The effects of inorganic nitrogen (N) nutrition (NH4+, NO3− or both) at equimolar (0.5 mM) concentration on growth, biomass allocation, mineral element concentration, N-uptake kinetics and nitrate reductase activity were assessed in hydroponically grown Cyperus laevigatus and Phormium tenax. Both species grew well with both NH4+ and NO3− as the sole N source, but the mean growth rate of C. laevigatus (RGR = 0.12 d−1) was significantly higher than that of P. tenax (RGR = 0.02 d−1). However, the RGR of C. laevigatus was higher when supplied with NH4+, either alone or in combination with NO3−, than when supplied with NO3− alone, whereas the RGR of P. tenax was indifferent to nitrogen source. The nitrogen uptake rate of C. laevigatus was generally higher (2–3-fold for NH4+ and 4–14-fold for NO3−) than that of P. tenax in concert with its higher growth rate. Both species had higher uptake capacity and higher affinity for NH4+ than for NO3−. The mean maximum uptake velocity (Vmax) for NH4+ was 99 and 41 μmol g−1 root DM h−1 for C. laevigatus and P. tenax, respectively, as opposed to a Vmax for NO3− of 24 and 3.4 μmol g−1 root DM h−1 for C. laevigatus and P. tenax, respectively. P. tenax had significantly lower affinities for both NH4+ and NO3− than C. laevigatus indicating that P. tenax are adapted to sites with relatively high N availability, as opposed to C. laevigatus, which via its high-affinity uptake systems can grow at sites with lower N availability. Both shoots and roots of both species had nitrate reductase activity, particularly when plants were fed with NO3−. P. tenax seemed to be more sensitive to NH4+ nutrition than C. laevigatus since tissue concentrations of mineral elements were lower in roots of plants supplied with NH4+. Our results suggest that the indifference to N nutrition provides C. laevigatus and P. tenax with the ability to grow well in periodically inundated areas with fluctuating and variable N sources. However, C. laevigatus may be better adapted than P. tenax to grow in permanently inundated soils with high NH4+ concentrations, whereas P. tenax may be better adapted to grow at high NO3− condition.