have higher initial PET values and benefit more from green interventions. Strategically placing two rows of large trees on main streets is more effective for cooling than a single row on both main and secondary streets on the high-radiation side. Neighbourhood-specific practical recommendations for strategically implementing street trees and grass are provided to improve urban cooling. • Fully covering streets with grass has only a marginal impact on thermal sensations. • Neighbourhoods with wide radial streets are easier to cool than others. • Street grass can lower air temperature more than small trees. • Planting on wider streets is more cooling-efficient. • Both PET and cooling efficiency should be considered when planning interventions.