Invasive Alien Species in Northern Ireland

Lemna minuta, Least duckweed

Lemna minuta Kunth
 
Introduction
The smallest of the duckweeds, only about 2mm across. Found floating in large quantities in lowland ponds, but still relatively rare and only recently arrived in Ireland. It may be under-recorded.
 
Description
This plant consists of a thin plate of green tissue 1-3mm in diameter which floats on the water surface. It resembles a small version of the common Lemna minor (common duckweed). Like lesser duckweed, with which it is often intermingled and which is native to Ireland, this American species can form a continuous green scum across the surface of ponds or lakes.
 
Country of origin
Western United States of America.
 
Current distribution
It is frequent in much of the southern half of Great Britain, but appears to be less frequent in the north or in Ireland.
 
Location in Ireland
It was first found in Ireland in County Cork in 1993 and County Sligo in 1995. Since then it has been reported from a growing number of lowland sites, mainly in the south of the island.
 
Life cycle
Like other species of Lemna, this plant can produce tiny flowers and seed. However, these have never been observed in Ireland and reproduction is by the highly effective process of vegetative budding. L. minuta appears to be able to overwinter successfully, although its prevalence in the more southerly portions of Great Britain and Ireland suggests that it may not overwinter so well further north where winter temperatures are lower.
 
Wildlife and habitat impacts
Like common duckweed it can form a continuous growth across still waters, and is often intermingled with that species. It has a capacity to grow in more shaded conditions than other duckweeds. This continuous scum is a nuisance in ornamental ponds and lakes, but its impact on native plants or wildlife generally is probably fairly minimal. Excessive growth is an indication of eutrophication.
 
Human impacts
The plant can be a visual annoyance to managers or owners of ornamental ponds and lakes. However, they are of high nutrient content and their composted remains may be seen as a potentially valuable horticultural resource.
 
Key vectors
The original introduction into Ireland has probably been in association with ornamental aquatic species in garden ponds. Its escape into wild situations may be through the agencies of birds, the tipping of garden waste into watercourses or natural overflowing of ponds in wet weather.
 
What you can do as an individual
Many garden ponds now have this plant. Dispose of any excess of this plant from garden ponds with care – do not throw or tip it into ditches or other watercourses, or into lakes or ponds.
 
Advice to specific groups relevant to this species
Managers and owners of aquatic plant retail outlets or nurseries, and of ornamental water gardens should be aware of the ease with which this species can be accidentally introduced into natural water bodies, canals and streams.
 
Management measures
The growth of this species (like common duckweed) is encouraged by nutrient enrichment. Measures designed to reduce enrichment are always good practice for a variety of reasons. Duckweeds can be removed mechanically quite effectively by dragging booms or brushes across the water surface, and booms at pond outlets will hinder the escape of floating duckweed species into streams. Herbicides (seek advice on which to use) only work on floating mats of duckweeds if they are only one layer of plants thick — thicker growths found in eutrophic conditions in warm weather will survive such treatments. Compost the excess material removed from ponds or lakes and do not allow it to fall into other ponds, streams or ditches from which it might get into other waterbodies.
 
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
Links
Centre for Aquatic Plant Management. The Robson Meeting 2002 on control of floating plants at www.ceh.ac.uk/sections/documents/RobsonProceedings2002.pdf
Centre for Aquatic Plant Management – Information Sheet on Lemna species
http://www.nerc-wallingford.ac.uk/research/capm/pdf%20files/16%20Duckweeds.pdf
Palomar College (USA). Wayne Armstrong's treatment of the Lemnaceae
http://waynesword.palomar.edu/1wayindx.htm
Osmosis: the SAPS (Science and Plants for Schools) Newsletter no. 5
http://www-saps.plantsci.cam.ac.uk/osmos/os5.htm
Identification of duckweeds: Plant Crib on line at
http://www.bsbi.org.uk/Lemna.pdf
Flora of North America on line
http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=222000206
 
Text written by:
Paul Hackney, Keeper of Botany, Ulster Museum