Abstract Farmers were paid to establish 6 m wide sown grass strips in arable field margins under the Countryside Stewardship Scheme in England, UK. One hundred and sixteen sites in eight regions were surveyed to determine if grass margins had been established successfully and the extent to which they were colonised by forbs. Sown margins had more grass and fewer weed species than naturally regenerated sites. Grass margins contrasted with normally cropped sites, having greater species richness of grasses, forbs and perennials and more bird, butterfly larva and bumblebee foodplants. Mesotrophic grassland forbs were scarce in margins established from basic grass seed mixtures but significantly more abundant if included in the seed mixture. Annuals were more prevalent in sites up to 2 years old but species composition was not related to age in sites over 2 years old. Variation partitioning showed that overall species composition was related to seed mixture type, region and soil properties but there was little overlap in the variation explained by these environmental variable sets. Habitat context and management practices did not explain any variation in species composition. Perennial grassy vegetation was established successfully using basic grass seed mixes but only competitive species colonised subsequently. Diverse seed mixtures containing mesotrophic grassland forbs merit support in agri-environment schemes because they do enhance the botanical diversity of sown grass margins.