Short description of Crassula helmsii, New Zealand Pigmyweed

New Zealand pygmyweed is a perennial with yellowish-green opposite ± succulent leaves < 20mm long and solitary white or pale pink flowers on pedicels >2mm in the leaf axils. Seed form found in GB but  may not be viable.

Impact summary: Crassula helmsii, New Zealand Pigmyweed

New Zealand pygmyweed may cover small ponds to a depth of 0.5m or cover the margins and bed of larger deeper waterbodies over many square metres. It can prevent recreational and commercial activities and may cause extensive declines in native plants.

Habitat summary: Crassula helmsii, New Zealand Pigmyweed

New Zealand pygmyweed grows in ponds, lakes, reservoirs, canals and ditches as well as on damp mud on the margins of ponds and reservoirs. It tolerates a wide range of conditions, from basic to acidic and oligotrophic to eutrophic.

Overview table

Environment	Freshwater
Species status	Non-Native
Native range	New South Wales, South Australia, Tasmania, Victoria, New Zealand North, New Zealand South
Functional type	Land plant
Status in England	Non-Native
Status in Scotland	Non-Native
Status in Wales	Non-Native
Location of first record	v.c.18 (Greensted)
Date of first record	1956

Origin

New Zealand pygmyweed is native to coastal regions of southern Australia, Tasmania and New Zealand.

First Record

New Zealand pygmyweed was first recorded in the wild in GB at Greensted Pond in Essex in 1956.

Pathway and Method

New Zealand pygmyweed was brought to GB from Tasmania in 1911 for sale as an “oxygenating plant” for ponds. The first dispersal into the wild was probably either by natural vectors from garden ponds or when the contents of aquaria or ponds were emptied out. Its continued spread throughout GB is almost certainly mainly by movement of vegetative fragments on boats, machinery used to manage water bodies, clothing and possibly wildfowl. It is possible that there are still some new introductions by people discarding pond plants.

Species Status

New Zealand pygmyweed is now present in Belgium, Denmark, Germany, the Netherlands, Portugal, Spain and the Baikal region of Russia. It is very local in France, Portugal and Spain where it is not yet considered invasive, however it appears to be spreading.

Dispersal Mechanisms

Dispersal in GB is almost certainly entirely by vegetative fragments, of which even a single node on 10mm of stem can root to form a new plant. It flowers abundantly and produces seed, but attempts to germinate these have failed and they may not be viable. In addition, turions; short shoots with very short internodes, which break off easily, are produced in the autumn and as these float, they are an effective means of colonization within wetland systems. It is likely that dispersal to new areas is mainly anthropogenic, by movement of vegetative fragments on boats, machinery used to manage water bodies, clothing and possibly wildfowl

Reproduction

The available evidence would suggest that reproduction of New Zealand pygmyweed in GB and elsewhere in Europe is vegetative, through fragmentation.

Known Predators/Herbivores

No species are known to preferentially graze C. helmsii, but grass carp (Ctenopharyngodon idella) will eat it.

Resistant Stages

None known.

Habitat Occupied in GB

New Zealand pygmyweed typically grows in ponds, lakes and reservoirs but will also grow in canals and ditches and will form extensive colonies on damp mud such as on the margins of ponds and the drawdown zones of reservoirs. It tolerates a range of pH, from acid to alkaline and even slightly salty water.

New Zealand pygmyweed is widespread and abundant throughout most of England, particularly the south, as well as Cumbria and scattered localities in Wales, the Isle of Man, Scotland and eastern Ireland.

Environmental Impact

Except in deep water, New Zealand pygmyweed tends to form dense mats, from 0.5m above water to depths of 3m under water, which apparently shade out other plants. These can also apparently cause oxygen depletion of the underlying water leading to a decline in invertebrates, frogs, newts and fishes. There is evidence to suggest that many plants (such as Luronium natans and Limosella aquatica) can either grow through deep-submerged stands, or survive and reproduce between mats growing on damp mud. However, in some areas such as the New Forest, it is highly likely that the increase of New Zealand pygmyweed has caused the decline of a number of rare taxa.

Health and Social Impact

The main reported health and social impacts of New Zealand pygmyweed involve disruption of water-based recreation and adverse effects on aesthetics and recreation associated with water bodies. There are also occasional reports of death of pets which mistake carpets of New Zealand pygmyweed for land, jump onto the surface and are then unable to get out of the water and drown.

Economic Impact

New Zealand pygmyweed may have adverse economic impacts where it forms dense mats in shallow water, obstructing water-borne transport, navigation and flood defences.

Identification

Stace, C.A. (2010) New flora of the British Isles, Third Edition, Cambridge University Press, Cambridge.

Lansdown, R.V. (2009) A field guide to the riverine plants of Britain and Ireland, Ardeola, Stroud, Gloucestershire.

Biology, ecology, spread, vectors

Preston, C.D. & Croft, J. (1994) Aquatic plants in Brtain and Ireland, Harley Books, Colchester.

Management and impact

Dawson, F.H. (1996) Crassula helmsii: Attempts at elimination using herbicides. Hydrobiologia, 340(1-3), 241-245.

Dawson, F.H. (1994) Spread of Crassula helmsii in Britain. In de Waal, L.C., Child, L.E., Wade, P.M. & Brock, J.H. (eds.), Ecology and Management of Invasive Riverside Plants, J. Wiley, New York.

General

Preston, C.D. & Croft, J. (1994) Aquatic plants in Brtain and Ireland, Harley Books, Colchester.

https://www.cabi.org/isc/datasheet/16463

https://secure.fera.defra.gov.uk/nonnativespecies/downloadDocument.cfm?id=1742