Adaptation of Salvia fruticosa, S. officinalis, S. ringens and interspecific hybrids in an extensive green roof under two irrigation frequencies

With the ultimate goal of introducing new species of sage to the green roof industry, the adaptation of the Mediterranean sage species Salvia fruticosa, S. officinalis, S. ringens and the interspecific hybrids S. fruticosa × S. ringens and S. officinalis × S. ringens was investigated, under normal and sparse irrigation in an extensive green roof in Athens, Greece. Rooted cuttings were planted (April 2021) on shallow (10 cm) substrate (grape-marc compost: perlite: pumice, 3:3:4, v/v) and irrigated when substrate moisture was 16-22% (normal irrigation) or 7-11% (sparse irrigation). All sage types established satisfactorily on the green roof, even under sparse irrigation, except S. fruticosa, which had the highest vegetative growth and at the end of the experiment (September 2021) showed the highest mortality rate regardless of irrigation frequency. Similar to the S. fruticosa response showed the S. fruticosa × S. ringens hybrid under normal irrigation. Both hybrids, S. fruticosa × S. ringens and S. officinalis × S. ringens had a lower mortality rate under sparse irrigation compared to the parental species. S. officinalis × S. ringens formed more lateral shoots than its parental species and had an almost spherical compact canopy, which is a nice feature for an ornamental plant. S. officinalis and S. ringens developed bigger horizontal diameter, which is valued for rapid substrate coverage in a green roof installation. All sage types flowered except S. fruticosa. All sage types showed higher values of stomatal resistance under sparse irrigation, while ΦPSIIo values under both irrigation frequencies indicated normal operation of the photosynthetic apparatus. S. officinalis, S. ringens and S. officinalis × S. ringens hybrid are highly recommended for sustainable extensive green roofs in arid/semi-arid regions.