Zandvlei Trust

Mexican Water lilies in the Keysers and Westlake Rivers
at Zandvlei 
by John Fowkes,  Zandvlei Trust Westlake Wetlands Project Co-ordinator  - August 2016.

photograph by John Fowkes.

Mexican water lily in the Keysers River at Zandvlei.

The Mexican water lily (Nymphaea mexicana) originates in Mexico and the south-eastern United States. It causes dense infestation of water bodies. This restricts water movement, contributes to siltation and decreases the economic utility, recreational and environmental values of water bodies. (Journal of Aquatic Plant Management No. 51 2013 pps 69 – 76.)

The Mexican water lily was first seen in the Westlake and Keyser’s Rivers in 2008. The significance of this sighting was not recognised at the time. Plants were seen concurrently in the Westlake River near the Main Road Bridge and the Keyser’s River near the sewerage conduit. They may have occurred upstream from these sites but no upstream observations were being conducted by this survey team. There was no obvious source of the plant or why they should have appeared concurrently in the two rivers well away from each sighting.

In 2010 a note was made of “extensive growth” in the Keyser’s River near the sewerage conduit. By 2016 plants were observed along the full length of the Westlake River from near the WESSA offices to the border of the Zandvlei main water body and along the Keyser’s River from Military Road to the Zandvlei.

The majority of invasive water weeds experienced in the Cape Town City area are floating plants. This means that they are relatively easily scooped up or are flushed out of the system during winter rains. In the Westlake Wetlands this flushing carries the plants into the saline water of the Zandvlei, where they do not survive salinity or they go out to sea. The major invasive water plants are also susceptible to biocontrol agents, which have been used very successfully throughout the City.

Unlike other invasive water weeds the Mexican water lily is bottom rooted. It has rhizome and creeping, fleshy stolons which spread through the bed of the water body. (see Note at end). This means that when a plant is pulled out unless the total root body is removed the plant will grow again. Because it is bottom rooted it is not flushed out of the system during heavy rains. There is also a reluctance to attempt biocontrol of the plant as any agent that attacks the invasive water lily may also attack indigenous water lilies. This means that historically successful means of removing invasive floating water plants in the City will not work with the Mexican water lily.

The potential threat of the Mexican water lily is severe. In the Westlake River there have already been large mats that have formed which cover a substantial area of the water surface. This cuts out light and the air/oxygen interface that occurs at the surface. The rotting plants also consume oxygen below the mat and can lead to anorexic conditions which would be fatal to fish. The covered surface also has an impact on any bird life which feeds on the fish in the rivers. Whilst these mats have been removed manually there is rapid regrowth.

In Australia the Mexican water lily has become a major problem. Chemical removal of the Mexican water lily has been successfully accomplished through the use of glyphosate spraying. Research suggests that this needs to be an ongoing process with two sprays per annum to achieve meaningful results. (Journal of Aquatic Plant Management No. 51 2013 pps 69 – 76).

The threat of the Mexican water lily is probably more severe than the presence of water hyacinth (Eichhornia crassipes). Like the hyacinth it covers the water surface in a solid mat. Unlike the hyacinth it is not easily harvested and cannot be removed through the use of biocontrol agents.

It is a Category 1b plant in terms of our legislation and the responsibility to control the species rests with the authorities in such water bodies (perscom Dr Guy Preston).

Prof Martin Hill, of the Rhodes university, reports “this species (is) becoming more and more invasive, especially in the Western Cape”. (Perscom Prof Martin Hill.

It is strongly recommended that the ZPAAC draws the attention of the Cape Town City Council to the potential threat of the Mexican water lily and requests a concerted programme aimed at its removal from local water bodies.

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. It is also noted that rhizomes, tubers and seeds are carried by water, and fragments can be spread by boats, fishing gear or machinery, or by planting.

At first glance rhizomes are like underground stolons, but there's an important difference between them: Each stolon is just one of what may be several stems radiating from the plant's center. Rhizomes, in contrast, are the main stem. If a tree grew with its trunk horizontal below the ground, with its side branches emerging aboveground, the buried trunk would be a rhizome. The thick, fleshy "roots" of irises, cannas, and water lilies are actually rhizomes.


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