Among the conventional methods in synthesizing nanoparticles, the methods that use biological resources, as reducing and stabilizing agents, can be considered eco-friendly methods. In this study, the leaf tissue of green tea (Camellia sinensis L.) and lavender (Lavandula anguistifolia) were utilized by the solid-state method as a one-step and low-cost method for the biosynthesis of copper oxide nanoparticles (CuO NPs). The results of the X-ray Diffraction (XRD), field emission scanning electron spectroscopy (FESEM) and transmission electron microscopy (TEM) showed that lavender is more productive in the synthesis of pure and uniform CuO NPs (50 nm). Comparing biogenic synthesized CuO NPs with chemically synthesized CuO NPs in terms of induction of phytotoxicity, exposed in treatments with concentrations of 40, 400 and 4000 μg ml⁻¹, green CuO NPs had less inhibitory effects on the seed germination factors (i.e., germination percentage, germination rate, shoot and root length, etc.) of lettuce (Lactuca sativa L.), and tomato (Solanum lycopersicum L.) seeds. However, both green/chemically synthesized CuO NPs at their lowest concentrations (4 μg ml⁻¹), had an effective role in root and shoot expansion of lettuce and tomato seedlings.