We examined the effects of foliar supplementation with three nitrogen sources (urea, NH4NO3, and CoRoN) at two different concentrations of each to evaluate their efficacy as spray fertilizers for the growth of hybrid poplar cultivar NM6 (Populus nigra L. x P. maximowiczii A. Henry;). Our goal is to reduce nitrogen fertilizer use and loss of nitrogen to the environment making it more economical and environment friendly for cultivated forestry practices. Our objectives were to determine if: (1) foliar nitrogen application is an effective method of fertilization for poplar cultivars; and (2) different nitrogen sources are metabolized similarly by the plants. The analyses involved free polyamines, amino acids, soluble protein, and chlorophyll content in the foliage, and overall plant biomass. The plants were started from vegetative cuttings grown in the UNH greenhouse facility. The data show that (a) the effects of foliar nitrogen application on leaf chemistry, biomass, and foliar nitrogen content varied according to the form and concentration of nitrogen source used, and (b) different nitrogen sources are metabolized differently by these plants. It can be concluded that the foliar nitrogen application method could become a practical and sustainable way to fertilize poplar. Future studies are aimed at genetically manipulating the metabolism of a group of nitrogen-rich metabolites (the polyamines) to further enhance the nitrogen use efficiency and in turn increase biomass production of these plants.