The invasion of floating pennywort (Hydrocotyle ranunculoides L.f.) in the Netherlands, 1996-1998.
The first local invasion of the alien subtropical aquatic H. ranunculoides in Dutch urban waterways during the summer of 1995 was seemingly controlled by a combination of mechanical removal of floating and terrestrial plant material and a severe winter. Recently, however, H. ranunculoides proved to be hardy to even severe frosts when protected by natural vegetation. As a result, a large area of waterways in and around 's-Hertogenbosch became covered with floating mats of H. ranunculoides, which required expensive removal. Despite this treatment, H. ranunculoides escaped to the river Meuse, and is predicted to spread to a former tidal nature reserve from there. H. ranunculoides is expected to cause problems in nature reserves and recreation areas. There are indications that flowering and seed production are stimulated by conditions unfavourable for vegetative growth. It is concluded that H. ranunculoides is very unlikely to be controlled by man.