The mechanism for the hydrothermal growth of ZnO nanoblades and nanoflowers is presented. ZnO nanostructures were grown on a copper strip substrate using zinc acetate dihydrate and ethanolamine as precursors at 65℃. The mechanism of growth was deduced as a function of substrate's exposure time to the reaction mixture. The growth was found to be nucleation driven and the presence of a seed structure promotes the formation of a nanoflower. Nanoblades grow due to the supersaturation of ZnO on the substrate surface.