The effects of substrate depth and irrigation regime, on seeded Sedum species grown on urban extensive green roof systems under semi-arid Mediterranean climatic conditions

Green roofs constitute a promising technology for reintroducing lost flora in degraded urban environments. The present study aimed to evaluate the growth and ground coverage capacity of seeded Sedums grown on shallow extensive green roofs under semi-arid Mediterranean climatic conditions. Treatments included: a) two types of seed mixtures (M1 containing seeds from 16 different Sedum species and M2 containing seeds from 28 different Sedum species; b) two substrate depths (6 cm and 12 cm) and c) two different irrigation regimes (high and low) during the dry summer seasons of the three-year study period: i) 15% and 0% of the cumulative reference evapotranspiration (ETo) in year 2012, ii) 60% and 30% of ETo in year 2013 and iii) 30% and 0% of ETo in year 2014. Growth performance was evaluated based on the total number of the Sedum plants and their ground coverage rate over the three-year study period. Out of the 16 and 28 Sedum species contained in mixtures M1 and M2, respectively, only four species (S. acre, S. album, S. reflexum, S. sexangulare) were able to survive for the duration of the study period. The seeded Sedum plants did not provide adequate growth or ground coverage. Their plant number decreased abruptly soon after the imposition of the first water stress in 2012 and remained at low numbers throughout the study period. The ground coverage rate was extremely slow. At the end of the threeyear study period, Sedum plants of the M1 seed mixture growing in both deep and shallow substrate depths and under high irrigation regimes provided ground coverage of 51.9% and 48.7%, respectively, while the same treatments of the M2 seed mixture provided ground coverage of 25.5% and 19.2%, respectively. All the remaining treatments provided minimal final coverage ranging from 3.1% to 0.5%. Although the seed mixture type exhibited minimal impact on total plant number and ground coverage, S. album and S. sexangulare were not able to grow in the M2 mixture. Overall, the seeded Sedum species exhibited reduced capacity to sustain growth under the Mediterranean climatic conditions, especially during the summer periods and under the imposition of water stress.