The Tamagawa floodplain of Japan experienced a series of changes in the structure of its plant community after 1960. Today, two dominant lianas -Pueraria lobata and Sicyos angulatus -have colonized the area and are competing for space. This study explores their competitive abilities in relation to soil factors and flooding disturbance downstream of the Tama River. Several colonies of these species were selected in three spots of the Tamagawa floodplain having different soil characteristics and flooding frequencies. Our results revealed that S. angulatus can only grow on fine sediment, whereas P. lobata can grow on soil with a wide range of particle sizes. Soil moisture and nitrogen requirements are higher for S. angulatus. S. angulatus grows only on fine sediment for two reasons: first, fine sediment stores a higher concentration of nitrogen and moisture; second, S. angulatus has a very shallow root system. P. lobata has an extensive root system with nodules, with which it can thrive on coarse and relatively dry soil with lower nitrogen. However, P. lobata requires higher soil phosphorus than S. angulatus does. The earlier patchy colonization of S. angulatus on the Tamagawa floodplain was provoked by flooding, when the perennial Miscanthus sacchariflorus and other species trapped fine sediments. Gradually, sediments became finer and nutrient-rich due to vegetative decomposition, and S. angulatus invaded the area. The soil at that time was not deep enough for the root system of P. lobata. Subsequently, P. lobata outcompeted S. angulatus on the various types of soil that developed. Severe washing of fine substrates by floodwaters makes the area unsuitable for S. angulatus, whereas P. lobata continues to thrive.