As presently circumscribed, Plagianthus includes two morphologically distinct species that are endemic to New Zealand. Plagianthus divaricatus, a divaricate shrub, is a dominant species in coastal saline shrub communities, whereas P. regius is a tree of lowland
and montane forests. Results from independent analyses of ITS and 5′ trnK/matK sequences are congruent, and when combined provide a robust framework to study character evolution. Our findings suggest the ancestor of Plagianthus originated in Australia where the sister
genera Asterotrichion and Gynatrix are presently distributed. The stem age of Plagianthus was estimated at 7.3 (4.0–14.0) million years ago (Ma) and the crown radiation at 3.9 (1.9–8.2) Ma. Most of the characters optimized onto the molecular phylogeny were
shared with source lineages from Australia and shown to be plesiomorphic. Only the divaricate branching pattern characteristic of Plagianthus divaricatus was acquired after the lineage became established in New Zealand and shown to be apomorphic. The initial Plagianthus founders
were shrubs or small trees with deciduous leaves and small inconspicuous dioecious flowers. Juvenile vegetative morphology and sexual maturation are decoupled in Plagianthus; heteroblastic vegetative development is well documented in Plagianthus and close relatives.