How do Lemna minor and Spirodela polyrhiza, two species that occupy similar ecological niches, coexist? We observe that the two duckweed species exhibit obvious seasonal dynamic patterns: in summer, the water surface is mostly occupied by S. polyrhiza, while in winter, L. minor becomes the dominant one. We investigated their competitive strategies and found that S. polyrhiza increases its inter-frond connection number via enhancing its antioxidant capacity, which in turn magnifies the attraction force between individuals. Furthermore, it releases more space for growing by calcium-ion-mediated apophysis of fronds, which indicates the tendency of S. polyrhiza toward consolidation strategy. Meanwhile, L. minor allows ROS (reactive oxygen species) to accumulate in the node area and trigger rupture, lowering inter-individual attraction force. With its tiny size and good blank-filling capability, L. minor adopts an invasive strategy. Furthermore, our research discovered that the critical factors underlying the seasonal dynamics of duckweeds are photoperiods and phytochrome. In summer, long-day causes accumulation of Pfr and in turn the elevation of connection number, which favors the consolidation strategy of S. polyrhiza. While in winter, short-day induces Pfr to turn back to Pr, stimulating the fractionation between fronds, which helps L. minor to occupy the surface of water. In conclusion, this discovery revealed how photoperiods regulate the seasonal dynamics of duckweeds, and provided a novel perspective on the competition of aquatic plants.