Hydrocotyle ranunculoides L.f.: infiltrant in waterland?

The appearance of the alien floating pennywort Hydrocotyle ranunculoides as an invasive weed of an urban waterway in Utrecht (the Netherlands) was the reason to do some research on the possible origin and the distribution of the weed. The plant, propagated in aquatic nurseries, probably escaped as a nuisance weed favoured by two successive abnormally hot summers, with little frost in-between. There are indications that the plant may withstand average Dutch winters. During the autumn of 1995 the species had been recorded on several localities in the Central, Western and Southern part of the Netherlands, one of which in a nature reserve.