Not to be semantically confused with : Fools Parsley (Aethusa cynapium) [a plant in the same umbellifer family with similar name]
Easily mistaken for : some growth forms (those which are dwarfish with procumbent stems) look very much like
Creeping Marshwort (Apium repens) but that now only occurs in South Essex, Berkshire and Oxfordshire.
Can be mistaken for Lesser Water-Parsnip (Berula erecta) which also likes to grow in similar damp places but that usually has several (4-7) bracts just beneath the umbel and which are
tri-pointed (whereas in Fool's-Water-cress bracts are usually absent, or with just one, or 2).
Can also be mistaken for Watercress (Rorippa nasturtium-aquaticum) but that is a Brassicaceae and not an umbellifer which tastes differently, has solid stems (rather than the hollow of Fool's-Water-cress) with flowers with only 4 petals, and with totally different fruits.
Hybridizes with :
Creeping Marshwort (Apium repens) to form Apium longipedunculatum (?) which is more procumbent and extensively rooting than Fool's-Water-cress with which it occurs in Cambridgeshire, SE Yorkshire and Fife.
Lesser Marshwort (Apium inundatum) to form Apium moorei which has but 1-3 rays and is sterile but occurs with both parents scattered over most of Ireland and the Outer Hebrides.
Grows in wet streams, ditches, pond-edges, in marshes, beside lakes and rivers. It is native and common in the British Isles.
The essential oil from the aerial parts of the plant may contain both
Dillapiol, the two frequently occurring together because the latter results from enzymatic methoxylation of the former. These should make the leaves poisonous, although there are reports of folk eating it in mistake for Water-cress (Nasturtium officinale) and getting away with it. The quantity of toxins probably depends upon the soil, what time of year it is harvested, the number and type of pathogens it encounters, and on the weather during their growing season.