Clearing tropical aquatic plants out of the Netherlands’ ditches and canals is costing water boards up to €3m a year, broadcaster Nos says on Monday.

Tropical plants are sold for garden ponds but many end up being dumped in the country’s waterways, where they grow extremely well, the broadcaster says. The mild winter and warm summer have only exacerbated the problem, a water board spokesman said.

In particular, water pennywort (Hydrocotyle ranunculoides) is such a pest that it can no longer be legally sold in the Netherlands.

A list is now being drawn up of other aquatic plants which are causing problems such as hydrilla (Hydrilla verticillata) and parrot feather watermilfoil (Myriophyllum aquaticum).

Aquatic plant breeders acknowledge the problem and hold regular talks with government officials, Nos says. They also make sure all plants carry a label warning buyers not to dump them in nature.