Crassula helmsii (Australian swamp stonecrop)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Biology and Ecology
- Latitude/Altitude Ranges
- Air Temperature
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Causes
- Pathway Vectors
- Impact Summary
- Economic Impact
- Environmental Impact
- Threatened Species
- Social Impact
- Risk and Impact Factors
- Uses List
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Crassula helmsii (T. Kirk) Cockayne
Preferred Common Name
- Australian swamp stonecrop
Other Scientific Names
- Bulliarda recurva Hook. f., 1847
- Crassula recurva (Hook. f.) Ostenf., 1918
- Crassula recurva N.E.Br., 1890
- Tillaea helmsii Kirk, 1899
- Tillaea recurva (Hook. f.) Hook. f., 1857
International Common Names
- English: Australian stonecrop; Crassula; helms crassula; New Zealand pigmy weed; New Zealand pigmyweed; New Zealand pygmy weed; New Zealand pygmyweed; New Zealand stonecrop; pigmy weed; pygmy weed; stonecrop; swamp crassula; swamp stonecrop; swamp stonecrop
- French: crassula des étangs
Local Common Names
- Denmark: krassula
- Germany: Helms Dickblatt; Nadelkraut; Watercrassula
- Netherlands: watercrassula
- Poland: grubosz
- CSBHE (Crassula helmsii)
Summary of InvasivenessTop of page
C. helmsii is an aquatic plant that can take on various growth forms depending on prevailing conditions, and is able to act as a submerged, emergent, or semi-terrestrial species. C. helmsii has the ability to form dense stands of 100% cover, which cause many negative environmental and economic impacts including displacing native plant species, reducing biodiversity, decreasing water quality and flow, impeding recreational activities, and diminishing aesthetic value. C. helmsii is extremely difficult and costly to control, and its ability to form new plants vegetatively from small fragments facilitates its spread to new locations. The trade and potential escape of C. helmsii through the aquarium and water garden industry may play a large role in its spread to new locations. In addition, the transportation of this plant on recreational equipment or by wildlife moving between water bodies may also play a role in local spread. C. helmsii is native to Australia and New Zealand, and has established itself as an exotic invasive in many parts of Europe, especially the United Kingdom. C. helmsii is declared a noxious weed in Florida and North Carolina, and although some sources report C. helmsii as occurring in these states (OEPP/EPPO, 2004; 2006), the status of these populations is unknown. C. helmsii is also declared a noxious weed in New Hampshire and Washington (USDA-GRIN, 2002), and is considered a prohibited species in Minnesota (MDNR, 2006) and Wisconsin (WDNR, 2009), though it is not known to occur in these states.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Rosales
- Family: Crassulaceae
- Genus: Crassula
- Species: Crassula helmsii
Notes on Taxonomy and NomenclatureTop of page
The genus Crassula (family Crassulaceae) contains approximately 200-300 species, of which only a few are adapted to wet conditions, and 23 are considered weeds (Sheppard et al., 2006; Winterton and Scher, 2007). The genus name Crassula is the diminutive of the Latin ‘crassus’ which means ‘thick’ or ‘fat’, referring to the fleshy, succulent nature of the genus as a whole. The genus is found almost worldwide, although the majority of species are from the southern parts of Africa. Crassula helmsii was originally classified as Bulliardarecurva Hook.f. in 1847, and revised to Tillaea recurva (Hook.f.) Hook.f. in 1857. Crassula recurva (Hook.f.) Ostenf. and Crassula recurva N.E.Br. were also both suggested as proper nomenclature. The taxonomy was further revised to Tillaea helmsii Kirk in 1899, and ultimately revised to Crassula helmsii. There are varying reports of authority, with both Crassula helmsii (Kirk) Cockayne, 1907, and Crassula helmsii A. Berger, 1930 reported in the literature (USDA-GRIN, 2002; USDA-NRCS, 2006; IPNI, 2008). Many of the unaccepted scientific names of C. helmsii are still often used in the aquarium trade (CAPM-CEH, 2004; OEPP/EPPO, 2007). As the English common names suggest, C. helmsii originates from Australia and New Zealand.
Previous classifications included the family Crassulaceae within the Rosales, but now this family is included in the order Saxifragales (Stevens, 2012); the Compendium taxonomic tree still remains to be updated in this respect.
DescriptionTop of page
C. helmsii is an aquatic or semi-terrestrial herbaceous succulent perennial plant with 1 mm thick round stems that are 10-130 cm long and creeping or floating (OEPP/EPPO, 2004). C. helmsii can grow in several different growth forms, establishing as a submersed plant in waters up to 3 m (10 ft) deep, and also as an emergent or semi-terrestrial plant on damp ground (Sheppard et al., 2006). The submersed form grows from a basal rosette with well-anchored roots, and can reach 1.3 m in height. The emergent form consists of short, densely packed stems in waters less than 0.6 m deep. The terrestrial form has creeping or erect stems with yellowish-green aerial leaves.
Plant TypeTop of page Aquatic
DistributionTop of page
C. helmsii is native to New Zealand and Australia, including the territories of New South Wales, South Australia, Tasmania, Victoria, and Western Australia (OEPP/EPPO, 2007). C. helmsii is known to occasionally be a nuisance in its native range (Sheppard et al., 2006). In New Zealand, it is reported as being naturally uncommon, and is known only from the west coast of the South Island from Karamea south to Haast (NZPCN, 2005). Randall (1999) also reports C. helmsii as being native to Papua New Guinea.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|USA||Restricted distribution||EPPO, 2014|
|-Florida||Present||Introduced||Invasive||OEPP/EPPO, 2004; OEPP/EPPO, 2006; OEPP/EPPO, 2007|
|-North Carolina||Present||Introduced||Invasive||OEPP/EPPO, 2004; OEPP/EPPO, 2006; OEPP/EPPO, 2007|
|Belgium||Present||NOBANIS, 2005; EPPO, 2014|
|Denmark||Present, few occurrences||Introduced||2003||Invasive||NOBANIS, 2005; EPPO, 2014|
|France||Present, few occurrences||Weber, 2003; OEPP/EPPO, 2007; EPPO, 2014|
|Germany||Restricted distribution||NOBANIS, 2005; OEPP/EPPO, 2007; EPPO, 2014|
|Ireland||Localised||Introduced||1984||Invasive||NOBANIS, 2005; OEPP/EPPO, 2007; Devlin, 2016|
|Italy||Localised||Introduced||Afferni and Tavormina, 2007||Trieste district|
|Netherlands||Restricted distribution||Introduced||1995||Invasive||OEPP/EPPO, 2007; EPPO, 2014|
|Portugal||Absent, invalid record||OEPP/EPPO, 2007; EPPO, 2014|
|Russian Federation||Absent, formerly present||Asovsky, 1981; OEPP/EPPO, 2004; EPPO, 2014|
|-Eastern Siberia||Absent, formerly present||EPPO, 2014|
|Spain||Present||OEPP/EPPO, 2004; EPPO, 2014|
|UK||Restricted distribution||OEPP/EPPO, 2007; EPPO, 2014|
|-Channel Islands||Present||Stace et al., 2005; EPPO, 2014|
|-England and Wales||Restricted distribution||Stace et al., 2005; EPPO, 2014|
|-Northern Ireland||Present||EPPO, 2014|
|Australia||Present||OEPP/EPPO, 2007; EPPO, 2014|
|-New South Wales||Present||Native||OEPP/EPPO, 2007|
|-South Australia||Present||Native||OEPP/EPPO, 2007|
|-Western Australia||Present||Native||OEPP/EPPO, 2007|
|New Zealand||Present||NZPCN, 2005; OEPP/EPPO, 2007; EPPO, 2014|
|Papua New Guinea||Present||Native||Randall, 1999|
History of Introduction and SpreadTop of page
C. helmsii was introduced from Tasmania to England in 1911, and sold throughout the 1920s as an ‘oxygenating plant’ in the aquarium trade (CAPM-CEH, 2004). The first population reported in the wild was in 1956, in Greensted Pond, Essex (OEPP/EPPO, 2007). Until the 1970s, C. helmsii was commercially available only through one supplier, Perry’s Hardy Plant Farm in Enfield, Middlesex, though it is now readily available through multiple suppliers (Dawson and Warman, 1987; Leach and Dawson, 1999). Since the initial introduction of C. helmsii to the United Kingdom, the number of sites invaded by the plant has doubled every two years, with over 1500 sites reported in the British Isles, though this is probably an underestimate of its true distribution (Environment Agency, 2003; OEPP/EPPO, 2007). It is reported from a pond in Co. Wexford, Irish Republic in 2014, which Devlin (2016) believed to be the first reported occurrence in the Irish Republic.
C. helmsii was first recorded in Germany in 1981, and has spread to many parts of the country including Hamburg, Hanover, Schleswig and the Pfalzerwald and Westphalia areas (Leach and Dawson, 1999). C. helmsii was first recorded in Belgium in 1982, and is described as being locally present (NOBANIS, 2005; OEPP/EPPO, 2007). C. helmsii was first reported in Northern Ireland in 1984 in a pool at Gosford (OEPP/EPPO, 2007). In Ireland, C. helmsii has a relatively restricted distribution compared to that of England and Wales (Kelly and Maguire, 2009). C. helmsii was first found in the Netherlands in 1995 in a nature reserve near Breda, and has also established localized populations in ponds in the provinces of Noord-Brabant and Zeeland (OEPP/EPPO, 2007). C. helmsii was first reported in Denmark in 2003, where the frequency of invaded sites is reported as low (NOBANIS, 2005).
C. helmsii has been recorded in Spain, but so far its presence has not been reported as invasive (OEPP/EPPO, 2004). Reports of C. helmsii occurring in Portugal have been invalidated (OEPP/EPPO, 2007). Localized populations of C. helmsii have been reported growing in several ponds in the Trieste district of Italy (Afferni and Tavormina, 2007). C. helmsii has also been reported as being present in France (OEPP/EPPO, 2004) and Austria (Minchin, 2008). C. helmsii has been recorded as a rare plant found in the Baikal region of Russia (Asovsky, 1981; OEPP/EPPO, 2004).
OEPP/EPPO (2004; 2006) report C. helmsii as occurring in Florida and North Carolina; however, the pathway of introduction and current status of these populations is unknown.
IntroductionsTop of page
|Introduced to||Introduced from||Year||Reason||Introduced by||Established in wild through||References||Notes|
|Natural reproduction||Continuous restocking|
|UK||Tasmania||1911||Escape from confinement or garden escape (pathway cause)
Ornamental purposes (pathway cause) ,
Pet trade (pathway cause)
|Yes||No||OEPP/EPPO (2007)||Sold in the 1920s, but not reported in the wild until 1956|
Risk of IntroductionTop of page
C. helmsii is continuing to expand its range and become more abundant (Minchin, 2008). C. helmsii is a very popular aquarium and water garden plant, and the increased availability of this plant over the internet and through mail order gives it the ability to travel to all parts of the world. In addition, C. helmsii is often found as a ‘contaminant’ or ‘hitchhiker’ plant with other species ordered through water garden catalogues (Environment Agency, 2003). C. helmsii has escaped confinement and has been intentionally or accidentally introduced on several occasions outside of its native range. C. helmsii is a highly competitive plant which is capable of rapid growth and spread. In the locales to which it has been introduced, it has often become the dominant plant species, outcompeting natives and displacing other species.
HabitatTop of page
C. helmsii can grow in several different growth forms, establishing in submersed waters up to 3 m (10 ft) deep, and also as an emergent or semi-terrestrial plant on damp ground (Sheppard et al., 2006). The morphology of the plant changes between these different growth forms according to the prevailing environmental conditions (OEPP/EPPO, 2007), although the submersed form of the plant is not known in its native range (Sheppard et al., 2006). C. helmsii inhabits lakes, ponds, gravel pits, inland and coastal wetlands, marshes, swamps, rivers, canals, and irrigation ditches.
Habitat ListTop of page
|Terrestrial ‑ Natural / Semi-natural||Riverbanks||Present, no further details|
|Wetlands||Present, no further details|
|Coastal areas||Present, no further details|
|Irrigation channels||Present, no further details||Harmful (pest or invasive)|
|Lakes||Present, no further details|
|Reservoirs||Present, no further details|
|Rivers / streams||Present, no further details|
|Ponds||Present, no further details||Harmful (pest or invasive)|
|Estuaries||Present, no further details|
Biology and EcologyTop of page
C. helmsii has a reported chromosome number of 2n=36 (Stace et al., 2005; Lockton, 2009). Studies of genetic variation show that it is likely there was only one introduction of C. helmsii into Britain, with the probable source population being the plants growing along the River Murray in Australia (OEPP/EPPO, 2007).
Genetic studies of New Zealand plants show a difference in chromosome number, with Australian plants being diploid (2n=14), and the smaller, more delicate plants from New Zealand being hexaploid (2n=42) (NZPCN, 2005). A more recent study by De Lange et al. (2008) refers to Australian material with a chromosome number of 2n=42 and New Zealand material with 2n=14.
C. helmsii has the ability to prolifically reproduce vegetatively through fragments, which can be as small as a single node on a 5 mm stem being capable of producing a new plant (CAPM-CEH, 2004). In addition, apical turions are produced in the autumn (in the United Kingdom), which then float on the waters’ surface (OEPP/EPPO, 2007). C. helmsii also can reproduce sexually and D’hondt et al. (2016) found that European populations can produce viable seeds, although with a low germination rate.
Physiology and Phenology
In its native range, C. helmsii flowers in November and December, with flowering continuing to February in New Zealand (OEPP/EPPO, 2007). In Europe, flowers appear between July and September, though the viability of seeds in Europe is uncertain (OEPP/EPPO, 2007). C. helmsii is able to grow throughout the year without a dormant period (CAPM-CEH, 2004).
C. helmsii can colonize a variety of waters, from static to gradually-flowing systems, and is able to withstand periods of extended drying. It colonizes waters ranging from acidic to alkaline, and has also been recorded in semi-saline water bodies (OEPP/EPPO, 2007). C. helmsii is associated with soft sediments and possibly also with iron-rich waters (Dawson and Warman, 1987). In its native range, C. helmsii appears to be confined to areas where summer temperatures are 20-25°C with 100-550 mm precipitation, and winter temperatures are 0-15°C with 200-3000 mm precipitation, including extended periods under snow (Leach and Dawson, 1999; OEPP/EPPO, 2007). C. helmsii can survive a wide range of climatic variation, from averages of 30°C in the summer to less than -6°C in winter (OEPP/EPPO, 2007). It is a lowland plant, occurring in altitudes from sea level up to 345 m (Lockton, 2009).
ClimateTop of page
|BS - Steppe climate||Tolerated||> 430mm and < 860mm annual precipitation|
|BW - Desert climate||Tolerated||< 430mm annual precipitation|
|Cf - Warm temperate climate, wet all year||Tolerated||Warm average temp. > 10°C, Cold average temp. > 0°C, wet all year|
|Cs - Warm temperate climate with dry summer||Preferred||Warm average temp. > 10°C, Cold average temp. > 0°C, dry summers|
Latitude/Altitude RangesTop of page
|Latitude North (°N)||Latitude South (°S)||Altitude Lower (m)||Altitude Upper (m)|
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Mean maximum temperature of hottest month (ºC)||30||0|
|Mean minimum temperature of coldest month (ºC)||-6||0|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Mean annual rainfall||100||3000||mm; lower/upper limits|
Notes on Natural EnemiesTop of page
Few natural enemies of C. helmsii are reported in the literature. However, CABI has been investigating the biological control of C. helmsii since 2009. Many natural enemies were observed causing damage to C. helmsii in the native range, for more information see Finding a biocontrol agent for Crassula'. Research is currently focussed on a host specific mite, Aculus sp. (Eriophiyidae) and scientists are at the final stages of pest risk assessment. If approval is granted, field trials will take place in 2018.
Means of Movement and DispersalTop of page
Natural Dispersal (Non-Biotic)
Hydrochory, the dispersal of disseminules by water currents, seems to be the main dispersal mode of vegetative fragments within a watershed.
Vector Transmission (Biotic)
C. helmsii can be transported via birds, wildlife, or mud and carried to new locations (OEPP/EPPO, 2007; Minchin, 2008; Denys et al., 2014). The spread of the plant by livestock may be a significant contributor to its continued spread (Wicks, 2004).
C. helmsii is often found as a ‘contaminant’ or ‘hitchhiker’ plant with other species ordered through water garden catalogues (Environment Agency, 2003). C. helmsii has been introduced through hobbyists emptying unwanted aquarium species directly into surrounding waterways, and can also be accidentally introduced by water garden ponds flooding into surrounding natural waterways. Due to the ability of C. helmsii to reproduce via small fragments, plants could also be spread accidentally to new locations by the movement of boats, trailers, nets, anglers, and other recreational equipment between water bodies.
The trade of this plant as a submerged ‘oxygenating’ aquarium plant through the internet and mail order has greatly increased its availability and facilitates its spread into new environments.
Pathway CausesTop of page
|Escape from confinement or garden escape||Yes||OEPP/EPPO, 2007|
|Flooding and other natural disasters||Yes||OEPP/EPPO, 2007|
|Hitchhiker||Yes||Yes||Environment Agency, 2003|
|Interbasin transfers||Yes||CAPM-CEH, 2004|
|Interconnected waterways||Yes||CAPM-CEH, 2004|
|Internet sales||Yes||Yes||Environment Agency, 2003|
|Nursery trade||Yes||Yes||OEPP/EPPO, 2007|
|Ornamental purposes||Yes||Yes||OEPP/EPPO, 2007|
|Pet trade||Yes||Yes||OEPP/EPPO, 2007|
Pathway VectorsTop of page
|Clothing, footwear and possessions||Yes||Leach and Dawson, 1999; OEPP/EPPO, 2007|
|Floating vegetation and debris||Yes||CAPM-CEH, 2004|
|Pets and aquarium species||Yes||Yes||OEPP/EPPO, 2007|
|Ship structures above the water line||Yes||Yes||Minchin, 2008; OEPP/EPPO, 2007|
Impact SummaryTop of page
Economic ImpactTop of page
C. helmsii has been found to limit water flow in irrigation channels and flood-control systems (Kelly and Maguire, 2009). In addition, the loss of recreational and aesthetic value associated with C. helmsii can also cause a decline in waterfront property values, as well as possible declines in tourism related revenue for communities. One recent estimate puts control costs of C. helmsii between 1.45 and 3 million euros (US $2.1-4.4 million) to manage 500 sites over 2-3 years (Leach and Dawson, 1999).
Environmental ImpactTop of page
Impact on Habitats
The dense stands and mats of vegetation that are characteristic of this species when introduced outside of its native range can decrease the oxygen levels by limiting water circulation and increasing decomposition of dead plants. Dense mats of C. helmsii also have the ability to change water hydrology and quality, negatively affecting the ecosystem in which it occurs. The plant is able to grow throughout the entire year without a dormant period, allowing it to occupy its niche year-round (CAPM-CEH, 2004). The very rapid growth of C. helmsii also allows it to uptake almost all the available nutrients (Environment Agency, 2003). The submerged form of C. helmsii is uniquely adapted to assimilate carbon dioxide for 20 hours of the day due to the plants’ ability to employ crassulacean acid metabolism (CAM) (Keeley, 1998).
Impact on Biodiversity
C. helmsii is winter hardy and has the ability to form 100% cover, giving it the capacity to outcompete and displace native plant species which typically die back in the winter (OEPP/EPPO, 2007; Habitas, 2009). A thin covering of C. helmsii can cause significant germination suppression in some plant species (Langdon et al., 2004). Dense mats suppress native flora and create a poor ecosystem for invertebrates, amphibians, and fish (CAPM-CEH, 2004; Minchin, 2008). Decomposing mats of C. helmsii also have the ability to cause fish kills by creating severe fluctuations in dissolved oxygen levels in the water (OEPP/EPPO, 2007).
Several rare or threatened species in the United Kingdom may be negatively impacted by the spread of C. helmsii (OEPP/EPPO, 2007). Reduced breeding success of a protected species, the great crested newt (Triturus cristatus), has been attributed to invasion of ponds by C. helmsii (Langdon et al., 2004). The rare starfruit plant, Damasonium alsima, is thought to be threatened by C. helmsii (Watson, 2001). C. helmsii may smother Callitriche spp., and outcompete charophytes (stoneworts) for space (Habitas, 2009). In addition, a study in England shows a significant reduction in the diatom Synedra delicatissima caused by the introduction of C. helmsii (Habitas, 2009). Smith and Buckley (2015), however, found in waterbodies in southeast England that invasion with C. helmsii did not reduce native species diversity, but rather that alterations in species assemblages after invasion often favoured rarer species.
Threatened SpeciesTop of page
Social ImpactTop of page
C. helmsii can form dense mats that impede recreational activities such as boating, fishing, swimming, water skiing, canoeing, and kayaking. In addition, unsightly mats of vegetation decrease aesthetic values, and can be mistaken as dry land which can present significant danger to animals and humans (Sheppard et al., 2006).
Risk and Impact FactorsTop of page Invasiveness
- Proved invasive outside its native range
- Highly adaptable to different environments
- Is a habitat generalist
- Tolerant of shade
- Fast growing
- Has high reproductive potential
- Has propagules that can remain viable for more than one year
- Reproduces asexually
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Modification of hydrology
- Modification of natural benthic communities
- Modification of nutrient regime
- Monoculture formation
- Negatively impacts cultural/traditional practices
- Negatively impacts human health
- Negatively impacts livelihoods
- Negatively impacts aquaculture/fisheries
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Threat to/ loss of endangered species
- Threat to/ loss of native species
- Transportation disruption
- Competition - monopolizing resources
- Competition - shading
- Competition - smothering
- Rapid growth
- Highly likely to be transported internationally accidentally
- Highly likely to be transported internationally deliberately
- Difficult to identify/detect as a commodity contaminant
- Difficult/costly to control
UsesTop of page
Uses ListTop of page
- Botanical garden/zoo
- Pet/aquarium trade
Detection and InspectionTop of page
Similarities to Other Species/ConditionsTop of page
C. helmsii is closely related to Crassula aquatica, though the two species can be distinguished based upon the size and position of their flowers (OEPP/EPPO, 2007). A similar species from South Africa, Crassula campestris, is reported as naturalized in Spain (Sheppard et al., 2006). The submerged stems of Callitriche speciesmay also be mistaken for Crassula helmsii, but Callitriche species are never emergent, and also have a distinct notch at the leaf tips (Environment Agency, 2003; OEPP/EPPO, 2007). The genus Crassula may also be confused with the genus Microcarpaea (Winterton and Scher, 2007).
Prevention and ControlTop of page
As with all invasive species management, prevention is better and more cost-effective than control.
Early detection and treatment is essential in the prevention of future invasions and spread of C. helmsii. Smaller, localized populations have better success at being controlled than those which have the opportunity to spread and become well-established (Environment Agency, 2003).
Several publications have been produced in areas with C. helmsii populations regarding the impacts of invasive species and the steps that aquarists and lake recreationists need to take in order to prevent introducing and spreading aquatic invasives.
Cultural control and sanitary measures
In several regions where aquatic invasives have established, governmental organizations have started requiring that recreationists drain all water and clean off all gear (boats, trailers, fishing equipment, nets, etc.) used on water bodies in order to minimize the chance of spreading aquatic invasive species, such as C. helmsii, to other areas. Dean et al. (2015) suggest that the presence of large grazing animals can facilitate a higher abundance of C. helmsii, and that managers of grazed wetland habitat should therefore enforce biosecurity measures, and prevent grazing livestock access to drawdown zones where the plant already occurs.
Control of C. helmsii has had limited efficacy due to its ability to propagate vegetatively through small fragments. Attempts to mechanically harvest only serve as means of creating and introducing more plant fragments, potentially aiding in dispersal to new locations (CAPM-CEH, 2004). C. helmsii is also tolerant of shade, extreme cold, and desiccation, making it very difficult to control. Small patches may be controlled with plastic shade material, but the material must remain in place for at least eight weeks, and often up towards six months (CAPM-CEH, 2004). This process is very labour intensive and causes much disturbance (Bridge, 2005). Freezing with liquid nitrogen has been effective on small populations, while surrounding medium sized populations with a fine wire mesh fence can aid in targeting removal and preventing further spread (OEPP/EPPO, 2007). Dredging of near shore or emergent vegetation throughout the year can also be an effective control mechanism. It is recommended that all dead plant material be removed to reduce potential oxygen depletion through decomposition.
Several countries have banned the importation or sale of exotic plants, such as C. helmsii, in attempts to minimize the chance of introduction to non-native regions. In the UK, C. helmsii has been added to Schedule 9 of the Wildlife and Countryside Act 1981, making it an offence to deliberately plant or cause this species to grow in the wild.
CABI has been investigating the biological control of C. helmsii since 2009, for more information see 'Finding a biocontrol agent for Crassula'. Research is currently focussed on a host specific mite, Aculus sp. (Eriophiyidae) and scientists are at the final stages of pest risk assessment. If approval is granted, field trials will take place in 2018. Grass carp will feed to a limited extent on small populations of C. helmsii but it is not its preferred food (Dawson and Warman, 1987). However, introduction of grass carp can negatively impact the coexisting native submerged vegetation, and introduction is even prohibited in some countries.
C. helmsii is susceptible to chemicals containing diquat and glyphosate (Dawson, 1996; CAPM-CEH, 2004). Diquat is best applied in the autumn or winter and water temperatures should be >12ºC (Minchin, 2008). In the European Union where diquat is banned for use in aquatic systems, early spring application of dichlobenil is often used when the plant is still entirely submerged (CAPM-CEH, 2004). Glyphosate should be applied from April to late November, when the majority of the plant is emergent. It is recommended that at least 70% of dense populations be chemically treated at one time to reduce potential re-colonization from untreated areas, and the remaining 30% should be treated one week later (CAPM-CEH, 2004). In an English nature preserve, a hot biodegradable foam made of coconut and corn sugars was reported as being able to control approximately 50% of the population, but did not eradicate it (Bridge, 2005). Hydrogen peroxide has been experimented with as a potential control method, but only plant scorching and temporary suppression of plant material was achieved (Dawson and Henville, 1991). Sims and Sims (2016) successfully eradicated C. helmsii from two ponds in Norfolk, England, using herbicide (glyphosate) application and in situ burial.
ReferencesTop of page
Afferni M, Tavormina G, 2007. Native Crassula in Italy. International Crassulaceae Network. http://www.crassulaceae.net/index.php?option=com_content&view=article&id=138:native-crassula-in-italy-uk&catid=43:speciescrassula&Itemid=5
Child LE, Spencer-Jones D, 1993. Treatment of Crassula helmsii - a case study. Plant invasions: general aspects and special problems. In: Workshop held at Kostelec nad Cernymilesy, Czech Republic, 16-19 September. 195-202
Dawson FH, Henville P, 1991. An investigation of the control of Crassula helmsii by herbicidal chemicals (with interim guidelines on control). Final report. Peterborough, UK: Nature Conservancy Council, 107 pp
Dean, C. E., Day, J., Gozlan, R. E., Diaz, A., 2015. Grazing vertebrates promote invasive swamp stonecrop (Crassula helmsii) abundance. Invasive Plant Science and Management, 8(2), 131-138. http://www.bioone.org/loi/ipsm doi: 10.1614/IPSM-D-14-00068.1
Denys, L., Packet, J., Jambon, W., Scheers, K., 2014. Dispersal of the non-native invasive species Crassula helmsii (Crassulaceae) may involve seeds and endozoochorous transport by birds. New Journal of Botany, 4(2), 104-106. http://www.maneyonline.com/loi/njb
Devlin Z, 2016. A new record for the alien invasive plant Crassula helmsii (Kirk) Cockayne (New Zealand Pigmyweed) in Co. Wexford (H12). Irish Naturalists' Journal, 35(1):51-52. http://irishnaturalistsjournal.org
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ContributorsTop of page
10/07/18 Updated by:
Sonal Varia, CABI, UK
16/12/09 Original text by:
Michelle Nault, Wisconsin Department of Natural Resources, 2801 Progress Rd, Madison, WI 53716-3339, USA
Distribution MapsTop of page
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