The synergistic impacts of climate change and biological invasions pose major threats to biodiversity. However, although previous studies have revealed that climate warming enhances the invasiveness of non-native plants, little is currently known regarding the eco-physiological mechanisms underlying the changes in invasiveness promoted by a changing climate. In this study, we aimed to clarify the effects of warming on the phenology and eco-physiological background of the invasive annual weed Bidens pilosa var. pilosa in temperate regions. We compared the timing of germination, seedling growth, and flowering phenology at two study sites characterized by different temperatures (difference in mean annual temperature: 2.8℃). From winter to early spring, germination commenced earlier and seedling growth was more rapid at the warmer site than at the cooler site. In the former site, some seedlings bore flowers in winter to early spring (spring flowering), whereas spring flowering was rarely recorded at the cooler site. The results of laboratory experiments indicated that spring flowering occurred when the daylength was sufficiently short for induction of flowering and when the growth stage exceeded a threshold size. To assess the impact of phenological shifts on invasiveness, we constructed a simple model for estimating the number of seeds produced annually (propagule pressure). Model estimates indicated that even partial spring flowering resulted in an exponential increase in propagule pressure. These findings indicate that phenological shifts caused by climate warming could result in a substantial increase in invasiveness, which is considerably more pronounced than that caused by growth promotion only.