Water hyacinths at the Hartebeersport dam. (Delwyn Verasamy/M&G)
Water hyacinth has once again exploded across Hartbeespoort Dam, carpeting about 35% of its surface.
This is the visible result of decades of pollution and a massive seed bank that lies buried beneath the water.
“Hartbeespoort Dam has a 50-year accumulation of dormant water hyacinth seeds in the sediment — millions and millions of them,” said Julie Coetzee, the deputy director of the Centre for Biological Control (CBC) at Rhodes University.
“These seeds can remain viable for 15 to 20 years.”
Since 2019, the CBC’s biocontrol strategy of inundating Hartbeespoort Dam with water hyacinth hoppers — a biological control agent and natural enemy of the aquatic superweed — has achieved less than 5% water hyacinth cover every summer.
These minuscule sap-sucking insects feed and lay eggs exclusively on water hyacinth, chronically stressing the plants, preventing flowering and suppressing growth — season after season.
“The reality is that we’re in peak growth season,” Coetzee said of the annual cycle currently underway.
“Every spring, perfect conditions trigger mass germination of water hyacinth. In summer/autumn, biocontrol reduces cover to less than 5%. In spring, seed germination creates 40% and higher cover again.”
“Dropping water levels expose sediment to sunlight, warm water temperatures activate seeds, rainfall washes in nutrients — nitrogen and phosphates — and seedlings establish rapidly before the bugs build up populations.”
The water hyacinth hoppers chronically stress the plants, prevent flowering (no new seeds) and slow growth rates, and only once biological control reduces growth, can physical removal actually get ahead, Coetzee said.
50-year problem playing out
Water hyacinth, native to South America, is described as the world’s worst aquatic weed. It thrives in polluted, nutrient-enriched waters like Hartbeespoort Dam, forming dense, impenetrable mats that affect boating, fishing, and other water-sport activities and harm aquatic biodiversity.
In May 2023, the department of water and sanitation officially appointed Magalies Water for a 36-month period to implement a remediation plan focused on improving raw water quality in the Crocodile West catchment upstream, including Hartbeespoort Dam.
Magalies Water has officially contracted the CBC to scale up its integrated management plan. Since August, over 100 000 hoppers have been released onto Hartbeespoort Dam, including 30 000 from community-rearing stations surrounding the dam.
The CBC has released 10 000 bugs per week, and over the past month has doubled that to 20 000 per week, overwhelming the dam with the tiny plant warriors.
“Since 2019, our biocontrol strategy has kept water hyacinth cover below 5% every summer,” Coetzee said.
“Every spring, seeds germinate again — but we’re not failing. You’re watching a 50-year problem play out in real time. Progress is happening at the microscopic level, as new seeds are prevented from entering the system.”
While Magalies Water’s contractors, Hya Matla Organics, are physically removing the weed using harvesters, excavators and a 30-member team working 22-hour days, Coetzee emphasised that “mechanical removal alone cannot win against the seed bank.”
Magalies Water is also trialling a new oxygen-infused nanobubble system at the dam inlet to target the nitrogen and phosphorus that feed the water hyacinth blooms.
Despite public calls for chemical spraying, Coetzee warned against the use of herbicides.
“There’s been no herbicide operation since 2017 — for good reason,” Coetzee said.
“We know spraying gives rapid cover reduction, but it encourages flowering and adds to the seed bank and worsens the problem for the next year.”
Herbicides trigger toxic blue-green algal blooms by promoting rapid plant decay, which provides sunlight for algal growth.
“We need sustainable solutions, not short-term fixes that make things worse,” she noted.
Instead, the CBC is focused on sustainable control.
“Biocontrol prevents flowering through chronic stress,” she explained. “Each year that we stop new seeds being produced, we move closer to exhausting that 50-year seed bank.
Biological control is the long game that works. “The CBC has been achieving excellent results since 2019 with water hyacinth hoppers. The truth is that this is going to take time.
“The pollution feeding this problem and the massive seed bank aren’t going away overnight. But integrated management through biocontrol, physical removal and nutrient reduction are the only path to lasting control.
“Until we exhaust that 50-year seed bank, we will have germination events every spring. There is no way to remove the seeds they’re buried in sediment across the dam, our only option is to prevent new seeds from biocontrol and starve out the seed bank over time,” she said.
This is a 10 to 15-year marathon, not a sprint. “Each year we prevent flowering, that’s one year closer to depleting the seed bank.”
Hartbeespoort fish kill probed
Meanwhile, Magalies Water confirmed a fish kill on 18 October at the canal of Venice Village Estate near Hartbeespoort Dam, with the affected fish later buried on a nearby vacant plot.
“Following this incident, Magalies Water responded promptly by collecting water samples on 22 October,” the utility said.
A joint site inspection with the department of water and sanitation and Madibeng Local Municipality found no visible signs or water-quality abnormalities to explain the cause.
“As the incident was reported three days after it occurred and water quality conditions can change rapidly, we urge the community to report fish kills immediately… with visual evidence and the exact location,” Magalies Water said.
Regular water sampling since the incident has revealed no further abnormalities. The utility acknowledged that the current surge of water hyacinth and salvinia has reduced oxygen levels in the dam — “which can also result in the death of other aquatic species, such as the fish found in the dam.”
It added that high pollutant loads entering the dam continue to fuel the spread of invasive plants, creating “other challenges in the preferred conditions of the raw water in the catchment and dam”.
“Work continues despite these challenges,” it said, adding that the water department and other stakeholders are being regularly updated on interventions and mitigation measures to slow the spread of the invasive plants.