Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix

Technology for establishment of vegetated roofs (green roofs) has developed rapidly over recent years but knowledge about how these systems will develop over time is still limited. This study investigates vegetation development on unfertilised thin extensive vegetated roofs during a 3-year period. The vegetation systems investigated were designed to be low maintenance and had a saturated weight of 50 kg/m2, a thickness of 4 cm and drought-resistant succulent and bryophyte vegetation. Vegetation development was investigated in relation to: establishment method, species mixture and substrate composition in a factorial experiment. Vegetation cover was investigated using point intercept. Moss was found to develop on most substrates and reached more than 80% cover on some plots. Sedum album and Sedum acre were the dominant species on the roofs. S. acre was found to decrease drastically after two years. The lack of difference found in this study between the establishment techniques shows that there are other possible marketable ways to construct vegetated roofs in Sweden, as an alternative to vegetation mats. Uniform extensive vegetated roofs with a high dominance of succulent species have limited value for plant biodiversity, as few species establish spontaneously.