Due to their diverse properties, zinc oxide nanoparticles (ZnONPs) have shown great potential in various applications. These nanoparticles exhibit unique characteristics, including antimicrobial, anti-inflammatory, wound healing, catalytic, magnetic, optical, and electronic properties, making them useful in various fields. The size and crystalline structure of the material are crucial factors affecting these properties. In recent years, using plant extracts to synthesize nanoparticles has emerged as a technique that allows for the control of the size, shape, and properties of these materials. The main objective of this work is to study the influence of extract concentration and temperature on the synthesis of ZnONPs using Flourensia cernua extract. Zinc nitrate hexahydrate was used as a precursor, and we tried different extract–solvent ratios, such as 0.5:10 and 1:10 w/v. The ZnNPs were synthesized at varying temperatures ranging from 300 to 500 °C. ZnONPs were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared spectrometry (FTIR). The XRD patterns showed a wurtzite hexagonal phase of ZnO. According to TEM characterization, crystalline particles with a semi-spherical morphology were obtained in a size range between 10 and 22 nm. Interestingly, increasing the amount of extract led to a decrease in the particle size. On the other hand, an increase in the calcination temperature caused the growth of the nanoparticles. This method showed that extract concentration and temperature greatly influence the size of the ZnONPs. The application of these particles in the photodegradation of organic dyes is being studied. Using plant extracts as a green synthesis method for ZnONPs can provide a sustainable and eco-friendly approach to developing customized nanoparticles for various applications.