Nitrogen removal in agricultural treatment wetlands is determined by the health of macrophytes. Nitrogen is removed by microbes dependant on an oxic-anoxic boundary layer created by oxygen flow through macrophyte culms and by nitrogen sequestration in plant biomass. Phytotoxic ammonia concentrations can limit plant growth. Orthophosphate fertilization to balance the nitrogen to phosphorus tissue ratio may increase biomass. One test of this hypothesis investigated <i>Juncus effusus</i> and <i>Typha latifolia</i> ammonia phytotoxicity: ammonia concentrations above 150 mgL-1 suppress <i>Typha</i> growth and above 300 mgL-1 do not affect <i>Juncus</i> growth. The second experiment altered mesocosm N:P ratios. Some alleviation of ammonia toxicity was shown; highest biomass production at toxic levels occurred at a 15:1experimental N:P ratio. The two species used N and P differently: the average N:P ratio was 12:1 in <i>Juncus</i> and 9.1:1 in <i>Typha</i>. More effective waste removal may be based upon more precise control of N:P ratios.