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New Zealand pygmy weed

What action is going on for this species?

The following will give you information on what work has been taking place through the Species Action Framework:

Species background

New Zealand pygmyweed (Crassula helmsii), also known as Australian swamp stoneweed, is a small aquatic plant of variable appearance with inconspicuous white flowers.

Why is this on the Species Action List?

This species meets criterion 2 of the Species Action Framework as an invasive non-native species which presents a great risk to biodiversity.

A native of Australia and New Zealand, its distribution in Scotland appears to be, as yet, relatively limited, but it is widespread and locally abundant in England, where it has spread rapidly, causing smothering and loss of native plant species and associated biota.  Although it is difficult to control, there are management options available and there are actions which could be taken to limit further spread.

It was recently added to Schedule 9 of the Wildlife and Countryside Act (1981), as amended, making it an offence to deliberately introduce this species to the wild.

Habitat, distribution and abundance

This perennial herb grows submerged in sheltered waters up to three metres deep or as an emergent on damp ground.  It can form dense, virtually pure stands.  It grows on soft substrates in a variety of lowland habitats, including ponds, lakes, reservoirs, canals and ditches and can tolerate a wide range of water chemistry.

The occurrence of New Zealand pygmyweed in Scotland is presently restricted, though it is under-recorded.  It is scattered mainly in the south of Scotland but with isolated records for Skye, Nairn and Aberdeen.  As this plant is associated with creation of new ponds, its occurrence is often close to centres of population.

History of invasion and expansion

This species was first introduced for sale in England in 1927, but was not recorded outwith water bodies which had been planted-up until 1956.  Since the late 1970s it has spread rapidly north and west. Between 1970 and 1986, it was recorded in 33 10 km squares in Britain and Ireland.  In contrast, between 1987 and 1999 it was recorded in 574 10 km squares.  It is now widespread in England, where it is causing difficulties on several designated sites.

Recently in Scotland it was found in a number of ponds which were planted-up, including those associated with Sustainable Urban Drainage Systems (SUDS).

It is believed that New Zealand pygmyweed has been introduced into Scotland as a result of its availability from plant suppliers such as garden centres and nurseries.  It is sold as a plant suitable for garden ponds, though plant retailers may refer to this species erroneously as Tillaea helmsii or Tillaea recurva.  However, it is likely that it is also spread through contamination of other plants and soil with viable fragments of New Zealand pygmyweed, within these retail outlets.  Once introduced, the species can then be spread by users of the affected water body.

Impacts on other biodiversity and conservation interests

New Zealand pygmyweed is a successful non-native species for a number of reasons:

  • It colonises water bodies of a variety of forms, substrates and water chemistry.
  • Although seeds may be produced, spread occurs through growth of small fragments of the plant, as individual nodes are viable.
  • It has both aquatic and terrestrial growth forms and does not die back in winter.
  • It generally grows vigorously in sites to which it is introduced, rapidly producing high biomass, at the expense of the indigenous plant species; stands may become extremely dense.

As a consequence, adverse effects on aquatic ecosystems may occur through light limitation, oxygen depletion, changes in pH, or a combination of these factors.  In addition, there may be suppression of the biota normally associated with the native flora e.g. species dependent on native plants as a substrate or food source. 

It is recognized as a potential cause of unfavourable condition for a wide range of freshwater habitat and species features on designated sites by the UK conservation agencies, and as a threat to Mesotrophic lakes and Eutrophic standing waters under UKBAP.

The 'Species Lead' at Scottish Natural Heritage   Tel 01786 450362

Last updated on Tuesday 19th November 2013 at 09:20 AM. Click here to comment on this page