Poyang Lake is one of the most important wintering habitats for waterbirds along the East Asian–Australasian Flyway. Carex spp., the dominant plant in Poyang Lake wetlands, is the main food source for overwintering geese, and its growth directly affects their foraging efficiency and energy intake. In recent years, hydrological regimes of Poyang Lake have been significantly altered by hydrological engineering and frequent extreme weather, which has disrupted the growth dynamics of Carex, leading to a mismatch with the migratory timing of overwintering geese. Mowing, a common management practice in grassland ecosystems, can stimulate plant regeneration and regulate growth rhythms, partially alleviating food resource mismatches and improving foraging conditions for geese. This study focused on Carex in the floodplains of Poyang Lake. In a field experiment, we selected exposed Carex during autumn and winter and applied different mowing frequencies across areas with varying soil moisture levels, with long-term management and monitoring. Results showed that increasing soil moisture significantly promoted plant height and aboveground biomass, while belowground biomass peaked only under low soil moisture 15 days after mowing. Multiple mowing significantly suppressed plant height and aboveground biomass and weakened the promoting effect of soil moisture, causing growth indicators across different moisture gradients to converge. Both soil moisture gradient and mowing frequency significantly affect plant height, aboveground biomass, and belowground biomass of Carex, with optimal growth observed under high soil moisture (≥ 33.79%) and low-intensity mowing (≤ 4 times). The results provide quantifiable parameters for water regulation and mowing frequency to guide the management of wintering waterbird habitats in the Poyang Lake wetlands.