Carbon nanomaterials (CNMs), comprising carbon dots, graphene-related materials, and carbon nanotubes, have significant potential for enhancing agricultural productivity. Their compositional compatibility and exceptional properties intrigue a great deal of explorations in agricultural applications, such as fertilizers, pesticides, and regulators of plant growth. However, the evaluation of their agricultural applicability often lacks quantitative sustainability metrics, with insufficient scrutiny on the carbon footprint and scalability of the manufacturing. This review attempts to provide a quantitative ranking system for evaluating the manufacturing processes of the CNMs by applying the twelve principles of Green Chemistry, particularly in the context of agriculture applications. The review also offers a systematically organized account of CNMs' effects on plant systems, encompassing nutrient enhancement, photosynthesis, soil amelioration, disease resistance, and phytotoxicity, which can provide design rationales for the further development of CNMs.