AbstractConstructed wetlands are built mainly for purifying waste water. As a new type of ecosystem, its plant diversity remains unstudied. We studied the seasonal change in plant diversity in a constructed wetland in Hangzhou Botanical Garden, which is located in the humid subtropical area of China. In the autumn of 2002, 72 species were found, of which 18 were assembled and 54 were natural immigrants. In early spring of 2003, the species number was 46 in total, including 33 immigrants. Although naturally recruited plants exceeded the assembled ones in the species number, they were still associated species in the community, in which the assembled species form the dominant components. Some valuable immigrant species should be reserved to enrich biodiversity and to keep the stability of the community. In addition, the co-existence of two functional groups, respectively growing in warm seasons and cold seasons, can effectively occupy the temporal niche-space to prompt the efficiency of nutrient removal and seasonal stability of the constructed wetland.