Invasive Kariba Weed Chokes Two Johannesburg Dams, Threatens Local Ecosystem

Two dams in northern Johannesburg have become completely covered by Kariba weed, a fast-growing and invasive aquatic plant that is threatening biodiversity and disrupting water flow.

The weed, scientifically known as Salvinia Molesta, is originally from Brazil and has spread across the Lonehill Dam and Witkoppen Spruit Park. It thrives in still or slow-moving water, where it forms thick mats that block sunlight and oxygen from reaching the water below, endangering native fish and plant life.

Environmental risk and health concerns grow

Johannesburg City Parks has warned that these dense mats not only threaten aquatic ecosystems but also provide breeding grounds for mosquitoes and snails that can carry bilharzia. Researchers have ranked Kariba weed as one of the most dangerous aquatic invaders in the world, second only to water hyacinth.

The weed is classified as a Category 1b invasive species under the National Environmental Management Biodiversity Act, meaning it must be removed and destroyed wherever it is found. Any trading or intentional planting of the species is illegal.

Initial efforts to remove the weed fail

Local councillor David Foley said residents first noticed the weed late last year. They tried to remove it manually, but it quickly grew back even thicker.

“In January, City Parks sent a small team with a rowboat who said they could clear the dam in a few hours. They worked for two weeks and barely made a dent,” Foley said. After two weeks, the project was halted due to budget constraints.

Frustrated residents, through the Lonehill Community Improvement Initiative, have since employed their own workers to continue trying to clean the dam.

Calls for a sustainable long-term solution

Daniel Masemola, director of Water Management and Biodiversity at the City of Johannesburg, said high nutrient levels caused by pollution, particularly from leaking sewers, have worsened the problem. He recommends a dual approach: using a weed harvester to remove the Kariba weed and introducing a natural predator—the water hyacinth planthopper—which feeds on the plant and eventually destroys it.

Masemola added, “If we don’t treat the root causes like sewer leaks and high nutrient levels, this problem will keep coming back.”

Meanwhile, Foley says he’s working with Joburg Water to ensure sewer leaks into the dam are addressed promptly. City Parks had not responded to requests for comment at the time of publication.

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Sourced: Ground Up

Picture: George.gov.za