Cattails have received considerable attention by scientists and agencies in Florida and around the U.S. over the past decades because of their dramatic expansion into native marshes and wetlands such as the Florida Everglades. Their expansion occurred as a result of nutrient enrichment, especially phosphorus (P), in the water column and sediment and altered hydrology. Early signs of this expansion are apparent in the lower Apalachicola River floodplain located in northwest Florida, USA. To understand why and how cattails grow in the floodplain ecosystem, the structure and function of cattail community including coverage and biomass and nutrient (C, N, and P) allocations; the effects of nutrient resources such as water and sediment, and the seasonal changes of nutrients in water and sediment were investigated. Hundreds of cattail, sediment, and water samples were collected from 24 locations within 8 sampling stations during wet, growing, and dry seasons during 2002 and 2003. Cattail coverage was mapped using GPS and ArcGIS. Samples were analyzed for total carbon, nitrogen, and phosphorus using a Carlo Erba CHNS/O elemental analyzer, a Carry UV/VIS-100 spectrophotometer, a Bran+Luebbe AutoAnalyzer III, and cold neutron prompt gamma activation analysis. Nutrients, especially P, in cattail tissues (shoot, root, rhizome, and shoot base) were correlated with nutrients in water and sediment. However, cattail coverage was consistent with biomass and nutrient allocations to cattail below-ground (root, rhizome, and shoot base) which depended on hydrology but not nutrients in water and sediment. These results indicate that cattail growth in the lower Apalachicola River floodplain depended on both nutrient availability from resources such as water and sediment and hydrological factors such as river inflow and tide. However, hydrology appeared to be a major driving force for cattail expansion in this coastal ecosystem.