Found on riversides, ditches and streams, this is a low-growing or prostrate perennial which has leaves vaguely similar to those of Watercress, although the latter's leaves are a darker shade of green. Fool's-water-cress has short-stalked umbels of very small white 5-petalled flowers which are opposite the leaves. These bloom in July and August. The leaves are pinnate having toothed, oval to lanceolate leaflets. The plant roots at nodes in its lower stems and the upper stems which are lightly ridged, are hollow. This is a native plant and it belongs to the Apiaceae family.
I found this plant growing on some small islands in the Caher River, Co Clare in July 2009 when I photographed it. One of the photographs depicts it totally submerged after very heavy rain had caused the river to flood.
If you are satisfied you have correctly identified this plant, please submit your sighting to the National Biodiversity Data Centre