Crop production and prevailing farming practices have greatly reduced biodiversity and nearly eliminated native prairie in the central USA. Restoring small areas of prairie on cropland may increase plant biodiversity and native species abundance while benefiting the cropland. In Iowa, we incorporated buffer strips composed of prairie vegetation within catchments (0.5 ha to 3.2 ha land areas in which precipitation drained to a collection point at the slope bottom) used for corn (Zea mays) and soybean (Glycine max) production. We planted prairie buffer strips in three designs, varying the proportion of the catchment converted to buffer and/or the continuity of the buffer. Within the catchments, we determined the identity and percent cover of buffer strip plant species during 2008–2011 and of weed species in cropped areas during 2009–2011. We found 380% more species in 6 m2 of buffer strip than in 6 m2 of crop, indicating that the presence of buffer strips greatly increased catchment diversity. Plant community composition did not differ among the three buffer designs. Despite being surrounded by cropland, the buffer vegetation was dominated by native perennial species—the targeted vegetation type for both ecohydrological functions (e.g., erosion control) and native species conservation—within four years of establishment. Furthermore, weed species richness and prevalence did not differ between cropped areas of catchments with buffer strips and cropped areas of catchments without buffer strips. These results indicate that converting 10–20% of cropland to prairie buffer strips successfully reintroduced perennial species characteristic of native prairie without increasing weeds in adjacent crops.