Effect of substrate type and irrigation frequency on growth of the bee-friendly plant Thymus citriodorus on an urban extensive green roof

The trend in recent years internationally has been to use bee-friendly plants in the urban landscape, given the large population decline of pollinators worldwide. In this study, growth of Thymus citriodorus (lemon thyme) was investigated on a green roof in Athens, Greece, of extensive type, in respect of two irrigation frequencies and substrate types. Rooted cuttings were planted beginning of November 2014 in experimental modules, with a green roof infrastructure (substrate moisture retention and protection of the insulation, drainage element and filter sheet), on a fully exposed flat roof at the Agricultural University of Athens. 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 (May-September, every 2 to 4 days (normal, 17-20% substrate moisture) and every 4 to 6 days (sparse, 8-11% substrate moisture), depending on the month. Results presented are after 11 months of culture. Plants developed larger horizontal diameter and dry weight of canopy in the substrate with soil, while irrigation frequency did not affect plant growth. Plant height and flower number were not affected by the treatments. Lemon thyme developed a dense, hemispherical canopy, with satisfactory soil coverage and extended flowering in all treatments resulting in strong attraction by bees. This could be suggested for use on urban extensive green roofs in semi-arid areas, employing sparse irrigation to save water, and lightweight substrate to support green roof constructions on old buildings.