Construction and maintenance factors affecting most the growth of shrubby Mediterranean native plants on urban extensive green roofs

Shrubby drought tolerant Mediterranean plants are a good choice for urban extensive green roofs in areas with arid/semi-arid Mediterranean climate. In this paper, results concerning the effect of substrate type and depth, irrigation frequency and green roof infrastructure/drainage system on growth of eleven such plants on an extensive green roof at Athens, Greece, are presented. The plants were Artemisia absinthium, Asteriscus maritimus, Atriplex halimus, Convolvulus cneorum, Helichrysum italicum, H. orientale, Lomelosia cretica, Origanum dictamnus, Origanum majorana, Santolina chamaecyparissus and Sideritis athoa. Rooted cuttings were planted in experimental modules, with a green roof infrastructure, on a fully exposed roof at the Agricultural University of Athens. Various substrate mixes containing perlite and pumice, amended or not with soil, peat or grape mark compost, were used at 7.5, 10 or 15 cm depth, and two irrigation frequencies were applied during the dry period(s) (May-September), when substrate moisture was 17-20% (normal irrigation) and 5-9% (sparse irrigation). Experiments lasted from 5 to 27 months. Compost and the deeper substrate promoted plant growth in all species, while irrigation frequency in most plant species did not affect growth. The addition of compost at 20% by volume to the substrate interacted with substrate depth and plants in low-depth compost amended substrate had similar growth to plants in deep substrate without compost. The addition of soil at 20% by volume to a compost-substrate, particularly in combination with normal irrigation, promoted slightly the growth of some plant species; however, plant growth was quite satisfying in the lighter soilless compost-amended substrate even under sparse irrigation. Concerning the drainage system of the green roof, a 3-cm thick perlite layer resulted in similar plant growth compared to a cell drainage system, while it reduced the water consumption. All plant species tested grew successfully on the green roof on a substrate with 20 or 30% grape mark compost, 50% perlite and 30 or 20% pumice, with 10 cm depth and sparse irrigation during summer months.