Objective The objective of this study was to indicate the impact of plant roots on physical property and moisture changing characteristics of green roofs.

Method The dynamic changes of rainfall, evapotranspiration, and substrate water content of green roofs with different vegetation covers (3 different planting density levels of Sedum spectabile, Portulaca pilosa and a non-vegetated substrate) were monitored under natural rainfall and artificial rainfall conditions. Combined with the characteristics of root system, substrate physical property and water content, the effects of plant roots on dynamic hydrological performance of green roof were quantitatively analyzed.

Result (1) The total porosity and capillary porosity of the green roof substrate increased with root length density, and the saturated water conductivity of substrate was positively correlated with root length density (P < 0.01). (2) The stormwater retention capacity of green roof increased with planting density. (3) The green roofs with P. pilosa had thicker root system than those with S. spectabile, and their stormwater retention capacities (24.4 mm) were greater than those of green roof with S. spectabile (19.9 mm). (4) The plant root system also affected the evapotranspiration of green roofs. Higher root length density promoted evaporation of the green roof substrate. Because of thicker root system, the evaporation rate of green roof substrate with P. pilosa was higher than those with S. spectabile.

Conclusion The root length density and root diameter of plant root systems can affect the physical property and moisture changing characteristics of green roof substrates, thereby impacting the substrate’s stormwater retention capacity and evaporation capabilities. The results are expected to provide scientific support for green roof plant/substrate configuration and management.