New Zealand pigmyweed (Australian swamp-stonecrop) Crassula helmsii

Status Non-native, invasive
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Banned From Sale after April 2014. A non-native invasive plant.

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.

Habitat

Aquatic habitats.

What's the problem?

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.

Rapid Risk Assessment

***** Critical Risk

Plantlife's position

Plantlife campaigned to have this species banned from sale. As of April 2014, in England and Wales, it will be. This species is also listed on Schedule 9 of the Wildlife and Countryside Act in England and Wales therefore, it is also an offence to plant or otherwise cause to grow these species in the wild.

Removing New Zealand pigmyweed

Early and regular treatment is highly recommended. In small ponds, regular digging out of the plant may be an effective control technique, but care needs to be taken as small fragments can disperse and regenerate. New Zealand pigmyweed does seem to be less vigorous in ponds with plants that provide some shade, like the native bogbean Menyanthes trifoliata for example. On the edges of ponds, covering infestations with black polythene for at least three months during the growing season can also be effective, although this will have adverse effects on other species covered up.