Speciation is a central process in evolutionary biology and is responsible for the diversity of life on Earth. Although there has been much progress in evolutionary research over the past 150 years, understanding the many facets of speciation remains a challenge. In this synthesis, I focus on the use of an interdisciplinary approach to examine speciation and species delimitation in a group of closely related eucalypts called the green ashes (Eucalyptus subgenus Eucalyptus section Eucalyptus). The green ashes comprise tall trees on fertile soils (e.g. the tallest angiosperm in the world, E. regnans), as well as medium trees and mallees on low-nutrient soils. Previous phylogenetic and population-genetics analyses based on genome-wide scans showed that species boundaries in the green ashes are not always consistent with classifications based on morphology and there was evidence of gene flow across lineages. Genomic analyses also suggested that the green ashes were at varying stages of speciation, with some species being highly genetically differentiated, whereas others were at earlier stages on the speciation continuum. A previous common garden study showed that inter-specific differences in seedling traits were significant, with traits such as leaf width being highly plastic across resource treatments for most species. Overall, this synthesis demonstrated that an interdisciplinary approach incorporating phylogenomics, population genomics and a common garden experiment can provide insights into speciation and species delimitation in the green ash eucalypts. Such an approach may be useful in understanding the evolutionary history of other closely related species in Eucalyptus, as well as other groups of organisms.