Optimising green roofs in the Netherlands: effects of mineral wool and compost additions on flowering success

The continuous increase of the global human population living in cities has significantly impacted various aspects of nature and climate worldwide. One initiative to promote the urban green space and its biodiversity is the construction of green roofs. Despite the basic structure of these green roofs being similar around the world, the optimal design can depend greatly on local environmental conditions. In the Netherlands, it remains unclear how the flowering performance of green roofs can be enhanced beyond the relatively short flowering period of commonly used Sedum species. This study investigates the effects of adding mineral wool and compost to experimental green roof systems in the Netherlands. Mineral wool was applied as a water-retentive substrate layer, while compost was added as a nutrient-amending component. Flower and seed production, as well as plant species richness, were used as indicators of flowering performance on the green roofs at ground level. The results showed that mineral wool significantly increases the number of flowers and seeds, and the overall richness of plant species, stimulating both sown and spontaneously colonising species. The addition of compost did not affect these parameters. These results reveal that green roof designs incorporating water-retentive substrate components can enhance flowering performance, and suggest an expansion of the flowering period of green roofs in the Netherlands. This study evaluates these results in the context of the current state of green roof construction and management in urban areas.