The effects of growth form on the impact of companion planting of nectar-producing plant species with Sedum album for extensive green roofs

Abstract Pollinators provide crucial urban ecosystem services. Introducing nectar-producing species on green roofs is one method to support pollinator populations. However, novel cultivation methods may be required to improve plant health with minimal irrigation on extensive green roofs, which are severely restricted in soil thickness. Although companion planting with CAM plants such as Sedum has been reported to improve the performance of neighboring plants, the growth form of the beneficiary species seems important in determining the effectiveness of the technique. In this study, eleven species of nectar-producing plants were classified into two growth forms: carpet species (Phyla canescens, Trifolium repens, Thymus serpyllum and Thymus longicaulis), non-carpet species (Calamintha nepeta, Lavandula officinalis, Lavandula stoechas, Melissa officinalis, Mimosa pudica, Rosmarinus officinalis ‘Erectus’, Rosmarinus officinalis ‘Prostratus’), and were planted together with Sedum album in a greenhouse or on a rooftop space. There were no net positive effects in the impacts of companion planting with S. album on plant biomass both in carpet and non-carpet species. However, companion planting with S. album had net positive effects on leaf health in L. stoechas, M. officinalis, M. pudica, and R. officinalis ‘Prostratus’, classified as non-carpet species. In contrast, there were no positive effects on carpet species, planted together with S. album. These results suggest that complementary growth forms could be one of the factors to determine effective planting combinations to improve plant health conditions on extensive green roofs. Additionally, the increase in soil water content by suppressing evapotranspiration when water is limiting can be effective in improving leaf health conditions of neighbors in companion planting with S. album. While past studies have shown positive benefits of companion planting with CAM species, this experiment suggests the possibility that not all growth forms may benefit from facilitation by Sedum.