|Crassula helmsii (Crassulaceae – Australian swamp stonecrop or New Zealand pygmyweed)
Why The EPPO Panel on Invasive Alien Species made a first categorization of aquatic invasive plants and considered that Crassula helmsii should be added to the EPPO Alert List.
Description C. helmsii is an aquatic or semi-terrestrial perennial, with round stems of 10-130 cm length, floating or creeping (with roots forming at the nodes). Leaves are opposite, sessile and succulent (4-20 mm long, 0.7-1.6 mm wide). White or pinkish flowers are borne solitary in the axils of leaves (diameter 3-3.5 mm). Flowers appear in Europe between July and September. Fruits contain 2 to 5 elliptical and smooth seeds (0.5 mm long). In UK, C. helmsii produces flowers but no viable seeds.
Pictures can be viewed on Internet:
Where C. helmsii originates from Australia and New Zealand. It has been introduced intentionally into Europe (as an aquarium plant), and the main problems are so far reported from the British Isles.
EPPO region: Belgium (no details), France (mentioned as a new taxon in a list of plant species present, but no further details could be found), Germany (first reported in the early 1980s, now found locally in Hessen, Mecklenburg-Vorpommern, Niedersachsen, Nordrhein-Westfalen, Rheinland-Pfalz, Schleswig-Holstein), Netherlands (first found in 1995 and 1996 in a nature reserve near Breda), Portugal (its presence has been reported, but apparently not as an invasive), Russia (reported in the Baikal region), Spain (its presence has been reported, but so far not as an invasive), United Kingdom (first found in the 1950s in Greensted Pond, Essex and it then spread ; now present over 650 sites in the British Isles from sea level to 278 m, including Alderney (first noted in 1986), Guernsey (1989) and Northern Ireland (1984, in a pool at Gosford)).
North America: USA (South-eastern part). In some States, C. helmsii is subjected to regulations.
Oceania: Australia, New Zealand.
Habitat Wetlands, slow-flowing or standing freshwater (e.g. ponds, lakes, reservoirs, canals, ditches). It can grow in a variety of different aquatic habitats (acid to alkaline waters, even in semi-saline sites). It can grow on damp ground and in water down to depths of 3 m.
In its native range, it can stand a wide range of climatic variations: mean temperatures from 30°C in summer to –6°C in winter, precipitation levels from 0.1-0.55 m in summer (November-April) to 0.2 – 3 m in winter (May-October).
Damage Vegetative growth leads to dense mats that outcompete the native flora. C. helmsii presents vigorous growth through most of the year, without any period of die-back in winter. It can block ponds and drainage ditches. It impoverishes the ecosystem for invertebrates and fish. The mats can be dangerous to pets, livestock and children who mistake them for dry land.
Dispersal Local dispersal is mainly ensured by vegetative reproduction. Small fragments (as small as a single node on 10 mm of stem) can produce new plants. These small fragments are readily transported with water, mud, or by wildlife to new sites. In addition, asexual reproduction is achieved via the production in autumn (in UK) of short shoots with very short internodes, known as turions. These are produced apically, and float or are blown around the water surface. These turions appear to be very effective at colonizing new areas. At least in UK, C. helmsii produces flowers but no viable seeds. So apparently in Europe, sexual reproduction does not play a role in plant multiplication and dissemination. Over long distances, trade of C. helmsii can obviously disseminate this species.
Pathway Plants for planting of C. helmsii (soil/water containing viable plant fragments or seeds?).
Possible risks Control is very difficult (mechanical control should be avoided as it produces more fragments which are able to disseminate the plant, herbicides (e.g. diquat) are available but their use in the natural environment might be difficult, the use of dark shading material is reported as a possibility in certain circumstances). At least in UK, C. helmsii has shown a high potential for invasiveness. Further spread of this species should be avoided.
Source(s) Brouwer, E.; Den Hartog, C. (1996) Crassula helmsii (Kirk) Cockayne, an adventive species on temporarily exposed sandy banks. Gorteria. 22(6), 149-152.
Dawson, F.H.; Caffrey, J.M. (ed.); Barrett, P.R.F. (ed.); Murphy, K.J. (ed.); Wade, P.M. (1996) Crassula helmsii: attempts at elimination using herbicides. Management and ecology of freshwater plants. Proceedings of the 9th international symposium on aquatic weeds, European Weed Research Society, Dublin, Irish Republic, 1994. Hydrobiologia, 340(1/3), 241-245.
Dawson, F.H.; Waal, L.C. de (ed.); Child, L.E. (ed.); Wade P.M. (ed.); Brock, J.H. (1994) Spread of Crassula helmsii in Britain. In: Ecology and management of invasive riverside plants, 1-14. John Wiley & Sons Ltd; Chichester; UK
Dawson, F.H.; Warman, E.A. (1987) Crassula helmsii (T. Kirk) Cockayne: is it an aggressive alien aquatic plant in Britain? Biological-Conservation. 1987, 42(4), 247-272.
Weber, E. (2003) Invasive plant species of the world. A reference guide to environmental weeds. CABI Publishing, UK, 548 pp.
Alien Plants Ecology in Spain. Plant invaders in Spain (check-list). ‘The unwanted citizens’ by Dana, E.D.; Sanz-Elorza, M.; Sobrino, E. http://www.ual.es/personal/edana/alienplants/
Centre for Ecology and Hydrology Dorset, UK. Crassula helmsii. Focus on control – an update. http://dorset.ceh.ac.uk/River_Ecology/Botany_Research/Botany_Research_Pictures/Crassula_Helmsii.pdf
Flora of Northern Ireland. http://www.habitas.org.uk/flora/species.asp?item=4639
German Centre for Documentation and Information in Agriculture (ZADI). A map showing the distribution of C. helmsii in Germany.
INRA Web Site. Index synonymique de la flore de France par Michel Kerguélen. http://www.inra.fr/flore-france/index.htm
Invasive Non-Native Species in the UK. University of Liverpool. The Invasive Alien Species Project. Fact Sheet: 1. Invasive Alien Aquatic Plant Species. Crassula helmsii (Kirk) Cockayne, Australian Swamp Stonecrop by Dr Jon Huckle, 19th February 2002.
Invasive weeds. Crassula helmsii: an unwelcome invader by Jason Leach and Hugh Dawson. http://www.btinternet.com/~shsol/invasiveweeds/crassula/BRITWILD.DOC
EPPO RS 2004/042
Panel review date 2006-03 Entry date 2004-04