Stinging nettle (Urtica dioica) is a specialty crop with economic potential. Apart from being harvested and consumed as a leafy vegetable, stinging nettle has well-documented applications in alternative medicine and industry. However, research on stinging nettle mineral nutrition is insufficient and the current study is part of efforts to establish agronomic guidelines for managed cultivation. Greenhouse experiments were conducted over two seasons (summer and fall) to evaluate stinging nettle growth and dry matter partitioning in response to variations in the supply of nitrogen (N), and N in combination with potassium (K). In the first experiment, seedlings were transplanted into potted media amended with N applied at rates equivalent to 0, 15, 30, 45, 60, and 75 g·m−2, while Expt. 2 consisted of N (15, 45, and 75 g·m−2 equivalent) and K (4, 8, and 12 g·m−2 equivalent) applied in factorial combinations. In Expt. 1, stinging nettle growth was positively correlated with N supply up to 60 g·m−2 during the reproductive phase (summer) and 75 g·m−2 during the vegetative phase (fall), while there was a slight decline in growth and dry matter yield at the highest level of K (12 g·m−2) at all N levels in Expt. 2. In both experiments, growth and dry matter accumulation was higher in the fall than in summer, and high N accounted for significantly more vegetative growth with a concomitant increase in aboveground biomass. Our results suggest that K should be applied at a rate below the growth-limiting threshold of 12 g·m−2. In this study, N strongly stimulated aboveground growth suggesting it is the most important element in stinging nettle nutrition.