New Zealand Pigmy Weed (Crassula helmsii)

This plant was first recorded in Ireland in 1984. It has spread very rapidly in watercourses in Britain and has the potential to do likewise here.This plant is very widely available from nurseries and garden centres, being sold as an excellent oxygenator.

Identification

  • A perennial with yellowish-green opposite succulent leaves
  • The tiny flowers are white or pale pink in colour. They comprise four triangular shaped petals.
  • Flowers occur on a long stalk, originating at a leaf join. They are always found above the water’s surface.
  • The plant flowers from July to September.
  • Although seeds are produced, they are not viable.

Issues

  • 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.
  • 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 suitable aquatic habitats the biomass produced is sufficient to eliminate native plants and create poorer conditions for macro-invertebrates and fish
  • Just a tiny fragment of the stem can regrow and multiply into a dense mat of vegetation. 

Treatment

  • 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.
  • 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. 
  • Chemical control – the most effective control worth considering. Crassula is only really susceptible to herbicide formulations containing diquat and glyphosate.