Green roofs, stormwater mitigation and weeds

Green roofs, stormwater mitigation and weeds
Photo Courtesy of AIPH

Weeds might not be unwanted on green roofs. Green roofs are increasingly popular in cities across the globe. This type of green infrastructure’s top layer is constructed from substrate with plants seeded into it. The benefits of green roofs in cities includes air quality improvement, stormwater runoff reduction, aesthetic value and biodiversity for insects. Green roofs can intercept and store rainfall in the substrate, thereby reducing stormwater runoff and delaying peak flow rates. The vegetation in green roofs can further increase rainfall retention by uptake and transpiration of the stored water. Sedum is commonly planted as it is tolerant of shallow substrates and variable water access. In the absence of routine maintenance, green roofs are often colonised by ‘weedy’ spontaneous vegetation which can become dominant.

However, because green roofs are often implemented with limited resources, this raises concerns that these benefits may be lost due to the challenges associated with management and maintenance of the planted area. The initial expense and resource demand can also be a barrier to adoption in lower socio-economic areas. Research from Schrieke et al. in Melbourne, Australia, considered  the role of ‘weeds’ in the function of green roofs to reduce rainfall runoff  in circumstances where low management could change the plant species mix. This research was conducted to understand if ‘weeds’/spontaneous vegetation establishment led to a loss in the ecosystem service benefits.

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