EPPO Global Database

EPPO Reporting Service no. 03 - 2004 Num. article: 2004/042

Addition of invasive plant species to the EPPO Alert List: Crassula helmsii, Hydrocotyle ranunculoides and Solidago nemoralis

As explained above (EPPO RS 2004/040 and 2004/041), the EPPO Panel on Invasive Alien Species has considered that the following plant species should be added to the EPPO Alert List.

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 grown 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.

EPPO RS 2004/042
Panel review date        -        Entry date 2004-04

Hydrocotyle ranunculoides (Apiaceae – floating (marsh) pennywort)
Why: The EPPO Panel on Invasive Alien Species made a first categorization of aquatic invasive plants and considered that Hydrocotyle ranunculoides should be added to the EPPO Alert List.

Description: H. ranunculoides is a stoloniferous, perennial, aquatic plant, with floating and emergent leaves. It is rooting in the shallow margins of slow flowing waters. It has round-reniform leaves (diameter 2-6 cm), with 3-7 lobes (lobe divisions extend to about mid-leaf). Long petioles (5-35 cm) are attached to leaf edge (although they appear to be attached to the centre of the leaf). It has small white, greenish or yellow flowers (umbels) arising from the leaf base (each flower has 5 tiny petals). In North America, it flowers during July and October. Fruit is 1-3 mm long, elliptic to round, and flattened with faint ribs. Many fibrous roots emerge from the stems at the nodes. In Europe, can be confused with the native H. vulgaris.
Pictures can be viewed on Internet:

Where: H. ranunculoides is a native of North America, and has become naturalized in Central and South America.
EPPO region: Netherlands (first found in waterways in summer 1995, it is suspected that it escaped from garden ponds. Since then, its range rapidly expanded, and it is now considered that it will be very difficult to stop its spread), United Kingdom (first recorded as naturalised in the south east in the 1980s. Introduced by the aquatic nursery trade for tropical aquaria and garden ponds, by 2001 it was recorded at 71 sites, mainly in the south of England and south Wales, but it expands in northwest England). It is reported to occur in Portugal and Italy. According to van der Krabben and Rotteveel (2003), it also occurs in Belgium and France. In the Netherlands, it is now prohibited to sell and possess H. ranunculoides.
North America: Canada, USA (Alabama, Arizona, Arkansas, California, Delaware, Florida, Georgia, Illinois, Kansas, Louisiana, Maryland, Mississippi, New Jersey, New York, North Carolina, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, Tennessee, Texas, Virginia, Washington, West Virginia). In some States (Illinois, New Jersey, New York) it is considered as an endangered species.
South and Central America: Argentina, Bolivia , Brazil, Chile, Costa Rica. It is probably present in other countries but data is lacking.
Africa: Zimbabwe.
Oceania: Australia (Western Australia). First found in 1983, and measures are taken to eradicate it and prevent any further spread.

Habitat: H. ranunculoides can be found in slow-flowing water bodies, particularly ditches, dykes, lakes and ponds. Growth is more abundant on eutrophic sites with high organic matter availability.

Damage: H. ranunculoides forms dense interwoven mats of vegetation which can quickly cover the water surface. Plants die back in winter, but shoots and roots survive in the bank and wet ground, and then quickly grow into new plants in spring. In UK conditions, mats of vegetation have been observed to grown up to 15 m from the bank in a single season (so approximately 20 cm per day). Leaf matter extends up to 40 cm above the water surface and the interwoven mat of roots and stems can sink up to 50 cm into the water. H. ranunculoides outcompetes other plants species. Its dense mats of vegetation pose a problem for recreational users and may have a negative impact on the ecosystem (lower light penetration to the water, oxygen shortage, higher risks of flooding).

Dispersal: Local dispersal is ensured by rooting at nodes, root and stem fragments, and seeds. Fragments of plants can be transported with flowing water to new sites. Waterfowl are also able to spread plant fragments. Over long distance, trade of plants for aquaria and garden ponds can disseminate H. ranunculoides.

Pathway: Plants for planting of H. ranunculoides, (soil/water containing viable plant fragments or seeds?).

Possible risks: Control is very difficult. Several methods can be used but none is fully effective. As H. ranunculoides forms very thick mats, it is usually recommended to remove plants mechanically before applying herbicides (diquat, 2,4-D amine, MCPA). Shading with dark material may be a possibility, as well as netting to reduce spread. At least in UK and the Netherlands, H. ranunculoides has shown a high potential for invasiveness. Further spread of this species should be avoided.
EPPO RS 2004/042
Panel review date        -        Entry date 2004-04

Solidago nemoralis (Asteraceae –  Gray goldenrod)
Why: The EPPO Panel on Invasive Alien Species made a first categorization of alien invasive plants. As Solidago nemoralis is currently advertised for landscaping purposes in Europe without any thorough analysis of its potential invasiveness, the Panel felt that it should be added to the EPPO Alert List.

Description: S. nemoralis is a perennial, slender plant (0.3-0.5 m tall). Leaves are alternate. Basal and rosette leaves are long, oval, and tapered at base with serrations on the distal half. Stalk leaves are oval and tapered at both ends. All leaves have distinct midrib and web-like venation. The inflorescence is a cluster of yellow flowers all growing on one side of the stalk. In North America, it flowers in August-September. Achenes then develop with tufts of hair and are dispersed by the wind.
Pictures can be viewed :

North America: Canada, USA (Alabama, Arkansas, Connecticut, Delaware, Illinois, Indiana, Iowa, Kansas, Maine, Maryland, Massachusetts, Michigan, Missouri, New Hampshire, New Jersey, New York, Ohio, Oklahoma, Pennsylvania, Rhode Island, Texas, Vermont, Virginia, West Virginia).

Habitat: Prairies (growing on black soil, gravel, sand), dry and sandy fields, sandy roadsides, railroads, dunes, Quercus velutina savannas, eroded clay banks, abandoned fields.

Damage: In some Western states in USA, it is considered as a troublesome weed. Chemical treatments are mentioned in the literature in apple orchards, pastures, and soybean fields. In the USA, no environmental or social impact is mentioned. In its native range S. nemoralis is not considered as a noxious weed or invasive species. Data is lacking on possible hybridization with other Solidago species. However, S. nemoralis shares some similiarities with other Solidago species which are invasive in Europe (e.g. S. canadensis, S. gigantea). It is reported that, at suitable locations, S. nemoralis has a tendency to form colonies. It survives in difficult locations (slopes, poor soil) where little else will grow. It produces large number of highly viable seeds and vegetative spread is considered as rapid. It can stand low temperatures (-30C°).

Dispersal: Local dispersal is mainly ensured by seeds. Plants can also be spread by rhizomes when soil or plants are disposed of. Over long distances, trade of plants and seeds can disseminate S. nemoralis.

Pathway: Plants for planting, seeds of S. nemoralis (soil with viable rhizomes or seeds?).

Possible risks: It shares characteristics with other Solidago species which are invasive in Europe and in other parts of the world. Control methods are available (herbicides, mowing) but may be difficult to apply in natural or semi-natural environments. This plant is planned to be planted in Europe in public spaces, motorway verges, embankments for landscaping purposes. Considering its high seed production, rapid seed spread and vegetative spread, it could present a risk to the EPPO region as it could outcompete native species including endangered species (e.g. in natural grasslands competing with orchids). However, more data is needed on possible habitats at risk in Europe, and plant biology. Introduction of this species for landscaping purpose should be avoided, native species or alien plants with no history of invasiveness in Europe should be substituted for it.
EPPO RS 2004/042
Panel review date        -        Entry date 2004-04


Crassula helmsii
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. http://gis.zadi.de/scripts/esrimap.dll?name=florkart&cmd=mapflor&app=distflor&ly=gw&taxnr=6731
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

Hydrocotyle ranunculoides
Baas, W.J.; Duistermaat, L.H (1999) The invasion of floating pennywort (Hydrocotyle ranunculoides L.f.) in the Netherlands, 1996-1998. Gorteria, 25(4), 77-82.
Chikwenhere, G.P.; Julien, M.H. (ed.); Hill, M.P. (ed.); Center, T.D. (ed.); Ding-Jianqing (2001) Current strategies for the management of water hyacinth on the Manyame River System in Zimbabwe. Biological and integrated control of water hyacinth: Eichhornia crassipes. Proceedings of the Second Meeting of the Global Working Group for the Biological and Integrated Control of Water Hyacinth, Beijing, China, 9-12 October 2000, 105-108.
Ferreira, A.G.; Souto, C.F.M.; Goidanich, V.; Lipp J.K.; Del Pino, R. (1981) Density, position and dimensions of stomata of plants occurring at Taim Ecological Station, RS, Brazil. Anais do 32 Congresso Nacional de Botanica, Teresina, Brazil, January 1981, 229-232.
Krabben, K.P.M. van der,  Rotteveel, A.J.M. (2003) Draft Report of a pest risk assessment of Hydrocotyle ranunculoides. Unpublished document.
Lallana, V.H.; Barrett, P.R.F. (ed.); Greaves, M.P. (ed.); Murphy, K.J. (ed.); Pieterse, A.H. (ed.); Wade, P.M. (ed.); Wallsten, M. (1990) Dispersal units in aquatic environments of the middle Parana River and its tributary, the Saladillo River. Proceedings of the 8th international symposium on aquatic weeds, Uppsala, Sweden, 13-17 August 1990, 151-159.
Checklist of dicotyledonous species associated with Bolivian wetlands by Dr Nur Ritter, University of Fresno (US). http://www.botanize.com/bol_checklist/dicots.html
Invasive Non-Native Species in the UK. University of Liverpool. The Invasive Alien Species Project. Fact Sheet: 2. Invasive Alien Aquatic Plant Species. Hydrocotyle ranunculoides (L.f.), Floating Pennywort by Dr Jon Huckle, 19th February 2002. http://www.appliedvegetationdynamics.co.uk/IAAPwebsite/FactSheet/Hydrocotyl.doc
Instituto Nacional de Biodiversidad (Costa Rica). Lista de especímenes de Hydrocotyle ranunculoides. http://www.inbio.ac.cr/bims/k03/p13/c045/o0135/f01665/g006947/s019969.htm
Palma, B.; San Martín, C.; Rosales, M.; Zúñiga, Ramírez, C. (1986) Distribución espacial de la flora y vegetación acuática y palustre del estero marga-marga en Chile central. http://biblioweb.dgsca.unam.mx/cienciasdelmar/instituto/1987-2/articulo269.html
USDA Natural Resources Conservation Service. Plant Profile. Hydrocotyle ranunculoides. http://plants.usda.gov
Wageningen University. Environmental Science. Aquatic Ecology and Water Quality Management Group. Invasion of Dutch waters by floating pennywort (Hydrocotyle ranunculoides) (‘Grote Waternavel’). http://www.dow.wau.nl/aew/Projects/info_waternavel.html

Solidago nemoralis
Kapusta, G.; Krausz, R.F. (1993) Weed control and yields are equal in conventional, reduced-, and no-tillage soybean (Glycine max) after 11 years. Weed-Technology, 7(2), 443-451.
Peters, E.J. (1983) Effectiveness and safety of translocated herbicides applied to pasture weeds with a rope-wick applicator. Proceedings of the XIV International Grassland Congress, held at Lexington, Kentucky, U.S.A., June 15-24, 1981 [Smith, J.A.; Hays, V.W. (Editors)], 553-555.
Peters, E.J.; Lowance, S.A. (1978) Effects of multiple mowing on western ironweed (Vernonia baldwinii) and gray goldenrod (Solidago nemoralis). Weed-Science, 26(2), 190-192.
Schrader, G., Baker, R. (2003) Draft Report of a pest risk assessment of Solidago nemoralis. Unpublished document.
Weller, S.C. (1984) Post-emergence weed control in orchards. Proceedings, North Central Weed Control Conference, Vol.39, p 145.
Prairie Wildflowers of Illinois. Field Goldenrod – Solidago nemoralis. http://www.shout.net/~jhilty/plantx/fld_goldenrodx.htm
USDA Natural Resources Conservation Service. Plant Profile. Solidago nemoralis. http://plants.usda.gov/cgi_bin/plant_profile.cgi?symbol=SONEN
USDA Natural Resources Conservation Service. Conservation Plant Characteristics for Solidago nemoralis. http://plants.usda.gov/cgi_bin/plant_attribute.cgi?symbol=SONE