Invasive Species Compendium

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Datasheet

Crassula helmsii
(Australian swamp stonecrop)

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Datasheet

Crassula helmsii (Australian swamp stonecrop)

Summary

  • Last modified
  • 15 July 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Preferred Scientific Name
  • Crassula helmsii
  • Preferred Common Name
  • Australian swamp stonecrop
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Dicotyledonae
  • Summary of Invasiveness
  • 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...

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Pictures

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PictureTitleCaptionCopyright
Crassula helmsii (Australian swamp stonecrop); habit, showing a dense carpet growing along the margins of a freshwater reedbed system. Rye Harbour Nature Reserve, East Sussex, England. July 2010.
TitleHabit
CaptionCrassula helmsii (Australian swamp stonecrop); habit, showing a dense carpet growing along the margins of a freshwater reedbed system. Rye Harbour Nature Reserve, East Sussex, England. July 2010.
Copyright©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing a dense carpet growing along the margins of a freshwater reedbed system. Rye Harbour Nature Reserve, East Sussex, England. July 2010.
HabitCrassula helmsii (Australian swamp stonecrop); habit, showing a dense carpet growing along the margins of a freshwater reedbed system. Rye Harbour Nature Reserve, East Sussex, England. July 2010.©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing a thick, monospecific carpet. Rye Harbour Nature Reserve, East Sussex, England. July 2010.
TitleHabit
CaptionCrassula helmsii (Australian swamp stonecrop); habit, showing a thick, monospecific carpet. Rye Harbour Nature Reserve, East Sussex, England. July 2010.
Copyright©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing a thick, monospecific carpet. Rye Harbour Nature Reserve, East Sussex, England. July 2010.
HabitCrassula helmsii (Australian swamp stonecrop); habit, showing a thick, monospecific carpet. Rye Harbour Nature Reserve, East Sussex, England. July 2010.©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing terrestrial form. Studland Nature Reserve, Dorset, England. April 2010.
TitleHabit
CaptionCrassula helmsii (Australian swamp stonecrop); habit, showing terrestrial form. Studland Nature Reserve, Dorset, England. April 2010.
Copyright©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing terrestrial form. Studland Nature Reserve, Dorset, England. April 2010.
HabitCrassula helmsii (Australian swamp stonecrop); habit, showing terrestrial form. Studland Nature Reserve, Dorset, England. April 2010.©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing a close-up of the terrestrial form. Studland Nature Reserve, Dorset, England. April 2010.
TitleHabit
CaptionCrassula helmsii (Australian swamp stonecrop); habit, showing a close-up of the terrestrial form. Studland Nature Reserve, Dorset, England. April 2010.
Copyright©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing a close-up of the terrestrial form. Studland Nature Reserve, Dorset, England. April 2010.
HabitCrassula helmsii (Australian swamp stonecrop); habit, showing a close-up of the terrestrial form. Studland Nature Reserve, Dorset, England. April 2010.©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, growing in an emergent form within a shallow, freshwater pond. nr Cofe Castle, Dorset, England. April 2010.
TitleHabit
CaptionCrassula helmsii (Australian swamp stonecrop); habit, growing in an emergent form within a shallow, freshwater pond. nr Cofe Castle, Dorset, England. April 2010.
Copyright©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, growing in an emergent form within a shallow, freshwater pond. nr Cofe Castle, Dorset, England. April 2010.
HabitCrassula helmsii (Australian swamp stonecrop); habit, growing in an emergent form within a shallow, freshwater pond. nr Cofe Castle, Dorset, England. April 2010.©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing the transition between terrestrial and emergent morphological forms. Studland Nature Reserve, Dorset, England. Ocotober 2010.
TitleHabit
CaptionCrassula helmsii (Australian swamp stonecrop); habit, showing the transition between terrestrial and emergent morphological forms. Studland Nature Reserve, Dorset, England. Ocotober 2010.
Copyright©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing the transition between terrestrial and emergent morphological forms. Studland Nature Reserve, Dorset, England. Ocotober 2010.
HabitCrassula helmsii (Australian swamp stonecrop); habit, showing the transition between terrestrial and emergent morphological forms. Studland Nature Reserve, Dorset, England. Ocotober 2010.©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing terrestrial form. Note small, white, flowers. Studland Nature Reserve, Dorset, England. October 2010.
TitleHabit and flowers
CaptionCrassula helmsii (Australian swamp stonecrop); habit, showing terrestrial form. Note small, white, flowers. Studland Nature Reserve, Dorset, England. October 2010.
Copyright©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing terrestrial form. Note small, white, flowers. Studland Nature Reserve, Dorset, England. October 2010.
Habit and flowersCrassula helmsii (Australian swamp stonecrop); habit, showing terrestrial form. Note small, white, flowers. Studland Nature Reserve, Dorset, England. October 2010.©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing terrestrial form. Note small, white, flowers. Studland Nature Reserve, Dorset, England. October 2010.
TitleHabit and flowers
CaptionCrassula helmsii (Australian swamp stonecrop); habit, showing terrestrial form. Note small, white, flowers. Studland Nature Reserve, Dorset, England. October 2010.
Copyright©Clare Dean
Crassula helmsii (Australian swamp stonecrop); habit, showing terrestrial form. Note small, white, flowers. Studland Nature Reserve, Dorset, England. October 2010.
Habit and flowersCrassula helmsii (Australian swamp stonecrop); habit, showing terrestrial form. Note small, white, flowers. Studland Nature Reserve, Dorset, England. October 2010.©Clare Dean

Identity

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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

EPPO code

  • CSBHE (Crassula helmsii)

Summary of Invasiveness

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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 Tree

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  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Dicotyledonae
  •                     Order: Rosales
  •                         Family: Crassulaceae
  •                             Genus: Crassula
  •                                 Species: Crassula helmsii

Notes on Taxonomy and Nomenclature

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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.

Description

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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. 

The succulent linear to narrowly oval leaves are opposite and sessile, 4-24 mm long and 0.7-1.6 mm wide. Flowers have four petals, are white or occasionally pink, 3-3.5 mm in diameter, and are borne singly on stalks in the axils of the leaves. The fruits contain 2-5 smooth, elliptical seeds 0.5 mm long (OEPP/EPPO, 2007).

Plant Type

Top of page Aquatic
Herbaceous
Perennial
Seed propagated
Succulent
Vegetatively propagated

Distribution

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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. 

C. helmsii is currently naturalized in several areas of Europe, including the United Kingdom, Germany, Belgium, Ireland, the Netherlands, Denmark, France, Spain, Italy, Austria, and the Baikal region of Russia (OEPP/EPPO, 2004; 2007; NOBANIS, 2005; Afferni and Tavormina, 2007; Minchin, 2008). C. helmsii has been reported as being present in Portugal (OEPP/EPPO, 2004); however, this has since been invalidated (OEPP/EPPO, 2007). There are reports of C. helmsii occurring in Florida and North Carolina in the southeastern United States (OEPP/EPPO, 2004; 2006; Minchin, 2008), but the extent of distribution and current status of these populations is unknown.

Distribution Table

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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/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

North America

USARestricted distributionEPPO, 2014
-FloridaPresentIntroduced Invasive OEPP/EPPO, 2004; OEPP/EPPO, 2006; OEPP/EPPO, 2007
-North CarolinaPresentIntroduced Invasive OEPP/EPPO, 2004; OEPP/EPPO, 2006; OEPP/EPPO, 2007

Europe

AustriaPresentIntroducedMinchin, 2008
BelgiumPresentNOBANIS, 2005; EPPO, 2014
DenmarkPresent, few occurrencesIntroduced2003 Invasive NOBANIS, 2005; EPPO, 2014
FrancePresent, few occurrencesWeber, 2003; OEPP/EPPO, 2007; EPPO, 2014
-CorsicaPresentIntroducedMinchin, 2008
GermanyRestricted distributionNOBANIS, 2005; OEPP/EPPO, 2007; EPPO, 2014
IrelandLocalisedIntroduced1984 Invasive NOBANIS, 2005; OEPP/EPPO, 2007; Devlin, 2016
ItalyLocalisedIntroducedAfferni and Tavormina, 2007Trieste district
NetherlandsRestricted distributionIntroduced1995 Invasive OEPP/EPPO, 2007; EPPO, 2014
PortugalAbsent, invalid recordOEPP/EPPO, 2007; EPPO, 2014
Russian FederationAbsent, formerly presentAsovsky, 1981; OEPP/EPPO, 2004; EPPO, 2014
-Eastern SiberiaAbsent, formerly presentEPPO, 2014
SpainPresentOEPP/EPPO, 2004; EPPO, 2014
UKRestricted distributionOEPP/EPPO, 2007; EPPO, 2014
-Channel IslandsPresentStace et al., 2005; EPPO, 2014
-England and WalesRestricted distributionStace et al., 2005; EPPO, 2014
-Northern IrelandPresentEPPO, 2014

Oceania

AustraliaPresentOEPP/EPPO, 2007; EPPO, 2014
-New South WalesPresentNativeOEPP/EPPO, 2007
-South AustraliaPresentNativeOEPP/EPPO, 2007
-TasmaniaPresentNativeOEPP/EPPO, 2007
-VictoriaPresentNativeOEPP/EPPO, 2007
-Western AustraliaPresentNativeOEPP/EPPO, 2007
New ZealandPresentNZPCN, 2005; OEPP/EPPO, 2007; EPPO, 2014
Papua New GuineaPresentNativeRandall, 1999

History of Introduction and Spread

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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).

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.

Introductions

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Introduced toIntroduced fromYearReasonIntroduced byEstablished in wild throughReferencesNotes
Natural reproductionContinuous 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 Introduction

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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.

Habitat

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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 List

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CategoryHabitatPresenceStatus
Brackish
Estuaries Present, no further details
Freshwater
Irrigation channels Present, no further details Harmful (pest or invasive)
Lakes Present, no further details
Ponds Present, no further details Harmful (pest or invasive)
Reservoirs Present, no further details
Rivers / streams Present, no further details
Littoral
Coastal areas Present, no further details
Terrestrial-natural/semi-natural
Riverbanks Present, no further details
Wetlands Present, no further details

Biology and Ecology

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Genetics

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.
 
Reproductive Biology
 
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, though production of viable seeds is uncertain in Europe (OEPP/EPPO, 2007).     
 
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).       
 
Environmental Requirements
 
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).

Climate

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ClimateStatusDescriptionRemark
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 Ranges

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Latitude North (°N)Latitude South (°S)Altitude Lower (m)Altitude Upper (m)
40-60 20-50 0 0

Air Temperature

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Parameter Lower limit Upper limit
Mean maximum temperature of hottest month (ºC) 30 0
Mean minimum temperature of coldest month (ºC) -6 0

Rainfall

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ParameterLower limitUpper limitDescription
Mean annual rainfall1003000mm; lower/upper limits

Notes on Natural Enemies

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Few natural enemies of C. helmsii are reported in the literature. However, surveying for natural enemies has been started by CABI, for more information see 'Hunting for natural enemies of New Zealand pigmyweed'.

Means of Movement and Dispersal

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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). The spread of the plant by livestock may be a significant contributor to its continued spread (Wicks, 2004).    
    
Accidental Introduction
 
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.
 
Intentional Introduction
 
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 Causes

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CauseNotesLong DistanceLocalReferences
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

Impact Summary

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CategoryImpact
Cultural/amenity Negative
Economic/livelihood Negative
Environment (generally) Negative

Economic Impact

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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 Impact

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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).   

Threatened Species

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Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
Damasonium alismaNo details No detailsUKCompetitionOEPP/EPPO, 2007; Watson, 2001
Triturus cristatusNo DetailsEngland and WalesLangdon et al., 2004; OEPP/EPPO, 2007

Social Impact

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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 Factors

Top 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
Impact outcomes
  • 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
Impact mechanisms
  • Competition - monopolizing resources
  • Competition - shading
  • Competition - smothering
  • Rapid growth
Likelihood of entry/control
  • 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

Uses

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Economic Value

C. helmsii is sold in garden centres and nurseries as a submerged oxygenating plant for aquariums and water garden ponds (OEPP/EPPO, 2007). Overall, its economic value is minor, and alternative non-invasive aquarium and pond species are readily available.
 
Environmental Services
 
In Europe, C. helmsii flowers late into the winter and may provide a useful source of nectar (Lockton, 2009).

 

Uses List

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General

  • Botanical garden/zoo
  • Pet/aquarium trade

Detection and Inspection

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Infestations of aquatic invasive species are often first reported at boat launches and public access points and these areas should be monitored frequently in order to eradicate or control new invasions at an early stage. Users should inspect all recreational equipment before leaving any water body, and any visible plants, animals, or sediment should be removed. In addition, rinsing gear with hot water or steam may help in removing any additional non-visible organisms. 
 

Similarities to Other Species/Conditions

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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 Control

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Prevention

As with all invasive species management, prevention is better and more cost-effective than control.
 
Rapid Response
 
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). 
 
Public Awareness
 
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.    
 
Control
 
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.
 
Physical/mechanical control
 
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.    
 
Movement control
 
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.
 
Biological control
 
There are no known biological control agents for C. helmsii; more information on the plant in its native range needs to be collected (Gassmann et al., 2006). Work has been started by CABI on surveying for natural enemies, for more information see 'Hunting for natural enemies of New Zealand pigmyweed'. 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.
 
Chemical control
 
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). 

References

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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

Asovsky MG, 1981. Rare finds for east Siberia of littoral-aquatic and aquatic plants on the Baikalo-Amur Route. Botanicheskii Zhurnal (Lningr.), 66:1218-1220.

Bridge T, 2005. Controlling New Zealand pygmyweed Crassula helmsii using hot foam, herbicide and by burying at Old Moor RSPB Reserve, South Yorkshire, England. Conservation Evidence, 2:33-34.

CAPM-CEH, 2004. Centre for Aquatic Plant Management, Center for Ecology & Hydrology. Wallingford, UK. http://www.capm.org.uk

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, 1996. Crassula helmsii: attempts at elimination using herbicides. In: Hydrobiologia, 340(1/3) [ed. by Caffrey, J. M.\Barrett, P. R. F.\Murphy, K. J.\Wade, P. M.]. 241-245.

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.

Dawson FH, Warman EA, 1987. Crassula helmsii (T. Kirk) Cockayne: is it an aggressive alien aquatic plant in Britain? Biological Conservation, 42(4):247-272.

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

Environment Agency, 2003. Guidance for the control of invasive weeds in or near fresh water. Bristol, UK: Environment Agency.

EPPO, 2014. PQR database. Paris, France: European and Mediterranean Plant Protection Organization. http://www.eppo.int/DATABASES/pqr/pqr.htm

Gassmann A, Cock MJW, Shaw R, Evans HC, 2006. The potential for biological control of invasive alien aquatic weeds in Europe: a review. Hydrobiologia [11th International Symposium on Aquatic Weeds 'Macrophytes in Aquatic Ecosystems: From Biology to Management', Moliets et Maâ, France, 2002.], 570:217-222.

Habitas, 2009. Crassula helmsii, New Zealand Pigmyweed. Holywood, Ireland: National Museums Northern Ireland. http://www.habitas.org.uk/invasive/species.asp?item=4639

IPNI, 2008. The International Plant Names Index. http://www.ipni.org/

Keeley JE, 1998. CAM photosynthesis in submerged aquatic plants. The Botanical Review, 64(2):121-175.

Kelly J, Maguire CM, 2009. New Zealand Pigmyweed (Crassula helmsii) Invasive Species Action Plan. Prepared for NIEA and NPWS as part of Invasive Species Ireland.

Langdon SJ, Marrs RH, Hosie CA, McAllister HA, Norris KM, Potter JA, 2004. Crassula helmsii in UK ponds: effects on plant biodiversity and implications for newt conservation. Weed Technology, 18:1349-1352.

Lange PJde, Heenan PB, Keeling DJ, Murray BG, Smissen R, Sykes WR, 2008. Biosystematics and conservation: a case study with two enigmatic and uncommon species of Crassula from New Zealand. Annals of Botany, 101(6):881-899. http://aob.oxfordjournals.org/cgi/content/full/101/6/881

Leach J, Dawson H, 1999. Crassula helmsii in the British Isles - an unwelcome invader. British Wildlife, 10:234-239. http://uk.geocities/jjleach@btinternet.com/invasiveweeds/crassula/BRITWILD.DOC

Lockton AJ, 2009. Botanical Society of the British Isles. Crassula helmsii. http://species.bsbi.org.uk/html/crassula_helmsii.html

MDNR, 2006. Minnesota Invasive Species Laws. St Paul, Minnesota: Minnesota Department of Natural Resources. http://www.dnr.state.mn.us/ecological_services/invasives/laws.html

Minchin D, 2008. Crassula helmsii. Delivering Alien Invasive Species Inventories for Europe (DAISIE). http://www.europe-aliens.org/pdf/Crassula_helmsii.pdf

NOBANIS, 2005. Crassula helmsii (Crassulaceae)., Denmark: North European and Baltic Network on Invasive Alien Species.

NZPCN, 2005. Crassula helmsii. New Zealand Plant Conservation Network. http://www.nzpcn.org.nz/nz_threatenedplants/detail.asp?PlantID=254

OEPP/EPPO, 2004. EPPO Reporting Service 2004, No. 3. Paris, France.

OEPP/EPPO, 2006. Crassula helmsii (Kirk) Cockayne. Report of Pest Risk Analysis. http://www.eppo.org/QUARANTINE/Pest_Risk_Analysis/PRAdocs_plants/06-12801%20PRA%20report%20CSBHE.doc

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Sheppard AW, Shaw RH, Sforza R, 2006. Top 20 environmental weeds for classical biological control in Europe: a review of opportunities, regulations and other barriers to adoption. Weed Research (Oxford), 46(2):93-117. http://www.blackwell-synergy.com/servlet/useragent?func=showIssues&code=wre

Stace C, Meijden Rder, Kort Ide, 2005. Interactive Flora of NW Europe. http://ip30.eti.uva.nl/bis/flora.php

Stevens PF, 2012. Angiosperm Phylogeny Website. St Louis, Missouri, USA: University of Missouri and Missouri Botanical Garden. http://www.mobot.org/MOBOT/research/APweb/

USDA-GRIN, 2002. Germplasm Resources Information Network. Beltsville, Maryland, USA: National Germplasm Resources Laboratory. http://www.ars-grin.gov

USDA-NRCS, 2006. The PLANTS Database Version 3. Baton Rouge, USA: National Plant Data Center. http://plants.usda.gov

Watson WRC, 2001. An unwelcome invader!. Worcestershire Record, issue 10. http://www.wbrc.org.uk/WorcRedc/Issue10/invader.htm

WDNR, 2009. Wisconsin Department of Natural Resources. Chapter NR 40. http://dnr.wi.gov/invasives/classification/NR40.htm

Weber E, 2003. Invasive Plant Species of the World. A Reference Guide to Environmental Weeds. Wallingford, UK: CABI Publishing.

Wicks D, 2004. War against Crassula helmsii. In: 13th International Conference on Aquatic Invasive Species, 20-24 September, 2004, Ennis, County Clare, Ireland.

Winterton S, Scher J, 2007. Aquarium and Pond Plants of the World, Edition 2.0, Lucid v. 3.4. USA: USDA/APHIS/PPQ Center for Plant Health Science and Technology, North Carolina State University, and California Department of Food and Agriculture. http://www.lucidcentral.org/keys/aquariumplants2/

Contributors

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16/12/09 Original text by:

Michelle Nault, Wisconsin Department of Natural Resources, 2801 Progress Rd, Madison, WI 53716-3339, USA

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