Johannesburg is facing a new threat in its waterways — a fast-spreading aquatic weed known as red fern.
What’s happening?
Just weeks after clearing out the notorious Kariba weed, officials at Lonehill Dam in Johannesburg are fighting another invader — red fern, according to The South African. The sudden arrival of this floating plant has raised alarms about the health of the city’s dams and rivers.
“We’ve got a new breed of weed, called red fern, that has decided to show its face here,” said Ward 94 councillor David Foley.
Red water fern (Azolla filiculoides), classified as a Category 1b invasive species in South Africa, must legally be removed or controlled wherever it appears. Originally from tropical South America, it was introduced as an ornamental plant. But in local waterways, it spreads unchecked — thriving in conditions where native species struggle to compete.
Why is this plant concerning?
On the surface, a floating fern might not sound like a serious problem — until it starts affecting people’s lives. When invasive plants like red fern take over, they threaten clean water access, raise the risk of flooding, and drive up the costs of maintaining dams and stormwater systems. Local food chains also take a hit, as the plants and fish that communities depend on begin to disappear.
That’s because red fern mats grow so thick they block sunlight from reaching the underwater plants and drain oxygen from the water — suffocating fish and killing off native vegetation. The plant spreads fast through spores and fragments, quickly clogging water channels and overwhelming local waterways.
What’s being done about it?
Foley is collaborating with International Processing to deploy a rim-flow device — a floating skimmer — to cut and remove the red fern from the dam’s surface.
Similar restoration efforts have worked elsewhere. In Topeka, Kansas, the group Friends of Kaw led a two-year project to replace invasive plants along the Kansas River’s banks with deep-rooted native species. Their efforts helped stabilize soil, filter water, and reduce flooding risks for nearby communities.
Protecting native species remains one of the best defenses against invasives. Choosing native plants in gardens, supporting conservation efforts, and backing policies that prioritize native biodiversity all play a role in keeping waterways — and communities — resilient.
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