Growth and development of succulent mixtures for extensive green roofs in a Mediterranean climate

Green roof systems, aimed at reducing anthropic impact on the environment, are considered environmental mitigation technologies and adopted by many countries across the world to strengthen urban ecosystem services. This study evaluates two mixtures of succulent: one of Crassulaceae and the other of Aizoaceae, used in the creation of a continuous and homogenous plant groundcover in Mediterranean environments. To assess the species mixtures, the parameters plant height, growth index, cover percentage and flowering were observed. Hydrological observations were also carried out to evaluate the rainfall retained by the test system in any given month. All data were subjected to analysis of variance. Growth indicators in the study showed trends characteristic of xeric plants, which tend to slow down in dry, summer climate conditions to the point of halting plant vertical growth and ground cover development completely. The Aizocaeae mix, during the initial stage, showed prevalent horizontal growth, confirmed by greater a greater growth index (13,21) and cover percentage (45%) compared to Sedum (Growth index: 3,61; Cover: 36%). In contrast, the Sedum mix recorded greater vertical growth at the beginning (Sedum mixture: 7.53 cm; Aizoaceae mixture: 6,11 cm). During the final stages of observations, however, greater vertical growth in the Aizoaceae (7,88 cm) became apparent together with a recovery in horizontal growth in the Sedum (79%), albeit not sufficient to outperform the Aizoaceae mixture (87%). Flowering in the two mixtures occurred between late spring and late summer. The Sedum mixture guaranteed a longer flowering period (130 days) compared to the Aizoaceae (93 days), with a gradual start followed by steady flower emission. Regarding rainfall water retention, a comparison of the mixtures in late winter/early spring revealed that the Sedum performed best (44.9 L m2 vs 37.4 L m2), whilst the Aizoaceae outperformed the Sedum in Autumn (63 L m2 vs 55 L m2), in conjunction with favorable growth rates in both species mixtures. Both mixtures demonstrated satisfying results and are considered suited to a Mediterranean environment. Furthermore, based on the different growth rates of the species in the two test mixtures, this study suggests that new combinations of Sedum and Aizoaceae together might prove more resilient in Mediterranean environments.