EU Member States face a quandary: after decades of demanding powers to choose whether or not to cultivate GM crops, the EU has returned some limited but significant powers to them. A directive permits Member States to “opt-out” from GM cultivation, provided that they meet relevant criteria. Member States need to decide urgently and carefully whether and how to restrict GM crops, as the permeable nature of the environment facilitates the spread of genetically modified organisms (GMOs) once cultivated. One consideration is agri-sustainability. In principle, GM crops could promote agri-sustainability, including through increasing agrobiodiversity, as they facilitate introducing new traits or species into an ecosystem. However, the nature of their modifications allows for the applicability of patenting law, which enables the legal “enclosure” of the crops’ genetic make-up. This impacts negatively upon the long-term availability of plant genetic resources and agrobiodiversity, as farmers and other breeders operate in a context where accidental cultivation of patented material can still attract liability. This article argues that legal enclosure could justify imposing restrictions on GM cultivation in order to conserve agrobiodiversity as an exhaustible natural resource essential to agri-sustainability. To improve the likelihood of restrictions being upheld legally at both the EU and WTO level, such justifications must be distinguished clearly from any broader environmental concerns, as both the EU and WTO impose stringent restrictions where environmental objectives are raised.