Conservation of the gene pool of F. pennsylvanica (green ash), the species most susceptible to emerald ash borer (EAB), requires a range-wide genetic assessment of the population dynamics to guide seed collection efforts. An assessment of gene flow into natural stands from ubiquitously distributed green ash and F. americana (white ash) cultivars is also essential. We used 16 EST-SSR markers to genotype 1291 trees from 48 naturally regenerated populations of green ash, 19 green ash cultivars and 10 white ash from a Fraxinus species collection. We did not find evidence for latitudinal gradients in allele richness or higher differentiation in northern range edge populations. Analysis of population substructure revealed two major groups of green ash, one dominant in northwestern locations, one dominant in southern locations and extensive admixture in midwestern and eastern locations. We identified a third group as white ash, including 140 individuals with interspecific admixture in eastern and southern populations. We detected cultivar parentage with high confidence in 172 individuals in 34 of the 48 populations. We conclude that high dispersal capacity, broad adaptability, and gene flow from many cultivars into predominately northern regions, plays a role in the comparatively low regional population substructure in the Northwest. The high frequency of cultivar propagule dispersal and extent of interspecific admixture detected in our study suggests that under rapidly changing conditions, local populations may or may not be locally adapted and standing genetic variation may not be of long standing.