Gunnera perpensa L., a wetland herb, is extensively harvested and used as traditional medicine. In many areas of KwaZulu-Natal, South Africa, populations are under threat of overexploitation from the herb. Successful cultivation would reduce harvesting of natural populations and could help conserve the species. To provide information for cultivation, we assessed the effects of various flooding regimes on plant growth and biomass accumulation, as well as on morphological and physiological adaptations of this species to waterlogging stress. Plant rhizomes collected from the field were cultivated for three weeks, and then subjected to drained, 20%, 40%, and 80% flooded treatments for 12 weeks. Plant height, total dry biomass accumulation, and carbon dioxide exchange increased significantly with increase in level of flooding from the drained to the 40% flooded treatments, and thereafter decreased as level of flooding increased to 80%. Soil E_h and root specific gravity decreased with increases in flooding. In rhizotron experiments, in which plants were subjected to 40%, 80%, and 100% flooding, root growth as well as biomass accumulation decreased as flooding increased. Photosynthetic performance, monitored along a natural soil moisture gradient in the field, indicated that carbon dioxide exchange, quantum yield of photosystem II (PS II), and electron transport rate (ETR) through PS II were higher at a soil moisture of 45% than at 25% or 60%. This study demonstrated that G. perpensa grows best in moderately reduced soils that are moist but not waterlogged. Adaptations to waterlogging include increased aerenchyma in roots and petioles, and decreased below ground biomass accumulation.