Cumbungi or bullrush is a weed that commonly infests Tasmanian dams.
It is considered a pest, but could it be beneficial in some circumstances?
There is a global trend for even historically clean waterbodies to turn green, the result of warmer temperatures, more sunlight, lower water levels and added nutrients.
The colour is caused by blue-green algae, which is surprisingly nasty stuff.
Blue-green algae contamination of drinking water and food has been linked to fatal motor neurone disease and dementia, the suggested cause being a combination of exposure to the neurotoxic amino acid BMAA, along with genetic susceptibility in some individuals.
Might cumbungi be useful?
Cumbungi and silt had built up in my dams so I had them dug out with an excavator.
When cumbungi was present the dam water was clear most of the time, and without cumbungi the water went bright green in summer.
Cumbungi, being thick and tall, shielded much of the water from sunlight, which presumably lessened algal growth.
I expect cumbungi also pulled nutrients from the water, such as those from washed-in animal poo.
There was also life in the cumbungi – frogs and birds liked it. They went away with the cumbungi removal.
Cumbungi is deemed a problem because it “clogs dams, reduces capacity and may pollute water when it dies off in autumn/winter”.
Given that many farm dams only need to retain enough water for stock to drink in summer, reduced capacity from cumbungi may not be an issue.
As a livestock farmer, water quality is more important to me than water quantity, as long as it doesn’t run out.
Cumbungi may use water, but it may also reduce wind and light-driven evaporation.
There is evidence that using blue-green algae contaminated water in agriculture may introduce toxic BMAA into the food supply, something to consider if you are using green water to grow vegetables and feed livestock.
So, given the risks of BMAA exposure, is summer cumbungi growth potentially useful?
I have not noticed water quality problems when the cumbungi dies off in autumn/winter, but that does not mean all dams may be unaffected.
Here’s a few links meanwhile …
Late edit: BMAA from hot waterholes may be to blame in the deaths of dozens of elephants in Africa.