Surface water accumulation significantly affects the growth and structure of Phragmites australis plants. The depth of water on the soil surface impacts the width of rhizomes, internode length, and knot number. Rhizome width is widest in perennial drought conditions, followed by seasonal hydrocele and perennial hydrocele. The proportion of rhizome weight to total underground biomass is highest in drought conditions. Plants in drought are thickset but sparse, while those in seasonal hydrocele are less dense but have more biomass. In contrast, plants in perennial hydrocele are thin and densely packed.