Although there have been advances in synthesizing hierarchical semiconductor materials, few studies have investigated the fundamental nucleation mechanisms to explain the origins of such complex structures. Resolving these nucleation and growth pathways is technically challenging but critical for developing predictive synthetic capabilities for the synthesis and application of new materials. In this Letter, we use state-of-the-art in situ liquid phase scanning electron microscopy (SEM) and high-resolution transmission electron microscopy in a combination with classical density functional theory (cDFT) to study the nucleation of highly branched wurtzite ZnO nanostructures via a facile, room-temperature aqueous synthesis route. Using a range of precursor concentrations, we systematically vary the hierarchical organization of these nanostructures. In situ liquid phase SEM demonstrates that all branches form through secondary nucleation and grow by classical processes. Neither random aggregation nor oriented attachment is observed. cDFT results imply that the morphological evolution with increasing [Zn2+] arises from an interplay between a rising thermodynamic driving force, which promotes branch number and variability of orientation, and increasing barriers to interfacial transport due to ion correlation forces that alter the anisotropic kinetics of growth. These findings provide a quantitative picture of branching that sets to rest past controversies and advances efforts to decipher growth mechanisms of hierarchical structures in real solution environments.