The effect of substrate depth and supplementary watering on the growth of nine herbaceous perennials in a semi-extensive green roof

Semi-extensive green roof systems combine many of the environmental benefits of extensive green roofs with some of the aesthetic potential of intensive green roofs. Perhaps most importantly they enable a wide range of herbaceous plant material to be grown without significant structural alteration to buildings. An experiment was undertaken in Sheffield, northern England to determine the potential of semi-extensive roofs for diverse herbaceous plantings. A range of drought tolerant herbaceous perennials with different structural characteristics (low growing, medium height and taller 'emergents') were grown in two depths of lightweight substrate (100 mm and 200 mm) and with or without limited supplementary watering. Increasing substrate depth without supplementary watering produced relatively few significant improvements in plant performance. However, supplementary watering at both depths promoted growth in the majority of species. Low growing species (those typical of standard extensive green roofs) performed less well under the more productive conditions. The paper discusses the many opportunities for semi-extensive green roofs, particularly where opportunities for additional watering (perhaps from stored rainwater) are available and where public or user access is possible.