This dataset contains information on understory plant communities in across twenty urban reforestation sites planted as part of the Reforest the Bluegrass program in Lexington, KY. Urban reforested areas located in Lexington, Kentucky were evaluated over the course of summer 2020. At least three plots (and up to nine plots) were established in each site, with additional plots added if forested patches were sufficiently large. At each plot, we established a 0.008-ha (0.02-ac) circular sampling plot to survey understory plant species. Groundcover of all species, excepting tree- and shrub-forming species, was visually estimated in 10 grids, 0.6 m x 0.6 m. These data will contribute to understanding of understory plant community dynamics in developing urban forests.