Being a very simple and cost-effective way to improve crop growth, nanopriming has been widely adopted as an efficient and advanced technology in agriculture. In recent years, there has been a drive to use green synthesized metal oxide nanoparticles (NPs) for nanopriming, which promotes an eco-friendly approach to material synthesis. Among various factors that influence the properties of NPs, annealing plays a critical role in altering their structural and functional characteristics. However, the role of annealing in optimizing green-synthesized NPs for effective nanopriming remains largely unexplored. Here, we present the first report on the effect of annealing on green-synthesized ZnO NPs for effective nanopriming. We analyze surface morphology, crystallinity, elemental composition, and colloidal stability by varying the annealing temperature of as-synthesized ZnO NPs. We show that colloidally stable smaller NPs (annealed at 400 °C) exhibit optimal performance in nanopriming. This finding is supported by practical data showing a significant improvement in germination percentage (up to 61.7%) and shoot length (up to 85.3%) of Momordica charantia seeds treated with 120 mg L−1 of ZnO NP annealed at 400 °C. This study not only provides information on the specialized synthesis of ZnO NPs but also paves the way for sustainable agricultural practices to increase food production.