Plant-based systems, such as wetlands and other ecological treatment systems, are promising, sustainable alternatives to conventional wastewater treatment. One of the biggest drawbacks of these systems when compared with conventional wastewater treatment systems is a large land area requirement; thus greater efficiency is needed to reduce land requirements and consequent costs. This research identified plant species that promote greater nitrogen removal, and whose use could lead to increased efficiency of ecological treatment systems. Potential nitrification and denitrification rates of bacteria associated with the roots of herbaceous species (Cyperus papyrus, Colocasia esculentus), and woody species (Hibiscus moscheutus and Salix nigra) were measured in the lab. Potential nitrification rates were determined from the rate of NO₃⁻ accumulation in microcosms containing wastewater and live plants. Potential denitrification was measured using the acetylene inhibition technique on microcosm incubations of filtered wastewater and plant roots. Significantly more NO₃⁻ (mg l⁻¹ hr⁻¹) was produced by bacteria associated with S. nigra and C. papyrus roots than in microcosms containing only wastewater, or H. moscheutus, (P < 0.001). Potential denitrification rates were significantly greater in H. moscheutos and S. nigra root microcosms than in C. esculentus and C. papyrus root microcosms on dry weight, wet weight and root area bases (P < 0.001). These results demonstrate that the efficiency of ecological treatment systems treating wastewater high in NH₄ would be improved with a mix of herbaceous and woody species, whereas wastewater high in NO₃⁻ would be most efficiently treated with woody species. Thus, a diversity of species is needed to optimize ecological treatment system function.