Crassula helmsii, known as swamp stonecrop or New Zealand pigmyweed, is an aquatic or semiterrestrial species of succulent plant in the family Crassulaceae. Originally found in Australia and New Zealand, it has been introduced around the world.
This plant is very widely available from nurseries and garden centres, being sold as an excellent oxygenator. In slow-moving water such as ponds, lakes and canals it forms a dense, tangled growth of stems underwater, sometimes billowing up in cushions on the water surface. It is often mis-labelled as Tillaea recurva, Tillaea helmsii or Crassula recurva. Around the edges in shallow water it forms dense, impenetrable mats, and it can even grow in the open on damp mud around seasonal pools. In garden ponds it destroys all other life and is frequently then thrown out. Just a tiny fragment of the stem can regrow and multiply into a dense mat of vegetation. It is spreading rapidly across the country and is almost impossible to eradicate. The best method of control is to fill in the infected pond and excavate a new one nearby; control in lakes and canals is nearly impossible.