Phosphorus and nitrogen are limiting plant nutrients in young and old weathered soils, respectively. We investigated the N × P interaction on performance of Rumex dentatus and R. pictus. Plants were grown on washed sand and supplied with 11 mM N either as nitrate or ammonium and 0.01, 0.15, 0.40, 1.0 and 1.5 mM P. Rumex, particularly R. dentatus, preferred nitrate over ammonium as N source. Nitrate nutrition favored succulence of R. dentatus but the reverse was true in R. pictus. The optimum P supply of Rumex spp. under nitrate (0.4 mM) was lower than that under ammonium nutrition (1.5 mM P). R. dentatus exhibited less number of leaves but longer and wider blades compared with R. pictus. Allocation of plant biomass to root was favored under ammonium nutrition and P deficiency, particularly in R. pictus. Increasing P supply increased Chl a and carotenoid concentrations but reduced Chl b concentration. Soluble sugars were higher under nitrate nutrition compared with ammonium only in R. pictus, with limited effect of P supply. The higher proline concentration under ammonium compared with nitrate nutrition and under P deficiency is not a consequence of impaired protein synthesis and suggests that ammonium might be stressful, particularly to R. dentatus. Only in R. dentatus, nitrate nutrition led to higher phenolic concentration and DPPH scavenging activity but to lower malondialdehyde content relative to ammonium. Increasing P supply increased phenolic concentration and DPPH scavenging activity but reduced malondialdehyde content. The concentrations of K+ and Na+ in the shoot were non-significantly affected by the form of N but exhibited marked genotypic variability in favor of R. dentatus. Increasing P supply non-significantly affected shoot K+ concentration but reduced Na+ concentration. Shoot nitrogen concentration was higher in R. dentatus than R. pictus and under nitrate over ammonium nutrition only in R. pictus. The increase in P supply increased P concentration particularly in the ammonium-fed plants. Phosphorus concentration of the shoot was significantly higher in R. pictus than R. dentatus under ammonium nutrition but the reverse was true under nitrate nutrition.