Purple-top, or Argentine, Tall or Wild Vervain (Verbena bonariensis), an infamous presence in the lowland pastures of NSW and QLD from the 1850s or earlier, so pervasive at the time that von Mueller apparently described these areas as ’the Verbena plains’ (*).
Whether the introduction of this South American species to Australia was the result of its import as a (marginal) ornamental plant, or as a contaminant in other seed is not clear. The plant has not been quite as prevalent in Victoria where rainfall, climate and topography are less favourable, but it is still reasonably common in Melbourne’s urban floodplains and up north on tributaries of the Murray. First recorded in Victoria on the Hume River in 1891, or possibly at Heidelberg in 1883.
This is the variety Verbena bonariensis var. conglomerata. It is differentiated from V. bonariensis var. bonariensis by the form of its inflorescence, where the flowers are held in low clusters rather than small spikes.
(*) As related by Joseph Hooker, I haven’t seen the original text.