Most Unwanted: New Zealand pigmyweed
Crassula helmsii
Habitat: Freshwater
Threat: Clogs waterways and negatively impacts on native species and ecosystems
Status: Established
New Zealand Pigmyweed originally came to Ireland and sold as an ‘oxygenating plant’ for garden ponds and aquaria. This species has sometimes been mislabelled Tillaea recurva or Tillaea helmsii. It was first reported in the wild in Ireland in the 1980’s.
What is it?
This aquatic plant generally grows on the margins of freshwater systems and can tolerate waters up to 3m in depth. It is present all year round.
Crassula in Ireland appears to spread only by vegetative reproduction. This makes management of the species very difficult as small fragments can lead to new infestations.
The shoots are rather stiff, carrying narrow parallel-sided leaves in opposite pairs, each leaf being about 4 - 24mm. Small white flowers with four petals are produced in summer on long stalks arising from the upper leaf axils. The flowers are always above water.
Impact
This species is suited to a wide range of environmental tolerances and is well adapted to Irish conditions. Most slow moving freshwater systems are believed to be susceptible to invasion by C. helmsii. It forms dense carpets of monospecific vegetations and out competes native species choking ponds and ditches.
Where is it from?
Native to New Zealand and Australia.
Where is it in Ireland
Crassula helmsii is known in the wild from a number of sites both the North and South of Ireland. The number of garden ponds with this species is not known.
How did it get here?
It is believed that this plant got to Ireland via trade for garden ponds. It is possible that the species was disposed of or escaped into the wild from private gardens.
Secondary spread by animals and human activity from an infested site to new sites may occur.
What you can do
Don’t buy invasive plants.
Don’t dispose of garden waste in the wild.
Report all sightings.
Additional reading
Habitas Invasive Alien Species in Northern Ireland
