Crassula helmsii, known as swamp stonecrop or New Zealand pigmyweed,[1] is an aquatic or semiterrestrial species of succulent plant in the family Crassulaceae.

[4] The shoots are rather stiff, carrying narrow parallel-sided leaves in opposite pairs, each leaf being about 4–24 millimetres (0.16–0.94 in).

Small white flowers with four petals are produced in summer on long stalks arising from the upper leaf axils.

[6] The plant grows on the muddy margins of ponds where it forms carpets with 100% cover, semi-submerged in deeper water, or totally submerged with elongated stems.

Schedule 9 of the UK Wildlife and Countryside Act 1981 lists this plant as one that must not be caused to grow in the wild.