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Sowing of grass seed mixtures is a feasible and cost‐effective method for landscape‐scale grassland restoration. However, sowing only grasses usually leads to species‐poor and dense swards, where the establishment of target forbs is hampered by microsite and propagule limitation. To overcome these limitations and increase the diversity of species‐poor sown grasslands, we developed a novel method by creating “establishment gaps.”. We used tillage to open gaps of 1‐, 4‐, and 16‐m² size in the dense grass sward of six species‐poor restored grasslands in the Great Hungarian Plain. We sowed high‐diversity seed mixtures of 35 native species into all gaps. We analyzed vegetation development during the first 5 years after setting up the trial. We also studied the colonization dynamics of the sown species along four 20‐m transects around each gap, resulting in a total of 1440 plots of 1‐m² size that were studied. Our results indicated that most of the sown species were able to establish permanently in the gaps. The total cover and the cover of perennial sown species increased and the cover of short‐lived sown species decreased independent of gap size. There was only a moderate level of weed abundance in the gaps, and weed cover decreased over the years. The sown target species started to colonize the species‐poor grasslands surrounding the gaps within 5 years. The highest number of species and individuals dispersed from the 4‐m² gaps, as they had a more stable development than smaller gaps and were exposed to lower grazing pressure than large ones.