Control of Invasive Exotic Aquatic Plants in Weerribben-Wieden (LIFE-IAAS)

 Invasive alien species are plants and animals that do not naturally occur in the Netherlands but have arrived here due to human actions. They may have been introduced through horticultural sales, aquarium releases, or unintentionally transported via vehicles, aircraft, or boats. While not all exotic species cause problems in their new environments, some thrive to the extent that they outcompete native species. These invasive exotic species can cause significant harm to the indigenous plants and animals within the ecosystems they invade.

Cabomba caroliniana and Myriophyllum heterophyllum are examples of invasive aquatic plants that have recently appeared in the Weerribben-Wieden. Originally from America, these plants are often sold in the Netherlands as popular aquarium species. When aquarium contents are dumped into local waterways, these species find themselves in a completely different ecosystem, approximately 7,500 km from their natural habitat, with no natural predators.

The invasive aquatic plants, Cabomba caroliniana and Myriophyllum heterophyllum, are rapidly spreading throughout the Weerribben-Wieden. This leads to thick overgrowth in the water, making it difficult for native plants and animals to survive. This also threatens the existence of vulnerable species in the protected Natura 2000 area.
The impact of these invasive species extends beyond the natural environment. We are observing that increasingly larger areas of water are becoming completely overgrown, which affects recreational activities such as boating, fishing, and swimming which leads to economic damage to the region. It becomes difficult to boat in the region, as the invasive aquatic plant becomes entangled in propellers. Many fish species disappear when there is insufficient swimming space available. Additionally, swimming becomes unpleasant for people. Furthermore, an increase in Cabomba caroliniana can result in the rise of freshwater snails, which carry larvae. When these come into contact with a swimmer's skin, they can cause irritation, commonly known as swimmer's itch. Moreover, the blockage of waterways can create issues with water supply and drainage, particularly during heavy rainfall.

The Province of Overijssel and it’s partners are working to protect and preserve the unique natural values in the Weerribben-Wieden. To remove Cabomba caroliniana and Myriophyllum heterophyllum from this area, approximately 7 million euros will be needed for the next five years. The European LIFE subsidy will contribute about 4.3 million euros, which accounts for 60% of the total project costs.
How we  propose to manage the invasive aquatic plants in the area:

1. Cabomba caroliniana and Myriophyllum heterophyllum will be removed from the area both mechanically and manually. This approach aims to combat the population of exotic aquatic plants and ensure that the protected native aquatic plants do not continue to decline. The goal is to reduce the coverage of invasive alien aquatic species (IAAS) to a maximum of 2%. After achieving this target, regular maintenance will involve only manual removal from the remaining small growth areas
2. The mechanical removal of invasive exotic aquatic plants is a significant undertaking. Therefore, we are also conducting pilot projects in peat pits to investigate whether we can manage the invasive exotic plants through alternative methods while effectively protecting the native ecosystem.
3. We shall raise awareness among the residents and the tourism sector about this problem and encourage contributions to its solution. We will inform residents and tourists about measures they can take to prevent the spread of these exotic species.

Myriophyllum heterophyllum has finely divided, feathery leaves that resemble feathers. This species is particularly common in the Wieden area, especially around Belt-Schutsloot. It is highly competitive and grows very quickly, causing entire bodies of water to become overgrown. It is also an evergreen plant, meaning it does not die off in the winter. In spring, when many other plants are still growing, Myriophyllum heterophyllum can already be abundantly present. The thick layer of plants prevents light from reaching the bottom and depletes oxygen in the water. Light and oxygen are essential for many other plants, fish, mammals, and amphibians. The lack of oxygen also releases phosphate from the bottom, which Myriophyllum heterophyllum uses as a nutrient. But this harms other species and worsens water quality. This causes many native plants to struggle in these waters, along with all the species dependent on them. This includes dragonflies, fish, and other underwater creatures that rely on the plants that naturally grow there.

Myriophyllum heterophyllum is an aquatic plant with feathery leaves arranged in whorls. The plant can grow 3-15 cm above the water surface. The bracts of the inflorescence are toothed and much longer than the flowers. The flowering period is from June to September. Myriophyllum heterophyllum remains green in winter.

Cabomba caroliniana grows in and around the Weerribben area. With its fine, feathery leaves, Cabomba caroliniana may seem harmless, but appearances can be deceiving. Like Myriophyllum heterophyllum, Cabomba caroliniana forms dense mats that suffocate other aquatic plants. Additionally, Cabomba caroliniana breaks up quickly into pieces. These pieces are then carried by the current or attach to boats, allowing the plant to spread rapidly over a large area.

Cabomba caroliniana grows underwater. The stems range in length from a few tens of centimeters to several meters. The long, unbranched stems grow up to the water’s surface, with only the flowers sticking out above the water. These flowers are white with a yellow center. The blooming period is usually late summer. Due to the current, you often see the stems floating in the water.

As a boat owner, it is important to keep the propellers clean. If water plants get tangled in them and the boat moves to a new area, these plants can break into pieces and start growing in a new location.
Residents can help by not throwing aquarium plants or other garden waste into the water. You can also assist by removing aquatic plants from the water behind your house. In the video on this page, you can see how to do this without leaving any plant remnants in the water.

Invasive species can generally be placed in the green waste container (GFT). For some species, you should only do this before they have set seeds. These seeds can survive the composting process and later grow back as plants. Therefore, parts with seeds should be placed in the container with residual waste, where they will be burned.
An exception to this rule is Fallopia Japonica. This plant should NEVER go in the GFT and must ALWAYS go into the gray container for residual waste, which will be burned.

In this video, you can see how to remove invasive aquatic plants from the water: https://youtu.be/u6G69rd2RtA

In April 2025, the Municipality of Steenwijkerland will place seven containers at various locations within the national park. These will be situated at spots where invasive aquatic plants are most prevalent, the so-called hotspots in the area. These locations include the Ossenzijl Visitor Centre, the boat ramp at Koningin Julianaweg, the Blokzijl marina, the Zuidercluft marina in Giethoorn, Haagje Wanneperveen, the Vollenhove harbor, and at Arembergerweg.

Visitors who rent a boat will receive information that helps them recognize these plants. They will also be asked to take any plants they encounter along the way and dispose of them in the designated waste bins on the shore. This is how we all work together to combat these invasive aquatic plants. 
The map below shows where the hotspots are in the area and where the waste bins will be placed.