Context: There is an urgent need to stop the biodiversity loss in European agricultural landscapes. These landscapes, due to their fragmentation, include a lot of edges, many of them between habitats of different quality in terms of biodiversity. Objectives: Here, we ask how plant species richness is distributed from the interior of protected semi-natural grassland into the interior of adjacent crop fields, and which species groups determine the observed patterns. Methods: At grassland–crop field interfaces in two German landscapes, we sampled the vegetation along transects extending 32 m into both habitats. Based on theory, we fitted different models to visualize the species richness curve along transects and selected the best model via AIC. Results: The best fitting model for all species was monotone over the interface with a minimum in the field and a maximum in the grassland. This shape was driven by the generalists, showing the same pattern, while grassland specialists showed a sigmoid curve with an increase only in the grassland. Arable specialists had a peak at the field edge and decreased towards the grassland. Curves for Ellenberg indicator values for moisture, nutrients and light showed the same shape as the overall species curve, yet with an inverse pattern for light. Conclusions: This is the first study revealing gradual responses of plant species richness at the grassland–crop field interface with a high spatial resolution. As these adjacent habitats influence each other far into their neighbourhood, attention should be given when conserving biodiversity in agricultural landscapes, particularly in case of often small and scattered protected areas.