Cut-leaved Evening Primrose (Oenothera laciniata), an annual or short-lived perennial from the sand prairies of the eastern United States.
Introduced in 1770 to English cultivation (per Loudon), the plant was naturalised in northern Europe by the early 20th c., when collected in the UK, Netherlands and Scandi. It was recorded in China and South Africa in the 1920s, in C. and S. America in the 1940s, and in Australia on NSW’s Upper Hunter River in 1953. The species was also noticed on beaches around Perth in the 1980s.
I don’t think this has really been a common horticultural selection, but it may have occasionally shown up in Australia as a contaminant in imported seed packs of ornamental annuals. The plant may also have been brought to WA (and Vic) in contaminated ballast from the UK or South Africa, although the window for such a windjammer exchange was quite narrow and leaves a substantial lag before it was detected.
In Victoria, Cut-leaved Evening Primrose is known from a single naturalised population at Port Melbourne. This population, photographed here, has been recorded since 1995 and persists to the present day in the Sandridge dune scruff.
How it got here is almost a complete mystery. I say almost as there is a hint in the human observation records that the species is not restricted to Sandridge Beach, but has actually been present around Hobsons Bay.
Two past observations were made around the Webb Dock and Fisherman’s Bend for Oenothera stricta, a similarly yellow-flowered Evening Primrose with leaves toothed rather than lobed, and four more at various points from Altona down to Geelong. There isn’t a herbarium collection for O. stricta from any of those areas (indeed, the only collection of O. stricta in the Melbourne metropolitan was made at Box Hill in 1884). The earliest observation dates to 1942 near Seaholme, with the others made in the 1980s and 1990s.
It seems possible then that plants were misidentified in the field, and Cut-leaved Evening Primrose has been quietly about its annual business at several discrete points around the bay.