Three varieties of Bidens pilosa are recorded in Taiwan, among them only B. pilosa var. radiata is considered an invasive plant, while B. pilosa var. minor and B. pilosa var. pilosa are naturalized in Taiwan. To identify traits making var. radiata spreading faster than the other two varieties in sympatric areas, we grew these three varieties in summer and in fall and compared their morphology and traits related to growth. We found that all three varieties grew faster in summer than in fall and var. radiata grew faster and accumulated more biomass than the other two varieties in summer but not in fall. Further analysis revealed that the proportion of shoot biomass allocating to axillary shoots might contribute to the aforementioned differences. Shoots of all three varieties were capable of producing adventitious roots. However, the growing angle of the lowest axillary shoots was significantly more horizontal in var. radiata than in the other two varieties and the secondary axillary shoots were only found in var. radiata. Accordingly, the axillary shoots of var. radiata would have greater opportunity to contact soil, produce adventitious roots and generate ramets than those of var. minor and var. pilosa. We concluded that more biomass allocating to axillary shoots and more horizontally-oriented axillary shoots were important traits contributing to the faster growth and better clonal growth potential of B. pilosa var. radiata than the other two varieties in Taiwan.