Crassula helmsii | New Zealand Pigmyweed



Conservation status

Not Assessed

Legal status

Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011. (Note: Regulation 50 not yet enacted). Listed as a schedule 9 species under Articles 15 & 15A of the Wildlife Order (Northern Ireland) 1985 (Article 15A not yet enacted).

First reported in the wild



Invasive species - risk of High Impact

Irish status


Introduction pathways - 1

Release in Nature

Introduction pathways subclass - 1

Ornamental purpose

Invasive score


NAPRA Ireland risk assessed


Species Biology


"Crassula helmsii is a small succulent flowering perennial that grows rapidly to form an extensive lush-green ‘carpet’ that floats on freshwater or may be submerged. Growth can extend from margins of sheltered waterbodies to completely cover the water surface with tangles of stems and shoots. The stems are rather rigisd and round with leaves up to 2cm long and arranged on the stem in opposite pairs. The base of the leaves are joined around the stem to form a c'collar'. The leaves are fleshy when emergent and less fleshy and flat when permanently submerged. It has very small white flowers with four petals that are produced in summer on long stalks arising from the upper leaf axils. Petals are slightly longer than the sepals, and the flowers are always above water. Fruits are follicles containing 2–5 elliptical and smooth seeds about 0.5 mm long" (Minchin, 2009).


Crassula helmsii forms dense mats that smother other vegetation and may cause changes to the pelagic communities in lakes and it is extremely costly to control (Minchin, 2009; Stokes et al., 2009). However there are few empirical studies on the exact effects of Crassula helmsii on lotic communities. It has been shown to have significant effects on germination rates of pond plants, supressing them by up to 83%, though the seed banks appeared to be unaffected (Langdon et al., 2004) suggesting if removal was successful regenerations of plant communities could take place.


Inland surface waters


Can fully regenerate from stem and leaf fragments attached to a single within 3 weeks (Hussner, 2009), which allows the plant to spread rapidly by vegetative means.

Pathway and vector description

Imported for trade in Ireland and sold as a pond plant and ornamental (Reynolds, 2002). Further distributed in Ireland through movement of leisure craft and angling equipment.

Mechanism of impact

Competition, Bio-fouling, Other

Broad environment


Habitat description

Submerged or emergent aquatic plant, grows in lakes or slow moving water.

Species group


Native region



World distribution(GBIF)

Irish distribution

Established - Widespread, locally abundant (Minchin, 2007).

Native distribution

Native to Australia, New Zealand and Tasmania (Booy et al., 2015).

Temporal change

Date of first record category


Fifty year date category


Records submitted to Data Centre in 2023

The following map is interactive. If you would prefer to view it full screen then click here.

How can you help

Report any sightings to the National Biodiversity Data Centre. Obey good biosecurity practices: Check and clean leisure craft when moving from areas containing Crassula helmsii to uninfected river systems; disinfect, and when possible allow to dry, fishing/angling equipment; disinfect boats when moving from one river catchment to another or upstream within river systems.

Further information

Delivering Alien Invasive Species In Europe (DAISIE) project Crassula helmsii this as one of the 100 Worst Invaders in Europe.



Booy, O., Wade, M., & Roy, H. (2015). Field guide to Invasive Plants & Animals in Britain. Bloomsbury.

Hussner, A. (2009). Growth and photosynthesis of four invasive aquatic plant species in Europe. Weed Research, 49(5), 506-515.

Minchin, D. (2007) A checklist of alien and cryptogenic aquatic species in Ireland. Aquatic Invasions, 2(4), 341-366.

Minchin D (2009) Crassula helmsii In: Handbook of Alien Species in Europe DAISIE, Drake, J.A. (ed) . Springer Series in Invasion Ecology, Springer.

Reynolds, S.C.P. (2002) A catalogue of alien plants in Ireland. National Botanic Gardens. Glasnevin, Dublin.

Stokes, K., O'Neill, K. & McDonald, R.A. (2004) Invasive species in Ireland. Report to Environment & Heritage Service and National Parks & Wildlife Service. Quercus, Queens University Belfast, Belfast.

Inland Fisheries Ireland

Risk Assessment of Crassula helmsii

CABI Datasheet

DAISIE Factsheet

Global Invasive Species Database