Plant community composition may be limited by propagule availability, environmental constraints, or both. In the Nanticoke River watershed in Maryland and Delaware, herbaceous communities of tidal freshwater swamps in upstream areas are less diverse than in downstream areas. To test factors constraining plant community composition, a reciprocal transplant study was conducted between an upstream site with lower diversity, Broad Creek, and a downstream site with higher diversity, Deep Creek. Species composition, height, biomass, and below-canopy PAR were measured in sections of soil with rooted vegetation (sods) assigned to one of three treatments arranged in a randomized block design: transplant (excavated and moved between sites), disturbed control (excavated and replaced in same location), or undisturbed control (no excavation). Data were also collected on soil seed bank composition, hydrology, and salinity. Vegetation measures in sods transplanted from Deep to Broad Creek were not significantly different from control sods left at Deep Creek, but sods transplanted from Broad to Deep Creek developed a community intermediate to controls at either site. Broad Creek was inundated 10% longer than Deep Creek (P < 0.0001), and had lower light levels for four of five sample periods (P < 0.021); salinity was significantly higher in Deep Creek for all months (P < 0.001) and exceeded 3 parts per thousand in July. Seedling density (P  =  0.013) and taxa density (P  =  0.001) were four times higher in seedlings emerging from seed bank samples of Deep Creek than Broad Creek. The community that developed in sods transplanted into Broad Creek suggests that over short time scales, diversity may be increased by propagule additions. However, a combination of propagule availability and suitable environmental conditions over longer periods may be necessary to sustain a robust vegetation community.