Invasive Alien Species in Northern Ireland

Crassula helmsii, New Zealand Pigmyweed

Click on the thumbnails to enlarge the images
Crassula helmsii
© Graham Day
Crassula helmsii
© Graham Day
Crassula helmsii
Click on map to enlarge
Crassula helmsii (Kirk) Cockayne, (Hook.f.) Ostenf. non N.E.Br., (Hook.f.) Hook.f.
Australian swamp stonecrop is an aquatic or semi-aquatic species introduced into cultivation in England in 1911, but not reported from the wild in Great Britain until 1956. It was first reported from the wild in Ireland in 1985. It has been sold as an oxygenating plant for garden ponds since at least the 1920s in England, but its use in this capacity seems more recent in Ireland.
Its English names are Australian swamp stonecrop and New Zealand pigmy weed. It is sometimes sold by garden centres under the incorrect name Tillaea recurva.
The plant grows on the muddy margins of ponds where it forms carpets with 100% cover, or semi-submerged in deeper water, or totally submerged with elongated stems. It does not die back in winter.
The shoots are rather stiff, carrying narrow parallel-sided leaves in opposite pairs, each leaf being about 4 - 24mm. Small white flowers with four petals are produced in summer on long stalks arising from the upper leaf axils. The flowers are always above water.
The plant is able to absorb carbon dioxide at night and store it for use by day in photosynthesis.
Country of origin
Australian swamp stonecrop is native to New Zealand and Australia. The first introduction to England is thought to have come from Tasmania in 1911. It is not known if all the plants sold subsequently came from the same source or whether it has been repeatedly re-introduced from different geographical sources, or even if the Irish plants are ultimately from the same source as those in Great Britain.
Location in Ireland
The first notice of this species in Ireland was in 1984 from an artificial pond in Gosford Forest Park, County Armagh, where it had probably been deliberately planted. The first report from a wild habitat dates from 1985 from the flooded Glastry Clay Pits in the Ards Peninsula, County Down. These pits are managed by the National Trust as an educational nature reserve. Altogether there are now (March 2005) reports from around seven 'wild' sites, all in the east of Northern Ireland. The number of garden ponds containing this species is unknown.
In the Republic of Ireland, the recent New Atlas of the British & Irish Flora shows its presence in three 10 x 10 km squares in the east of the country.
Life cycle
Australian swamp stonecrop appears to be spread entirely by vegetative means, including fragmentation and the production of special short shoots called turions which break off and float away. Colonisation of uninfected waterbodies can be by deliberate or accidental dumping of plant fragments, such as the unwanted excess from garden ponds. The plant is winter green, enabling it to outpace native species which die back each winter. According to EPPO (European & Mediterranean Plant Protection Organisation), the plant does not produce viable seed in the UK.
Wildlife and habitat impacts
There have been relatively few systematic studies of the impact on wildlife but these have focused mainly on two possible effects: the effect on the breeding success of amphibians and the effect on the native flora and vegetation of infected waterbodies.
Some English studies have looked at the effect on the breeding of species of newts, but these suggest that the impact on the smooth newt (the only species in Ireland) is not obviously detrimental. No adverse effect on frog breeding has been detected.
The impact on flora is not easily predictable. A study in NW England suggests that there is no net reduction of the numbers of plant species, but there is a reduction of germination rates of native species, an increase in the proportion of emergent or marginal species and a reduction in aquatic species of open water. Smaller marginal plants such as some water starworts (Callitriche spp.) seem bound to be smothered, and competition for space seems likely to cause a reduction in stoneworts (charophytes).
There are other possible implications for wildlife one study in England has shown a significant reduction in the population of a diatom Synedra delicatissima caused by Crassula helmsii , although the precise mechanism of this impact is unclear. Since freshwater algae provide food for many invertebrates, this kind of effect may have a serious impact on freshwater invertebrate populations.
Human impacts
None known.
Key vectors
The precise origins of the colonies in Northern Ireland are unknown, but are assumed to be from discarded plant material from garden ponds. Further spread by animal agency such as wading birds is a possibility but unproven. Accidental spread by human agency - on fishing kit, waders etc. is also a theoretical possibility. Even very small fragments of the plant are apparently capable of developing new colonies.
What you can do as an individual
The best action is to refrain from purchasing or growing this plant in your own garden pond. If you do have it, take steps to exterminate it. Do not dump excess or unwanted material into streams, ponds or lakes, or into any dump or landfill site, or into your domestic refuse bin. Burn or compost any excess material. If you see the plant for sale in a garden centre or aquaculture centre, bring the dangers of the plant to the notice of the shop manager, and ask him/her to withdraw it from sale.
Advice to specific groups relevant to this species
Gardening societies should be aware of the undesirable nature of this plant. The Royal Horticultural Society, Britain's leading gardening organisation, actively discourages the sale or use of this species.
Management measures
For detailed advice on how to exterminate this species, see the Centre for Aquatic Plant Management web site (information sheet 11). It is important to note that attempts at mechanical or physical removal are bound to fail unless the infestation is very small and that there are only two other options: the use of a weedkiller, or blanketing the pond off for six months with a light-proof cover. There are no known agents of biological control; fish do not normally eat the plant.
Further information
Centre for Aquatic Plant Management, CEH, Wallingford - advice on identification and removal
Royal Horticultural Society - general information and advice on invasive alien plants, including Crassula helmsii
Royal Botanic Gardens Kew; information sheet T4 on invasive species
European & Mediterranean Plant Protection Organisation (EPPO)
See the Invasive Species Ireland web site for further information -
Text written by:
Paul Hackney, Keeper of Botany, Ulster Museum