FOR THIRTY YEARS or more, I have been urging people to grow the plants in their gardens that come from comparable soils and climates. For thirty years and more, I have been wrong, for these are the plants that are most likely to leap the garden wall, to get ‘out of bounds’, as they are now doing at an accelerating and frightening rate.

A change that has been taking place since the European invasion of the continent has gained momentum quite massively in the last few decades. That change is the introduction of exotic plants, animals and pathogens that make a takeover bid in their new environment. In the words of Alfred Crosby, European immigrants did not arrive alone in the new lands: they were accompanied by ‘a grunting, lowing, neighing, crowing, chirping, snarling, buzzing, self-replicating and world-altering avalanche’. As for the animals, so for the plants. Early in the last century, especially in semi-arid South Australia, the cultivation of wheat was attempted in country that was far too dry, in the mistaken belief that ‘Rain follows the plough’. In reality, weeds follow the plough. Wild oats (Avena spp.) arrived with the first wheat seed, and is now to be found wherever there is disturbed ground in southern Australia, smothering what is left of the indigenous plant cover, and ready to burn with the first cigarette butt in summer.