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Biological invasions are determined by interactions between resident plant communities and exotic plants. Time of invasion and species diversity of resident plant communities may greatly affect exotic plant invasions. We assembled low- and high-diversity resident plant communities by sowing seeds of four and eight grassland species, respectively, and at each of three time periods (1, 4 and 7 weeks after sowing), the resident communities were invaded by Hydrocotyle vulgaris or not. We also constructed a plant community with H. vulgaris alone. Presence of H. vulgaris had no effect on biomass of the resident communities or biomass of each component species. Community age significantly affected biomass and evenness of the resident communities, and their competition with H. vulgaris, but the priority effect of the resident communities was slight. Increasing species richness did not change the interaction between H. vulgaris and the resident plant communities. These findings suggest a weaker competitive exclusionary effect of H. vulgaris on the resident communities with early germination, and H. vulgaris tended to have no significant impact on intact resident terrestrial plant communities. Thus, the potential risk of H. vulgaris invasion is low, especially in the communities with young age.