Abstract: A new exotic weed invader, Flaveria bidentis, is spreading rapidly in central China. Aqueous extracts of F. bidentis plants growing in different habitats, and with different densities, and in vegetations with different replacement plants were used to evaluate the allelopathic effects of the plant on seed germination and seedling growth of Lolium perenne. The primary aim of the study was to clarify the invasion mechanism of F. bidentis. The results showed some degree of correlation between allelopathy and environmental factors. Inhibition effects on seed germination and seedling growth of L. perenne increased with increasing extract concentration. Significant differences were noted among different habitats and plant parts. Inhibition effects among different habitats were in the order of waterside > orchard > roadside with response indexes of allelopathy of ?2.83, ?1.31 and ?0.71 of 50 g·L^?1 leaf and stem aqueous extracts of F. bidentis on L. perenne seed germination rate, respectively. The inhibition effects of 50 g·L^?1 leaf and stem aqueous extracts of F. bidentis from wasteland plants with different community density were in the order of medium density > high density > low density, and with response indexes of allelopathy of ?4.89, ?3.91 and ?0.29 on L. perenne seed germination rate, respectively. The reverse was the trend for root extracts. In vegetations with different replacement plants, the inhibition effects of leaf and stem aqueous extracts of F. bidentis were in the order of mixture with replacement plants > mono-culture. A similar inhibition pattern was observed for root extracts, regardless of the pattern of the replacement plants. The findings suggested that allelopathic active component distribution in different plant organs was changed and associated efficacies varied with environmental conditions. This was a critical factor that influenced the invasion mechanism of F. bidentis.