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The Most Unwanted

Last updated: 4 August 2011

Zebra Mussels

picture of Zebra MusselsThe zebra mussel is a stripey, freshwater mussel native to Eastern Europe.
They spread naturally in water currents within connected lakes and river systems.
Outside connected waterways they are spread mainly by recreational activities such as boating and fishing.
They form large colonies that attach to almost any hard surface such as rocks, boat hulls and jetties.
Northern Ireland is one of the most recent regions to have been invaded by Zebra Mussels.

picture of Zebra Mussel Prevention raising awareness poster

They originate from the Caspian and Black Sea and spread across canal networks in Europe in the late 18th century, reaching England by 1824
They didn’t arrive in Ireland until 1994, and spread rapidly throughout the Shannon-Erne waterway and connected navigable water bodies.
In 2005 they were found in Lough Neagh.


Further details on Zebra Mussels can be found in the following documents:

Japanese Knotweed

picture of Japanese Knotweed leavesJapanese knotweed (.PDF 392.83Kb)Opens in New window, an invasive perennial plant, was first introduced into the UK in the 1820s as an ornamental garden plant.
It was subsequently spread intentionally to support steep banks and to provide screening before the negative impacts associated with it came to be known.
In it's home country of Japan it was an early coloniser of volcanic slopes where it is controlled by natural predators.
It’s natural enemies are not present in Northern Ireland so Japanese Knotweed plants thrive in our environment.
It is a tall ornamental plant growing to heights of 2.5 meters,  forming dense thickets.
Only female Japanese knotweed plants are known to occur in the UK.
It is spread entirely by the movement of plant material or by the movement of contaminated soil material containing fragments of its extensive rhizome (underground roots) system.

It has many direct and indirect impacts on the environment and the economy, ranging from out-competing our native species to causing structural damage to buildings and bridges. It is very difficult to control but there are a few methods available.
Through the ‘Invasive species in Ireland Project'Opens in New window both a best practice guide on controlling Japanese Knotweed and. an information leaflet aimed at assisting gardeners has been produced.

Giant Hogweed

picture of Giant Hogweed plantGiant Hogweed was first introduced as an ornamental garden plant from south west Asia in the 19th century. It is usually found along river banks, wasteland and road sides. It bears a close resemblance to Common Hogweed and Cow Parsley, but it is distinctive due to its huge size, often 3 to 5 metres tall and the leaves can grow up to 1.5 metres in width.It flowers between June and September, producing several thousand seeds per plant. These seeds are then subsequently dropped near the adult plant leading to its gradual spread and invasion of an area.
It is best known to the public for the sap which it produces.
When the sap comes into contact with human skin, in the presence of direct sunlight, it can lead to severe blistering of the skin.
Serious infestations can lead to the closure of public footpaths and recreational areas.
Information on best practice control for Giant HogweedOpens in New window

picture of Nuttall's Pondweed in Lough Erne

Nuttall's Pondweed

Nuttall’s pondweed is an invasive alien species of aquatic plant (macrophyte) that originated from North America. It was first recorded in Northern Ireland in 1984 on the west side of Lough Neagh. In more recent times it has been recorded in Lough Erne where it has grown in vast quantities. The following Nuttall’s Pondweed Commonly Asked Questions Booklet (.PDF 5.08Mb)Opens in New window provides a range of information on the species.


Common Cord Grass – Spartina anglica

picture of Common Cord Grass in Strangford LoughCommon Cord-Grass is an invasive perennial grass that grows mostly on soft muddy or sandy intertidal habitats. It is the product of a hybridisation event that occurred on the south coast of England approximately 140 years ago between the native Small Cord-Grass (Spartina maritima) and the introduced Smooth Cord-Grass Spartina x alterniflora from America.

Common Cord-Grass has tough, pointed leaves, with flower stems that can grow from 50 cm to 1 metre tall. It also forms a dense mat of underground roots which enable it to spread quickly.

It poses a real threat to the development of native saltmarsh, one of our most interesting and rarest habitats, with plant species such as the colourful Sea-Aster, Sea-Pink and Sea-Lavender easily outcompeted by S. anglica. It also grows in areas that are important feeding and roosting grounds for large flocks of wildfowl and wading birds which migrate here to overwinter, greatly reducing available habitat.

The following Common Cord-Grass – Frequently asked Questions leafletOpens in New window provides a range of information on the species.