Objective  Green roofs have various functions of stormwater management (e.g., rainwater retention, peak discharge reduction). However, the effects of plant roots on the hydrological performance of green roofs are unclear. The objective of this study was to indicate the impact of plant roots on hydrological performance of green roofs. The results of this study were expected to provide scientific support for green roof plant configuration and management.

  Method  Four green roofs with different vegetation cover (Sedum lineare, Portulaca grandiflora, Sedum spectabile and a non-vegetated substrate) were set up in Beijing, and the rainfall-runoff process and water content dynamic changes of green roofs were monitored. The root characteristics such as root length density and root diameter of plants were measured and the influence of plant roots on the stormwater retention of green roofs was analyzed.

  Result  The stormwater retention capacity of green roof substrate with plant roots was 2.1−4.1 mm larger than that of the non-vegetated substrate. The average time of runoff generation from green roofs containing plant roots was 6−10 min earlier than the non-vegetated substrate, and the average peak present time was 9−26 min earlier than the non-vegetated substrate. When the total root length density was similar, the stormwater retention performance of the S. spectabile green roof with thick root system was better than that of the P. grandiflora with thin root system. The S. lineare had the largest total root length density and average root diameter among the other plants, and the S. lineare green roof had the least amount of the proportion of the initial loss. So as the rainfall increased, it may have a more obvious decline of stormwater retention performance than the other plants.

  Conclusion  Although the plant roots could increase the stormwater retention capacity of green roofs, it may advance the generation time of runoff. Meanwhile, the radial distribution of the root system also affects the runoff regulation function of green roofs.