The substrate used for phytosation consists of sand, gravel, gravel and organic materials (compost).
The permeability of the substrate affects the circulation of the water thus allowing the retention of suspended matter and waste that it transports. The substrate not only supports the vegetation, but also becomes the support for a large number of living organisms, and the location of many chemical and biological (especially bacterial) transformations forming a source of carbon and energy for biological activity. Finally the substrate ensures the storage of many pollutants.