Eastern States’ wattles
Cootamundra wattle (Acacia baileyana), Sydney golden wattle (A. longifolia) and Flinders Range wattle (A. iteaphylla) are not a problem in their own natural geographic locations where local seed-eating insects keep them in check. But this natural control does not occur in Western Australia where these wattles reproduce prolifically by seed. If fire sweeps through bushland that they have invaded, it promotes faster germination of their seeds.
These and other invasive Eastern States species are often labelled as ‘native’ so make sure you shop at a reputable plant nursery and do some research before you buy. This website may be useful.
Wattles are nitrogen fixers. When they grow in an area that has been disturbed — for instance, by someone driving in to dump garden rubbish — the combination of disturbed soil and extra nitrogen encourages exotic grasses to invade. As the grasses dry out in summer, they increase the risk of bushfire. Fire stimulates more wattle seeds to germinate. The resultant wattles fix more nitrogen in the soil, more exotic grasses invade and act as fuel for further fires — and thus a destructive cycle is set in motion.
Pruning the flowers before they set seed is not a practical option once these wattles reach a certain size. The only sure way to prevent seed spreading is to fell the tree.
Elephant ears and taro
These plants belong to the same family (Araceae) as the arum lily. Elephant ears (Alocasia brisbanensis) are popular in gardens because of handsome foliage and a capacity to thrive in shade. However, its fruits are poisonous to people and pets, though they can be eaten safely by birds, which then disperse the seeds.
Tubers of taro (Colocasia esculenta) are commonly eaten in Polynesia, but they are edible only when boiled or baked in the correct way — otherwise they are poisonous.
As taro tolerates permanent inundation, fancy forms have become popular for ornamental ponds and swamp gardens in partial shade. However, taro also tolerates dense shade and intense sunlight. This flexibility allows it to naturalise easily in a wide range of situations.
Taro is increasingly causing concern in the Swan and Moore River catchments and is very common on parts of the Canning River. Flowers are infrequent and seed is uncommon and of low viability, but taro spreads aggressively by corms and stolons. Its ability to inhibit the growth of other plants makes it undesirable in the garden.
Elephant ears and taro can be removed by pulling or digging out, but taro in particular is likely to resprout. If this occurs, follow up with herbicide.