Sown grass strips have been widely established on field margins across France, because of their expected environmental benefits. They may also enhance the diversity of plant species in the adjacent boundary by buffering them from agricultural practices. However, the strips could act as an extended reservoir for weeds and increase their dispersal into the field. This study evaluated the impact of a 5-m-wide sown grass strip on (i) the weed richness, abundance and diversity of both boundary and field adjacent habitats and on (ii) the width of the edge zone within the field, defined as the zone where boundary habitats impact the flora in the field. We surveyed the flora in 10 fields along continuous transects running from the boundary toward the centre of the field on opposite sides of each field, one side with a sown grass strip and one side without. The establishment of sown grass strips did not change the overall species richness at the field scale, but modified its distribution in the different habitats. In the boundaries, plant species richness, total abundance or diversity (Shannon index) remained unchanged when a sown grass strip was present, whereas these indicators sharply decreased in the crop edge (-10.9 species and -86.4 individuals per 1.25 m2) and in the field margin (-3.6 species and -9.6 individuals per 1.25 m2) where the grass strip was present. Sown grass strips also significantly decreased the width of the edge zone, from 6.5 to 1.5 m for species richness and from 12 to 1 m for species diversity, demonstrating a decrease in weed ingress into the crop