Assessing chilling and drought tolerance in different plant genera for extensive green roofs in an arid climatic region in Mashhad (Iran)

The harsh and stressful growing environment of extensive green roofs means that only a limited range of plant species is routinely used. This paper describes an experiment for investigating suitable plants for extensive green roofs in Mashhad during the natural cold of autumn and winter in the form of a complete block design and, after that, during the warm season (drought stress) with three irrigation interval treatments (48, 72 and 96 hours) in the form of a factorial experiment based on a complete block design with four replicates. The results of both experiments showed a significant effect in the different seasons. Agropyron cristatum, Frankenia thymifolia and Carpobrotus edulis showed the best visual quality among the plants examined. From the survival point of view, Carpobrotus edulis showed the best performance for both cold and warm seasons and Frankenia thymifolia and Potentilla sp. follow the same manner. Vinca minor was good under cold conditions and Sedum spurium was observed to be an excellent choice for the warm season. Grass plants and groundcovers need regular irrigation during the warm season and showed significant differences regarding watering treatments. However, a very good response to drought was shown in the Sedum spurium plant. Overall succulent and groundcover plants are, respectively, considered good for the warm and cold seasons while grass plants are not properly suited. According to the results of this study, green roofs can be advised as sustainable green structures for Mashhad’s climate with certain considerations.