It can occur in quite substantial stands.

Alisma plantago-aquatica NS8761 13May2018 PW

It usually is in free still water at the edges of lakes or ponds.

Alisma plantago-aquatica 7June2017 NS7171 MP

Note the range of shapes of bases, but that there is always a clear angle between the base and the stem, rather than it emerging from a widening petiole.

Alisma plantago-aquatica NS7066 07Jul2018 PW

The flower only opens after midday.

Alisma plantago-aquatica NS7066 07Jul2018 PW

Over-wintering and last season’s flower spikes are distinctive.

Alisma plantago-aquatica NS6373 20Apr2019 PW

Alisma lanceolatum NS5768 10Aug2019 PW

The leaf blade is clearly tapering into the petiole.

Alisma lanceolatum NS5768 10Aug2019 PW

Alisma species - Water-Plantain

These two similar plants grow in mud, whether in or beside water. The leaves often rise vertically out of water, and the flowering stem has straggly branches with white, or pale pink, 3-petalled flowers.

Alisma plantago-aquatica, Water-plantain, has broad leaves whose base is rounded.

Alisma lanceolatum, Narrow-leaved Water-plantain, has longer, narrow leaves whose base tapers into the petiole.

The flowers of Alisma lanceolatum are said to open in the morning, while those of Alisma plantago-aquatica open in the afternoon: this may to some extent be true, but should not be regarded as definitive.