De opmars van Grote waternavel (Hydrocotyle ranunculoides L.f.) in Nederland, 1996—1998

The first local invasion of the alien subtropical aquatic Floating Pennywort (Hydrocotyle ranunculoides) in urban waterways during the hot summer of 1995 seemingly ended due to a combination of mechanical removal of floating and terrestrial plant material and a subsequent severe winter. Recently however the plant proved to be hardy to even severe frost when protected bij natural vegetation. As a result a large area of waterways in and around the city ’s-Hertogenbosch became covered with floating mats of plants which had to be removed at the expense of high costs. This however could not prevent the plant from escaping to the river Meuse, and probably from there to a former tidal nature reserve. Although the species will probably be confined to eutrophic watersystems, it may cause problems in nature reserves and recreation areas. It is very unlikely that it can be eradicated by man. There are indications that flowering and seed production are stimulated by conditions unfavourable for vegetative growth.