Dense arrays of ultra-thin zinc oxide (ZnO) nanorods have been fabricated on c-oriented ZnO seed layers by hydrothermal growth, with particular emphasis on exploring the effects of annealing or plasma pre-treating the seed layer. Such pre-treatments influence the density, size and surface defects of particles within the nucleating seed layer, which impacts on the subsequent nanorod growth via sequential reaction with OH- and Zn2+ species. Oxygen-rich defects at the seed layer surface are deduced to have particular impact – affecting both the c-axis growth rate and the photoluminescence properties of the as-grown nanorods.