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Crassula helmsii - New zealand pigmyweed

Synonym: Tillaea recurva, Crassula recurva
French name:  Crassule des étangs
Dutch name: Watercrassula
Family: Crassulaceae
Group: Vascular plants
Origin: Australia
Habitat: freshwater
Introduction:  aquariums and ponds
ISEIA Score : 12
Naturalization in Belgium
First observation in the wild: 1982
Invasion stage: spread
Spatial distribution: isolated
Reproduction in the wild: yes
Dispersion potential: high
Natural habitats: high
More on invasiveness: New Zealand Pygmiweed colonises a wide variety of freshwater habitats with chemistry ranging from acid to alkaline, including water bodies, slow-flowing rivers, marshes and peat bogs. Due to its specific metabolism (Crassulacean Acid Metabolism), it is adapted to live in moderately fertile shallow temporary pools that experience extreme diel fluctuations in carbon availability and is able to take advantage of elevated nighttime CO2 levels. Plant ability to grow from fragments of stem enables it spread from ponds and ornamental pools into the wild. Waterfowl, mammals, downstream movement and flooding play an important role in plant dissemination. Plant fragments may also be moved with mud.
Distribution in Belgium
Established populations
absent from district
isolated populations (1-5 localities per district)
widespread (>5 localities per district)
Endangered areas
low risk
medium risk
high risk

Endangered Natura 2000 habitats ():
freshwater habitats: 3130315032603270
Impacts on Species
Predation / Herbivory: low
Competition: high
Disease transmission: low
Genetic effects: low
Impacts on Ecosystems
Nutrient cycling: likely
Physical alteration: high
Natural successions: high
Food web alteration: low
More on impacts: Swamp stonecrop causes major environmental problems in freshwater habitats. It forms dense vegetation mats that deplete oxygen, outcompete native species and may affect the breeding success and survival of threatened aquatic plants and amphibians. Mats choke ponds and drainage ditches, impede water flow and adversely affect recreation activities.
Data Source & References
Authors: Branquart Etienne, Stiers Iris, Triest Ludwig, Vanderhoeven Sonia, Van Landuyt Wouter, Van Rossum Fabienne, Verloove Filip
Published on:  22 November 2007
Last update:  11 December 2013
Adriaens, T., Packet, J. & Denys, L. (2010)
Bestreiding over watercrassula, een lastige invasieve exoot.
Natuur.Focus 9(3): 128-129.
Booy, O., Wade, P.M. & White, V. (2008)
New Zealand Pygmyweed.
In: Invasive species management for infrastructure managers and the construction industry. CIRIA C679, CIRIA, London.
Dawson F.H. & Warman E.A. (1987)
Crassula helmsii (Kirk) Cockayne : is it an agressive alien aquatic plant in Britain?
Biological conservation 42: 247-272.
Denys, L., Packet, J. & Van Landuyt, W. (2004)
Neofyten in het Vlaamse water : signalement van vaste waarden en rijzende sterren.
Natuur.focus 3(4): 120128.
EPPO (2007)
Data sheet on quarantine pests: Crassula helmsii.
OEPP/EPPO Bulletin 37: 225-229.
Huckle J. (2002)
Invasive alien aquatic plant species, Crassula helmsii.
Invasive Alien Species Project. Fact Sheet 1. English Nature & University of Liverpool.
Keeley, J.E. (1998)
CAM Photosynthesis in submerged aquatic plants.
The Botanical Review 64(2): 121-175.
Lambinon, J., Delvosalle, L. & Duvigneaud, J. (2004)
Nouvelle fore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des régions voisines.
Editions du Patrimoine du Jardin botanique national de Belgique, Meise.
Langdon S.J. et al. (2004)
Crassula helmsii in UK ponds: effects on plant biodiversity and implications for newt conservation.
Weed Technology 18: 1349-1352.
Slembrouck J. & Molenaar E. (2001)
Crassula helmsii (Kirk) Cockayne, een nieuwe bedreiging voor onze waterflora?
Dumortiera 78: 20-23.
Van Landuyt, W., Hoste, I., Vanhecke, L., Van den Bremt, P. Vercruysse, W. & De Beer, D. (2006)
Atlas van de Flora van Vlaanderen en het Brussels gewest.
Nationale Plantentuin en het Instituut voor Natuur- en Bosonderzoek i.s.m. Flo.Wer vzw.
Verloove, F. (2006)
Catalogue of the Neophytes in Belgium (1800-2005).
Scripta Botanica Belgica 39, 89 pp.
Watson W.R.C. (1999)
Amphibians and Crassula helmsii.
Froglog 31.
Weber, E. & Gut, D. (2004)
Assessing the risk of potentially invasive plant species in central Europe.
Journal for Nature Conservation12: 171-179.

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