What is it?
Wild Taro is a fast-growing aquatic weed with dark reddish purple stalks and very large, arrowhead-shaped, dark-green, velvety leaves. The leaves are shaped somewhat like elephant's ears.
In some parts of the world wild taro is harvested for its potato-like tubers. (Esculenta means edible.) The plant is actually poisonous if ingested raw because of needle-shaped raphides in the plant cells - severe gastrointestinal distress can occur. The initial affect on the mouth is an immediate painful irritation, burning and swelling of the lips, tongue and other mouth parts with a subsequent feeling of local anaesthesia.
The Taro in Brisbane is mostly the common purple-black stemmed, peltate (stalked from back of blade) cultivar known as Colocasia esculenta 'Fontanesii'. There is another Taro (Blue Taro, Xanthosoma violaceum) which does not have a peltate leaf - the leaf stalk comes in at the edge of the leaf blade. Blue Taro is much less edible than Colocasia esculenta, and is also a nasty weed.
Wild Taro pollutes water by trapping organic matter in its roots, producing a foul-smelling sediment. Disturbed plants can release toxins harmful to native wildlive and its aggressive growth outcompetes native species. In the Enoggera catchment it's a high priority candidate for removal.
How do you get rid of it?
First of all let's admit we are not weed experts. We are just local bushcare volunteers with no special technical knowledge (just our own informal experience). That said, in terms of our local experience we have been pretty successful.
In dry times when the water is low we favour hand pulling by getting into the creek with waders. The whole plant can be lifted very gently and carried up the bank and piled well above any possible floodline. These piles can be cooked out under black plastic.
The whole plant is toxic to humans so you have to be careful not to break plants leading to distribution of toxic sap. Wear gloves, long sleeved shirts and long trousers. While on the topic of health and safety, it's worth noting that the plants can be very heavy so removing by hand requires strength and sometimes teamwork, such as a human chain or the use of wheelbarrows.
Removal by hand is a painstaking process. Often some material will be left and new plants will continue to emerge, which have to be treated gradually over months and years. Sometimes it is hard to lift plants intact without getting really close to the root mass, and sometimes gently prizing the root mass up with a pick or mattock is necessary to get it free from the sticky mud. This process obviously can also dislodge and harm frogs, fish and invertebrates, so the trick is to be as gentle as possible and do a bit at a time in order for fauna to relocate. That said, the Taro clumps do not seem to have a lot of native animals in their midst, possibly due to their stinky silt and toxicity.
To battle erosion generally, planting with suitable native species is the only method that we have found works. If the ground is suitable, you can sometimes plant Carex, Lomandra, Juncus spp where you have removed the Taro, and some riparian tree species like Sandpaper Fig, Waterhousea, etc -- whatever grows naturally in your area that likes occasional or regular inundation.
The leaves of Taro are quite moisture repellent, so foliar spray with herbicide is not effective and runs off. Extra surfactant would be dangerous to frogs etc, so we avoid foliar spray altogether.
In recent years we had several stretches of low rainfall - by 2006-2007 some sections of our creeks had effectively dried up leaving just occasional pools. This gave us access to Taro plants effectively not in the stream any more, allowing us to work without danger to the waterway. Here, we sliced them across near the base with a knife and dabbed a tiny amount of Glyphosate on the stem, on the green part of the new leaf in the centre of the stem (not the white fleshy part). We use a type of Glyphosate rated safe for use in waterways, however we avoid any herbicides where they might get into the water, so we only use this method when the creek has dried up. The cut sections of the plant are carried off to avoid them leaching their toxins into the stream. The poisoning of the "stump" seems to kill the plant. For this method to be safe, not only does it have to be dry, it also has to be fine for several days - if it rains there could be run off. The herbicide not absorbed by the plant is supposed to break down quickly, but we wouldn't risk this is rain was due any time soon.
As always with bushcare, methods evolve as we find better and safer solutions. Even though the weed problem in our catchment is immense and daunting, we tend to favour low impact methods -- that is, a little bit often rather than a lot all at once. If you come up with some insights and have some success, please let us know.
A final word
When considering removal of Wild Taro, you should probably contact your local government authority and other environmental authorities as they may have advice and rules on what can and can't be done in waterways.
Good luck and happy weeding.