Growth of golden oregano on an urban extensive green roof as affected by substrate type and irrigation frequency

Internationally, the construction of green roofs in the cities, as well as urban agriculture are expanding. In this study both these trends were combined and the growth of Origanum vulgare 'Aureum' (golden oregano), a culinary and simultaneously an ornamental herb, was investigated on an extensive green roof under the effect of two substrate types and irrigation frequencies, in Athens, Greece. Beginning of November 2014, rooted cuttings were planted in experimental modules, with a green roof infrastructure (substrate moisture retention and protection of the insulation mat, drainage element and filter sheet) on a fully exposed flat roof. The substrate used was 10 cm deep and consisted of either grape marc compost:perlite:soil:pumice (3:3:2:2, v/v) or grape marc compost:perlite:pumice (3:3:4, v/v). During the hot, dry period (May-September) the plants were watered either every 2-4 days (normal irrigation, 17-20% substrate moisture) or every 4-6 days (sparse irrigation, 8-11% substrate moisture). After 11 months of culture, plants developed larger horizontal diameter, height and dry weight of canopy in the soil containing substrate, while irrigation frequency did not affect plant growth. Inflorescences number was similar in all treatments. Golden oregano, although did not develop very dense canopy, had satisfactory soil coverage in all treatments, and thus could be suggested for use on urban extensive green roofs in semi-arid areas employing sparse irrigation, as well as lightweight substrate in case of green roofs on old buildings.