Geraldton Carnation Weed (Euphorbia terracina), a Mediterranean perennial established across southern Australia as a weed of pasture, roadsides and bushland.
Also called False Caper Spurge or Terracine Spurge, the species is particularly prevalent in South Australia, where it was first collected in 1903 and proclaimed noxious in 1911, and in Western Australia, where it was introduced before 1914 and earned its Australian common name in the vicinity of Geraldton. It has also naturalised in Southern California.
By the 1920s, the Western Australian infestation had grown to become a serious menace. In an open letter to the Minister for Agriculture, a Millewa resident declared it to be ‘a plant that… possesses more devastating potentialities than any plant that has ever before lodged on the shores of Western Australia.’ In demanding drastic action against the species, the writer related the story of a neighbour who had ‘got some goods from Geraldton… the packing in which these goods came was left out in the open, after the rain this packing sprouted out in Carnation weed. Lucky for this district Mr. Rumble was familiar with the plants and destroyed them.’
The seeds of Terracine Spurge have been found to be spread by ants and doves, but in WA dispersal appears to have been accelerated in contaminated soil (containing seed or adventitious root material) spread by construction vehicles.
Within Victoria, Terracine Spurge was first detected south of Mildura in 1966. It is now well-established there and on the Bellarine Peninsula south and east of Geelong, where it was identified in 1982. A few other scattered records have been made across western Victoria in the years since.
The photographed plant or plants were found in a single cluster, within a grassy patch of River Red Gum woodland on floodplain of the Maribyrnong River in Sunshine North. This would appear from digital sources to be the first recorded observation of the species within Melbourne; I have dutifully added it to iNaturalist.