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Green roofs provide important ecosystem services in urban areas. In Mediterranean and other semi-arid climate regions, most perennial plants on green roofs need to be irrigated during the dry season. However, the use of freshwater in such regions is scarce. Therefore, the possibility of using grey water should be examined. Coal ash, produced primarily from the burning of coal in power plants, constitutes an environmental contaminant that should be disposed. One option is to use ash as a growing substrate for plants. Here, we compare the effects of irrigating with grey- versus tap-water and using ash versus perlite as growing substrates in green roofs. The study was conducted in northern Israel in a Mediterranean climate. The design was full factorial with three factors: water-type (grey or tap-water)×substrate-type (coal ash vs perlite)×plant species (Phyla nodiflora, Convolvulus mauritanicus or no-plant). The development of plants and the quality of drainage water along the season, as well as quality of the used substrates were monitored. Both plant species developed well under all the experimental conditions with no effect of water type or substrate type. Under all treatments, both plant species enhanced electrical conductivity (EC) and chemical oxygen demand (COD) of the drainage water. In the summer, EC and COD reached levels that are unacceptable in water and are intended to be reused for irrigation. We conclude that irrigating with grey water and using coal ash as a growth substrate can both be implemented in green roofs. The drainage from tap water as well as from grey water can be further used for irrigating the roof, but for that, COD and EC levels must be lowered by adding a sufficient amount of tap water before reusing.

Green roof research is a multidisciplinary and new research area. We conducted a bibliometric quantification to assess the rate of publications in specific areas of research for this novel research area based on the scientific literature as available from the Web of Science. Bibliometric research can provide valuable information about changes in the trends within a particular area of research. For example, we found that the number of publications in this field increased in the last two decades at very similar pace to other pre-established academic disciplines. We also found that papers on green roofs were classified into 32 research areas. There was very little change in the frequency of most research areas through time. The percentages of plant sciences, forestry, marine and freshwater biology and biodiversity conservation of the total research areas classifications used each year increased significantly with time, while architecture decreased significantly with time signifying an increased interest in environmental issues and less focus on architectural issues. The distribution of publications between countries has been skewed, with the USA and the EU conducting 66% of the research, and thus allocation of research effort is focused in those continents and predominantly in temperate ecosystems. However, there has been a sharp increase in the number of countries that conduct green roof research. Our work provides a suite of indicators that can be combined to give a useful picture of the development of green roof research and identifies the challenges which lie ahead for this novel research area.