Corymbia species have a high propensity to form interspecific hybrids. Dickinson et al. investigate the locations and activity of reproductive isolating barriers between species, and identify pre-zygotic barriers as the main mechanisms for reproductive isolation. Crossing Corymbia torelliana with 14 Corymbia taxa and single species from related Angophora and Eucalyptus, they find reproductive success decreases with increasing taxonomic distance between parent species. The findings support the monophyly of Corymbia and the close relationships of infrageneric clades. The hybridizing propensity of Corymbia species provides opportunities for forestry breeding but suggests risks of environmental gene flow.