Effect of substrate type and irrigation frequency on growth of Mediterranean xerophytes on green roofs

The possibility of using native xerophytes at extensive green roofs in semi-arid Mediterranean areas was investigated aiming at the preservation of local character and biodiversity, as well, as to reduce water consumption and construction weight. Convolvulus cneorum L., Origanum dictamnus L. and Sideritis athoa L. plantlets were planted beginning of July 2011 in experimental modules, on a fully exposed flat roof at the Agricultural University of Athens, with a green roof infrastructure (substrate moisture retention and protection of the insulation, drainage element and filter sheet). Two types of substrate with 10 cm depth were used, grape marc compost: perlite: soil: pumice (3:3:2:2, v/v) and grape marc compost:perlite:pumice (3:3:4, v/v), and two irrigation frequencies during the dry period, every 5 days (normal) and every 7 days (sparse). Results presented are for the first 14 months (two dry periods) of cultivation. The growth of plant diameter of C. cneorum and S. athoa was promoted by the soil substrate during the first dry period, whereas at the end of the second dry period plant diameter of all three plant species was not affected by the substrate type. Irrigation frequency did not affect the diameter of C. cneorum and S. athoa, whereas O. dictamnus diameter was benefitted by normal irrigation particularly when grown in the soil substrate. During the dry period, leaf stomatal resistance the day before irrigation was increased in plants under sparse irrigation indicating water stress. Total yield of photosystem II one day after irrigation was not affected by irrigation frequency or substrate type. Plant size and roof coverage of the three species after 14-month cultivation was more or less similar in all the experimental treatments and thus the soilless substrate with sparse irrigation is suggested.