Effect of substrate type and irrigation frequency on growth of Pallenis maritima on an urban extensive green roof at the semi-arid Mediterranean region

The possibility of using Pallenis maritima, a native drought tolerant plant, on an urban extensive green roof at the semi-arid Mediterranean region was investigated under sparse irrigation and light substrate. Rooted stem cuttings were planted in November 2012 in experimental modules with a green roof infrastructure on a fully exposed flat roof at the Agricultural University of Athens, Greece. Two substrate types with 10 cm depth were used, one with soil, i.e., grape marc compost:perlite:soil: pumice (3:3:2:2, v/v) and a lighter one without soil, i.e., grape marc compost:perlite: pumice (3:3:4, v/v). The experiment lasted 16 months, including one hot and dry (summer) and two cold and wet (winter) periods. Two irrigation frequencies were applied during the dry period (May-Sept. 2013), i.e., every 3 days (normal) and 5 days (sparse) when substrate moisture was 17-20% and 5-9%, respectively. Plant growth was evaluated by plant horizontal expansion recorded monthly during the whole experimental period (Nov. 2012 - Mar. 2014) and the shoot number recorded after the dry period (Sept. 2013). The combination of soil substrate with normal irrigation gave slightly bigger horizontal expansion compared to the other treatments and the soil substrate promoted slightly the production of shoots. However, in general plant growth was quite similar in all treatments. Stomatal resistance (Rleaf) measured in July 2013, one day before irrigation was increased in plants under sparse irrigation indicating water limitation and the maximum quantum yield of PSII (ΦPSII) was smaller in those plants, but no evidence of damage to the photosynthetic apparatus was recorded in any of the treatments. Taking into account that plants had similar growth in all treatments, Pallenis maritima is recommended for use on extensive green roofs of the arid/semi-arid regions under sparse irrigation and soilless substrate with the desirable benefit of reducing water consumption and construction weight.