Acta Hortic. 1189. ISHS 2017. DOI 10.17660/ActaHortic.2017.1189.39 Proc. VI Int. Conf. on Landscape and Urban Horticulture Eds.: M. Papafotiou et al. 197 Designing green roofs for arid and semi-arid climates. the route towards the adaptive approach P.A. Nektarios a and N. Ntoulas Laboratory of Floriculture and Landscape Architecture, Department of Crop Science, Agricultural University of Athens, Iera Odos 75, 118 55 Athens, Greece. Abstract It is widely accepted that green roofs provide multiple benefits to the urban environment and the well-being of the cities’ residents. Despite their acknowledged contribution to the contemporary cityscape and environment, their broad implication in arid and semi-arid regions is extremely limited compared to northern countries. Thus, it is of great interest and importance to identify the hurdles that delay the broad implication of green roofs in challenging climatic zones. These hurdles can be categorized either as governmental or technical or psychological even though they all intermingle and interact. More specifically, in the arid and semi-arid climatic zones, such as those of the Mediterranean basin, the governmental incentives are absent forcing the private sector and the homeowners to take the initiative. In those cases, investing in green roofing can only be justified if green roofs are usable and aesthetically pleasing since the preference of the public has been documented towards the intensive green roof types. However, taking into account that the majority of existing buildings are old and can bare minimal additional loading, it is obvious that green roof substrate depth must be minimal. All the above parameters are contradicting and have led us to the introduction of a new green roof category, namely the adaptive type. Adaptive green roofs have minimal substrate depth ranging from 5 to 15 cm but utilize a broad palette of plant species such as groundcovers, turfgrasses, medicinal and aromatic plants coppices and small shrubs. All the above are feasible due to minimal and prudent irrigation inputs. The current presentation provides a thorough review of the plant species evaluated in adaptive green roof systems concerning their response to different substrate types and depths, their tolerance to either deficit irrigation or drought and their irrigation requirements. Keywords: adaptive green roof systems, Mediterranean zone, substrate depth, substrate type, native plant species, deficient irrigation, drought tolerance INTRODUCTION Green roofs formulaic classification separates them into extensive, simple intensive and intensive. Such a classification is based on substrate depth, the potential plant material that can be utilized and the intensity of maintenance procedures. Intensive green roofs are equipped with deep substrates and can host a broad variety of plant forms such as trees, shrubs and groundcovers creating an aesthetically pleasing urban landscape. However, their maintenance intensity and the input of resources such as irrigation and fertilization are demanding. In contrast, extensive green roofs aim mainly to provide environmental benefits with minimal construction cost and maintenance requirements. They are characterized by shallow substrate depths and are usually planted with succulent plants mainly of Sedum and Delosperma species, which are characterized by extreme drought stress tolerance in conjunction with shallow root systems. Even though extensive green roofs seem to have a satisfactory market share in the northern climates, their application in the arid and semi‐arid Mediterranean climates is hindered due to the extreme summer temperatures and drought conditions along with the minimal adaptability a E-mail: pan@aua.gr