【Background】 Bidens alba and B.pilosa var.radiata (Compositae) are native weeds from tropical America, that have recently invaded roadside, cropland, waste ground etc.in South China, causing losses to local agriculture and the environment. Mechanisms facilitating their invasion are not well understood. We conducted comparisons between these two alien weeds and their native congener, aimed to know better the invasive ability and invasiveness. 【Method】Morphological and growth parameters, biomass and its allocation, the duration of reproductive phase and seed production per plant, as well as phenotypic plasticity index of B.alba and B.pilosa var.radiata were measured and compared to those of the native congener B.biternata, grown under different soil nutrient levels.【Result】Most growth parameters except the duration of reproductive phase and seed production per plant had no significant difference between two alien weed and native weed under low soil nutrient level. At high soil nutrient level, the total leaf area as well as specific leaf area, leaf area ratio, duration of reproductive phase and seed production of B.alba were significantly higher than those of native B.biternata. However, the total leaf area, specific leaf area, total biomass and relative growth rate of alien weed B.pilosa var.radiata were significantly lower than those of B.biternata under high soil nutrient levels. [JP2]The plasticity indexes of plant height, total leaf area, specific leaf area, total biomass, leaf area ratio, relative growth rate, mean leaf area ratio and seed production in B.alba were higher than those either in B.biternata and in B.pilosa var.radiata. Most plasticity indexes were similar between in B.pilosa var.radiata and in B.biternata.[JP]【Conclusion and significance】B.alba was an alien weed with strong invasive ability. Long reproductive phase and high seed production in B.alba and B.pilosa var. radiata which were different from those in native weed B.biternata, were some of the important characters for invasiveness.This research provide evidence for revealing the invasive mechanisms and influence factors of B.alba and B.pilosa var.radiata, as well as provided basic data for prediction and control of invasions.