Annual Saltmarsh Aster or Aster-weed (Symphyotrichum subulatum), naturalised on brackish wetland areas, riparian banks, irrigation canals, and seasonally wet sites throughout Victoria.
Native to saline environments throughout much of the Americas, Aster-weed has also naturalised in the UK and continental Europe, China, Japan and New Zealand. I have not seen a theory for how and from where it reached Australia, however the species produces copious wind-dispersed seed and has the potential to have been a repeated contaminant of ballast or imported seed and hay.
A history of taxonomic confusion about the appropriate classification of this species complex makes its botanical records something of a mess; it was, and by some authorities still is, assigned as Aster subulatus. There is a single 1877 record of the species in Western Australia, and then a succession of records from the vicinity of Sydney beginning in 1880. In Victoria, collections were made in 1906 from damp places within the Domain at Melbourne and from the Mallee district, and in 1908 from Gippsland. Early newspaper mentions offer other common names: Wild Aster, Shrub Aster, Bushy Starwort.
In the 1920s, new infestations of golf courses and farms in South Australia garnered concern in the popular press, with columns decrying the failure of policy to achieve control of weeds declared noxious as much as they demanded that Aster-weed be added to those statutory lists. In Victoria, the problem of Aster-weed was noted repeatedly in 20th c. newspapers, in context of irrigation districts throughout the state where sufficient summer moisture combined with increasingly problematic soil salinity, for which the species was well-suited.
Today, Aster-weed is recognised as an environmental weed in key salt marsh complexes and other coastal habitat across much of Australia. The photographed plants will have completed their life cycle now, but were seen in April along a section of Edgars Creek in Reservoir where thin grassy banks lay atop exposed rock.