The red invasive plant species taking over Johannesburg's waterways

The South African

31-07-2025

Red fern, a fast-spreading inasive weed, is beginning to take hold in Johannesburg's waterways.
Just weeks after a costly cleanup to remove Kariba weed, the arrival of red fern at Lonehill Dam has raised new concerns about the long-term health of the city's dams and rivers.
In South Africa, red fern is classified as a Category 1b weed under the NEMBA Act. This means it must be eradicated or controlled wherever it appears.
Kariba weed, or Salvinia molesta , is a floating aquatic fern originally from South America.
It forms thick mats on water surfaces, blocking sunlight, depleting oxygen, and killing fish and native plants.
After a nine-week course of getting the invasive plant under control at Lonehill Dam, a new threat has emerged.
The Kariba weed cleanup cost Joburg City Parks and Zoo (JCPZ) R300 000. It involved 20 days of hired equipment with an operator and fuel, as well as the use of eco granules to combat the infestation.
'The Kariba weed was pretty much under control,' Ward 94 councillor David Foley told the Fourways Review .
'But we've got a new breed of weed, called red fern, that has decided to show its face here.'
Last year The South African reported on how a local community group had tackled a water hyacinth problem at the dams inside Johannesburg's Delta Park.
Hyacinth has also been an issue at Hartbeespoort Dam, while the Vaal River suffered from an infestation of water lettuce (Nile cabbage).
These invasive aquatic plants generally thrive in polluted, nutrient-rich waters.
Red fern or Azolla filiculoides is a free-floating aquatic fern native to tropical South America.
It is considered an invasive species. Much like the Kariba weed, it spreads rapidly across water surfaces and poses a threat to local ecosystems.
Red fern spreads via spores and detached plant segments. According to researchers, the weed was first brought into the country as an aquarium or ornamental plant, and then discarded into natural waterways.
Left unchecked, red fern can also reduce water quality and increase the risk of flooding by clogging up channels and stormwater systems.
To prevent the plant from taking over at Lonehill Dam, Foley is working with Ian Ross from International Metal Processing to introduce a rim flow device.
The floating mechanism would skim the dam's surface, cutting away the red fern and depositing it along the banks for removal.
Foley noted that the proposal still requires approval from JCPZ.
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