Segetal plants, which grow preferentially or exclusively in cereal fields, experienced a strong decline during the last century. Among them, Bromus grossus received particular attention, as it is highly threatened in Europe. Its decline is thought to be due to crop seed cleaning among other causes. Re-establishing the sowing of uncleaned crop seeds should therefore be considered as a tool for the conservation of this species. In this study, we aimed to evaluate (i) how the conservation of B. grossus relies on transfer in uncleaned crop seed, (ii) how this practice may help to restore new populations of this species, and (iii) the contribution of this practice to the dispersal of other segetal plants. From 2012 to 2016, we monitored eight fields from three farms in Southern Belgium where uncleaned spelt seed containing B. grossus was sown. We found that B. grossus grew in the year following seed sowing, but disappeared in the second year in most cases. This highlights the extreme dependence of B. grossus upon uncleaned spelt-seed sowing. We also showed that, through associated management practices, B. grossus acted as an ‘umbrella species' to other arable-dependent plants. Transfer of uncleaned seed led to an increase in species richness in an experimental field from 12 species in 2015 to 43 species in 2017. Based on the germination of uncleaned seeds in a greenhouse, we concluded that it was likely to account for the dispersal of at least nine species, and possibly 15 others.