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

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.

[8] C. helmsii is able to grow fully submerged in a cool-water aquarium or as a submersed or marginal plant in a pond.

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.