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Although weeding is one of the most time consuming tasks in green roof maintenance, there have been few studies of weed phenology and it is not clear how planting design affects weed colonisation on green roofs. This study investigated weeds including self-sowing planted species during the establishment period in a semi-extensive green roof in Rotherham, UK. This green roof was installed in the summer of 2005, and 54 plant species were planted in 10 cm (areas with gravel mulch) and 20 cm (areas without mulch) of the substrate. The planting density was 18–22 plants/m². Thirty-two quadrats (50 × 50 cm) were set up through the combinations of plant species diversity (high and low), planting density (high and low), four aspect and covering 2.5 cm of gravel mulch (with and without). Drip irrigation was installed for supplementary watering in dry seasons. All weeds and self-sowing in each quadrat were not …