Evaluating the growth of several Mediterranean endemic species in artificial substrates: Are these species suitable for their future use in green roofs?

The study of the growth of several Mediterranean endemic plant species in new artificial substrates is becoming necessary for seeking alternatives in green roof design in Mediterranean regions. In this regard, we evaluated the growth of six native species on three different artificial substrates. These species were Silene vulgaris, Silene secundiflora, Crithmum maritimum, Lagurus ovatus, Asteriscus maritimus, and Lotus creticus. A mixture of a green compost with a clay-loam soil (C + Soil), the same compost mixed with expanded clay (C + Clay), and the compost mixed with crushed bricks (C + Bricks) - all in 1:4 volumetric proportions - were the assayed substrates. Physichochemical and biochemical properties were studied in each mixture at the beginning and end of a three-month assay. Besides, the germination and growth of all plant species in each substrate type were evaluated. The mixtures C + Clay and C + Bricks showed greater porosity than C + Soil, leading to a high root: shoot biomass ratio in all species tested. Conversely, the mixture C + Soil possessed better biochemical properties (higher humic substances content and enzyme activity), but they decreased with experimental time. Contrarily, the humic substances and the dehydrogenase activity increased in the most porous mixtures. Regarding plant behaviour, S. vulgaris and L. ovatus showed greater germination and growth than the other species, especially above C + Clay and C + Bricks mixtures. Accordingly, we strongly recommend the use of lightweight and highly porous substrates as the basis for the growing of Mediterranean native herbaceous species, since they specially enhance their root development. The combined use of perennials and annuals species in that kind of substrates could provide a permanent plant cover in a green roof scenario.