The invasive Mexican water lily has taken root in Constantiaberg waters and residents have been warned not to be tempted by the plant’s pretty blooms.
“When it’s flowering it looks quite attractive, and people
might want to pick it and put it in their garden pond, but don’t do
that because it will spread,” said
Philippa Huntly, the project manager at the Wildlife and Environment Society of South Africa (Wessa).
Chandre Rhoda, of the City’s invasive species management unit, said the Nympheae mexicana is also called the yellow water lily. It was first recorded in the Westlake and Keysers rivers and has since been spotted in Maynard Park, Wynberg Park and Yellowood Dam, in Somerset West.
Ms Rhoda said the weed was very difficult to eradicate and was uprooted by hand in shallow waters. The unit was, however, conducting trials to find a suitable herbicide to fight it.
The plant posed a flood risk, she said, because it clogged up waterways.
“Controlling the yellow water lily in the Sand River catchment is very difficult, since we are not allowed to work in the river systems during the endangered western leopard toad season.”
John Fowkes, the Westlake Wetland’s project co-ordinator, compiled a report on the weed for the Zandvlei Trust.
According to the report, there is “no obvious source of the plant”.
It says the lily was first spotted in the Westlake and Keysers rivers in 2008. Extensive growth in the Keysers River was noted near the sewerage conduit in 2010. By 2016, the plants were seen along the full length of the Westlake River, from the Wessa offices to the edge of Zandvlei, and along the Keysers River, from Military Road to Zandvlei.
The weed’s creeping runners on the river bed make it hard to control. If the total root body isn’t removed when it’s pulled out, it carries on growing.
“Because it is bottom-rooted, it is not flushed out of the system during heavy rains,” the report says, describing the weed’s potential threat as “severe”.
Large mats of the weed already cover a big part of the Westlake River, blocking light and leaching oxygen from the water, causing anoxic conditions which could be fatal to fish.
“The covered surface also has an impact on any bird life which feeds on the fish in the rivers,” the report says.
Removed “mats” regrow quickly, and the report urges the trust to flag the threat with the City.
“The major concern is that the plant, if left to spread, will move into other fresh water bodies, like farm dams and irrigation systems, and cause severe economic damage or cost.”
Mayoral committee member for energy, environmental and spatial planning, Johan van der Merwe, said the City was working with scientists at Stellenbosch University to figure out how the weed spread and how to stop it.
“If we are able to determine the movement between water bodies, measures to prevent movement can be put in place,” Mr Van der Merwe said.
Ms Rhoda urged the public to visit www.capetowninvasives.org.za to report any sightings of the weed.