Invasive Alien Species in Northern Ireland

Lagarosiphon major, Curly waterweed

Lagarosiphon major (Ridley) Moss
Curly waterweed is also known as curly water thyme. Its correct scientific name is Lagarosiphon major but is often sold in aquatic garden centres under the incorrect name of Elodea crispa (there is no such species). It can be an aggressive weed in still or slow–flowing waters and has tended to increase at the expense of two other alien submerged aquatics, Canadian pondweed and Nuttall’s pondweed (Elodea canadensis and E. nuttallii). At present this is a rare species in the wild in Ireland but climate warming could encourage an increase in abundance at the expense of native flora.
A totally submerged aquatic with rhizomes rooted in the bottom mud and shoots which reach the water’s surface – these can grow in waters up to a depth of around three metres or more and still reach up to the surface. It is somewhat similar, superficially, to Elodea species, but they have whorls of leaves while Lagarosiphon major has leaves which are spirally arranged on the lower part of the stem. These leaves are more strongly recurved and the shoots are much more robust than in Elodea. Flowers are somewhat similar to those of Elodea and are unisexual, borne on separate male and female plants. Only female plants are recorded from Ireland and Great Britain.
Country of origin
Southern Africa.
Current distribution
Now fairly frequent across the southern half of Great Britain (first report 1944); also on the European continent, New Zealand and Australia.
Location in Ireland
First recorded in Ireland in the 1980s and still rather rare, scattered across the island, but gradually increasing.
Life cycle
Only female plants have been recorded in Ireland, so seed production plays no part in dispersal which is entirely by means of fragmentation of the shoots or rhizomes.
Wildlife and habitat impacts
Lagarosiphon produces dense pure growths in suitable conditions and can thus shade out other aquatic plant species and damage associated invertebrate communities. It is particularly adapted to growth in alkaline waters such as those found in limestone areas and can locally affect the water’s pH, making it even more alkaline to such a degree that photosynthesis by other plants is inhibited.
The worst experiences with this species have occurred in New Zealand, where aquatic habitats have been severely and probably irreversibly damaged. In Ireland it has recently spread into Lough Corrib which has unique assemblages of aquatic invertebrates and flora.
Human impacts
Curly waterweed can cause choking of canals and lakes. It is reported to have blocked the intakes of hydroelectric power plants in New Zealand, where the plant has proven very difficult to eradicate. In Ireland there is concern about its potential impact in Lough Corrib which supports economically significant recreational fishing as part of the local tourism industry.
On a minor scale, the plant can become a nuisance in ornamental garden ponds, completely taking over from all other plants and even filling the entire water basin completely.
Key vectors
Birds, other animals, boats, fishing equipment, waders can all spread fragments of this plant to potential new sites of infestation.
What you can do as an individual
To avoid accidentally spreading this or any other aquatic plant always thoroughly clean your boat, trailer, engine, angling equipment and waders after use.
Avoid growing this plant in garden ponds. If you do have it, make sure any excess is not introduced into any watercourse. Never dispose of excess material in the vicinity of any watercourse – dispose of it by composting, burying or burning.
Avoid disturbing or breaking up established growths of this plant – broken fragments will float away and form new colonies.
Advice to specific groups relevant to this species
Managers and users of waterbodies should be aware of the ease with which this plant can be transferred to new sites by accidental transport of fragments of the plant.
Gardeners should avoid purchasing this plant for garden ponds (probably the original source of infestation in Ireland). The Royal Horticultural Society recommends avoiding this species. Take no notice of any ‘expert’ advice that this is an essential ‘oxygenator’ – so–called ‘oxygenators’ are unnecessary in garden ponds.
Management measures
New Zealand has the greatest experience of dealing with infestations of this species. Methods used (none are completely successful) include physical removal by hand or machine, use of herbicides and suction dredging.
In the UK, the Centre for Aquatic Plant Management at Wallingford, Oxon, has also provided information on control methods in its Lagarosiphon major information sheet.
Management groups
In Northern Ireland a number of bodies are responsible for various aspects of watercourse and water body management, water quality, recreational use and aquatic habitat management. These include the Rivers Agency, the Water Service, Environment and Heritage Service and Waterways Ireland.
Further information
Centre for Aquatic Plant management – Information Sheet on Lagarosiphon major
Republic of Ireland’s National Plant Conservation Strategy at of Ireland’s Central Fisheries Board – brochure on curly waterweed
Management of Lagarosiphon major in the Waitaki catchment (New Zealand) for the Environment, New Zealand. Lake Managers’ Handbook – alien invaders 2002 Horticultural Society advice on invasive alien water weeds
McKee, D. et. al. (2002). Effects of simulated climate warming on macrophytes in freshwater microcosm communities. Aquatic Botany 74(1): 71–83.
(Abstract on line at cpsidt=13816587)
See the Invasive Species Ireland web site for further information -
Text written by:
Paul Hackney, Keeper of Botany, Ulster Museum