The modernization of agriculture in Northern countries has led to a loss of crop diversity, as well as a loss of knowledge, know-how and rights of farmers regarding on-farm seed breeding. In France, the Réseau Semences Paysannes (RSP) brings together collectives of actors (farmers, bakers, citizens, gardeners) mobilized in a quest to reclaim these aspects. Within the framework of the decentralized participatory breeding program conducted in collaboration with INRAE, farm- ers have co-constructed knowledge in terms of dynamic management of heterogeneous wheat populations. Yet, as the RSP network grows, these actors face new types of challenges regarding collective action. To address them, we proposed to consider peasant seeds as « common unknowns » rather than « common goods » and we adapted and applied an innova- tive design approach drawing upon the KCP (Knowledge-Concept-Proposals) method. This approach consisted in imple- menting, from 2019 to 2021, five decentralized design workshops with local seed breeding collectives. The replication of design workshops in different contexts provided a framework for interaction among RSP members, allowed the sharing of both scientific and experiential knowledge. Noteworthily, it fostered the generation of many proposals for enhancing collective action, and led to the development of operational proposals with a view to implement them in the future. This methodology-oriented paper mainly describes the decentralized design approach implemented, its outputs and outcomes, as well as its interests and limits in the context of crop diversity management. It also analyses how the workshop outputs enriches previous works in the field; in particular how such a participatory design approach leads to produce original operational solutions to respond to challenges identified in the literature.