Invasive alien species

Invasive exotic species: Giant water pennywort, Japanese knotweed, Water crassula

Invasive alien species

Exotic species are plants or animals that do not occur naturally in our country, but have been introduced into the Dutch environment through human activity. Due to the absence of natural predators, they can displace native species and become a pest. These are referred to as invasive exotic species.

Problems caused by exotic species

There are various types of invasive alien species in our management area that cause problems for the tasks and objectives of the water board.

Watercourses can become blocked by explosive growth of non-native aquatic plants, which can cause problems with water supply and drainage (example: giant water pennywort, water hyacinth).
Dikes and quays can be affected by the growth of non-native plants (e.g., Japanese knotweed, giant hogweed).
Non-native aquatic plants and animals can also be harmful to water quality because they displace or damage native species (example: water crassula, American crayfish).

What does HHNK do?

We have specially trained employees who remove invasive exotic plants. Removal must be done very carefully because any remnants left behind can grow again. By taking timely action, we reduce the risk of spread and high control costs. Known locations are checked regularly.

We are working to create a healthy water system in which native species can thrive. This reduces the likelihood of invasive species becoming dominant.

Our own employees keep track of locations where invasive alien species have been found in waterways or on our flood defenses. We also use information about the presence of invasive alien species from the National Database of Flora and Fauna (NDFF).

We talk to other owners and residents living near waterways about their responsibility to prevent problems. Where possible, we work together with other landowners to combat or control invasive species.

In the Netherlands, water boards are legally obliged to protect dikes and banks against damage caused by muskrats and coypu. HHNK has outsourced the control of these animals to Muskusrattenbeheer West- en Midden Nederland (Muskrat Management West and Central Netherlands). See the page Reporting muskrats.

Preferred strategy for species

We have established a preferred strategy for the exotic species found in our management area. A number of land plants and aquatic and riparian plants are actively controlled. These are species that cause significant nuisance (e.g., Japanese knotweed, giant hogweed, pearlweed) or species that are still limited in our area and can therefore be completely eradicated (e.g., lesser water primrose, water hyacinth, water fan). In addition, there are species that are fairly common and for which efforts are being made to manage their extent (e.g., Azolla or narrow-leaved water milfoil). Non-native aquatic animals, such as exotic crayfish, Chinese mitten crabs, or exotic gobies, are not currently being actively controlled. There is no effective control method available for these species.

What can you do?

Report

Have you spotted invasive alien species in or near the water? Please report them via Waarneming.nl. You can also use the NDFF Input App (ObsMapp / iObs or ObsIdentify) or the Exoten app (ndff.nl). Please include a clear photo with your report so that the species can be easily identified. This will help to map the spread of invasive alien species. Your report will be included in the NDFF's national database.

Prevention

Do not release plants or animals from your garden, pond, or aquarium into the wild. Dispose of your surplus (aquatic) plants in the green waste container and choose native species when purchasing plants.

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Invasive alien species