Water hyacinths: pretty for some, a nightmare for others

Water hyacinth plant depicting beautiful purple flower. Photo credit (D. Muir - DFFE)

Water hyacinth plant depicting beautiful purple flower. Photo credit (D. Muir - DFFE)

Published Jun 28, 2024


By Debbie Muir

PONTEDERIA crassipes, commonly known as water hyacinth, might be admired for its striking purple flowers, but it is also one of the most aggressive and problematic invasive plants globally.

Originating from the Amazon basin, the species was first recorded in South Africa at the beginning of the 20th century and has since become a significant environmental threat in this country.

Water hyacinths’ rapid growth and reproduction capabilities pose severe challenges to water bodies in South Africa.

Under ideal conditions, water hyacinths can double their biomass every 5 to 10 days under ideal conditions.

Each plant can produce up to 22 daughter plants, which mature, break off, and produce their own daughter plants, creating a vicious cycle. A single inflorescence with 20 flowers produces up to 3 000 seeds depending on the site and time of year.

Seed dispersal is also an important factor to consider as seed germination from seed banks or reservoirs takes only three days on average, with a combination of contributing factors such as water fluctuation, eutrophication and decomposition influencing the dispersal of seeds.

The predominant dispersal strategy used by water hyacinths is via the daughter plant propagation through the formation of stolons.

Water hyacinths’ rapid growth and reproduction capabilities pose severe challenges to water bodies in South Africa. It easily overwhelms small ponds, leading to desperate pond owners removing the plants and improperly disposing of them in nearby rivers or dams, exacerbating the spread.

Water hyacinth covering the Roodeplaat dam, Gauteng. (photo credit: D. Muir - DFFE)

Despite this bleak scenario, there is hope. Biological control agents, specifically insects, mites or even pathogens that feed on water hyacinths, serve as our ecological warriors.

There are eight biocontrol agents known to attack water hyacinths effectively. However, their success is closely tied to water quality; the more eutrophic the water, the longer it takes for these agents to control the invasive plants effectively.

Some biocontrol agents are also limited by environmental conditions such as temperature and seasonality. For example, the mirid (Eccritotarsus catarinensis) does not respond well to excessive cold or heat, so releases of these biocontrol agents should be in coastal areas or areas with sufficient canopy cover, whereas the planthopper (Megamelus scutellaris) is much more adaptable and can withstand colder climates – meaning it can be released inland and in the coastal areas.

This is the reason why M scutellaris was chosen for the testing of the integrated approach at the Roodeplaat Dam in Pretoria.

The biocontrol agent, Megamellus scutellaris, showing adults and nymphs (photo credit: Julie Coetzee)

In September 2020, the department applied a biocontrol with sub-lethal spraying of an aquatic-safe herbicide, such as a glyphosate-based herbicide (GBH) without the carcinogenic co-formulant polyethoxylated tallow amine (POE-T), to the Roodeplaat Dam. Large numbers of the biocontrol agent, M scutellaris, were released over three months. Initially, the water hyacinth infestation covered approximately 60% of the dam.

Roodeplaat dam showing water hyacinth covering about 68% of the surface area – Sentinel 2A

The intervention at Roodeplaat Dam demonstrated the potential of this method. Mass rearing stations were set up around the dam to boost biocontrol agent numbers, facilitating quicker establishment. By the third month, the water hyacinths cover had dropped below 45%.

The biocontrol populations were monitored until the water hyacinths’ mass dropped to below 20% at which stage a sub-lethal strip spray was applied by spraying two strips in the centre of the floating water hyacinth mass, ensuring the herbicide did not harm the biocontrol agents.

Care was also taken to ensure that the swaths that were sprayed were not within 5m of the shoreline to mitigate against riparian contamination and spray drift (riparian contamination occurs when pollutants from nearby land enter rivers, streams, or other water bodies. This can harm aquatic ecosystems, affect water quality, and disrupt the natural habitat of riparian zones).

This approach not only controlled the water hyacinths but also made the plants more palatable for the biocontrol agents, accelerating their feeding and control efforts. Within three months, the water hyacinths were effectively managed without disrupting the aquatic ecosystem.

Water hyacinth cover over the 3 months from August to December (credit: RU-CBC).

Given the success of the integrated approach on Roodeplaat Dam, the department embarked on the process of formalising this approach as the standard policy for controlling floating macrophytes nationally. The integrated approach would then serve as the protocol to be applied on all water bodies, especially dams and rivers battling water lettuce and water hyacinth, amongst others.

The Roodepoort Dam is now “free” of alien invasive plants with 1.99% of water hyacinth biomass (pic: RU-CBC)

The protocol stipulates that any control method used should be the combination of a sub-lethal herbicide spray with a polyethoxylated tallow amine free glyphosate-based herbicides, either aerial or foliar. This ultimately means that the use of a full dose spray of herbicide is no longer necessary or used as this combination is registered under the Fertilizers, Farm Feeds, Stock Remedies and Agricultural Remedies Act, (Act No 36 of 1946) as a registered agricultural application.

And it’s envisioned that this strategy will be deployed on the Vaal River and Dam, which is battling water lettuce (Salvinia molesta) growth. Scientists assigned with dealing with the Vaal River are currently in the process of researching the appropriate biocontrol agent for the water lettuce, in order to be able to calculate the effective sub-lethal dosage to apply in this novel integrated approach.

By embracing an integrated strategy that combines biocontrol and targeted sub-lethal herbicide use, we can protect our water bodies and ensure the health and productivity of our ecosystems. It is a shared responsibility that requires cooperation among various stakeholders, including government, research institutions, and local communities.

During this Environment Month, we call on all stakeholder to work together so we can combat the spread of alien and invasive alien plants and safeguard our natural resources for future generations. Let us unite in our efforts to restore and protect our precious terrestrial and aquatic ecosystems.

Debbie Muir is Biodiversity Officer and Pesticide Risk Manager at the Department of Forestry, Fisheries and the Environment.

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