Invasive species are a huge problem, internationally and within Ireland. They have had a dramatic impact on our biodiversity, economy, countryside and quality of life. Invasive species pose a massive threat to the survival of native species. Learn more about the species threatening Irish biodiversity below.
Japanese Knotweed
IDENTIFICATION
Japanese knotweed (Fallopia japonica) leaves are curved, with a pointed tip and a flat-base. They are a lush green color and are arranged in a zig zag pattern. In summer, the plant forms a thick canopy 3-4m high and by early autumn, has produced many small, creamy-white flowers.
The mature stems are hollow, purple speckled and have nodes on them, like bamboo.
The roots can be up to 10cm thick, with white shoots. When cut, they are bright orange and snap like a carrot.
The above-ground plant parts are very frost sensitive and die back in late autumn, but the root system can live for many years.
THREAT
Japanese knotweed grows so vigorously that it can push through tiny cracks in foundations, paving, and tarmac and cause massive structural damage to buildings. In the UK, people can be refused mortgages because the plant is on their property!
It has massive underground root (rhizome) system, much larger than what you see growing above the surface. It can survive lack of sunlight, drought and extreme heat. Mature plants grow into immense stands that block the growth of native species, severely reducing biodiversity. This species has spread throughout Ireland extremely rapidly.
ACTION
If you find Japanese knotweed, DO NOT CUT OR BREAK it! Stem and root fragments can easily regrow as new plants.
DO NOT TRANSPORT Japanese knotweed anywhere. It is illegal to move knotweed waste except to bring it to a licensed waste facility that has been given prior notification.
Any eradication or control of Japanese knotweed must be undertaken by a reputable invasive species control company. Poorly planned treatments will increase the plant’s resistance to future control methods!
Other Knotweeds
IDENTIFICATION
Giant knotweed has very large leaves, up to 40cm long, with a heart shaped base.
Bohemian knotweed (hybrid) has leaves up to 25cm long, with a slightly curved base. Its leaf looks midway between that of a Japanese and giant knotweed leaf.
Himalayan knotweed has lance-shaped leaves, up to 20cm long. They have a red mid-vein. The flowers of Himalayan knotweed are held on red stalks.
THREAT
There are three other types of knotweed present in Ireland. They all pose the same threats as Japanese knotweed. The three types are: Giant knotweed (Fallopia sachalinensis); Bohemian knotweed (Fallopia x bohemica), which is a hybrid between Giant and Japanese knotweed; and Himalayan knotweed (Persicaria wallichii). The various types can be identified by their different leaf shapes.
ACTION
If you find Japanese knotweed, DO NOT CUT OR BREAK it! Stem and root fragments can easily regrow as new plants.
DO NOT TRANSPORT Japanese knotweed anywhere. It is illegal to move knotweed waste except to bring it to a licensed waste facility that has been given prior notification.
Any eradication or control of Japanese knotweed must be undertaken by a reputable invasive species control company. Poorly planned treatments will increase the plant’s resistance to future control method.
Giant Hogweed
IDENTIFICATION
Giant hogweed (Heracleum mantegazzianum) is a massive 2 – 5m tall plant with leaves more than 1m wide. The leaves are divided and sharply pointed with soft hairs on the underside.
Its stems are large and green with reddish, purple spots. They are ribbed lengthways and covered in bristles.
The flowers open from June to August. They are very small and white, and grouped together in a shape like an umbrella (similar to cow parsley, only much larger).
The plant dies back in early autumn each year but can leave brittle stems behind.
THREAT
Giant hogweed sap is dangerous to humans. If left on the skin and exposed to sunlight, it can cause massive blistering. Even 7 years later, exposure to the sun can trigger new blisters.
Giant hogweed forms huge stands that block the sunlight from our native plant species, greatly reducing native biodiversity. A single giant hogweed plant can produce up to 50,000 seeds, which disperse by wind, water and human activities.
ACTION
DO NOT TOUCH giant hogweed! The sap can cause severe skin damage! Everyone operating in infested areas should ensure they are wearing the appropriate PPE to protect from chemical burns.
If sap comes into contact with the skin, WASH with soap and water A.S.A.P. and keep the area away from sunlight for at least 48h. If the sap comes into contact with the eyes, or a reaction occurs, seek immediate medical advice.
Himilayan Balsam
IDENTIFICATION
Himalayan balsam (Impatiens glandulifera), AKA Indian Balsam, Policeman’s Helmet, can grow up to 3m tall. It flowers from late May to October.
The pink/purple bonnet shaped flowers are 2.5 – 4cm long.
The leaves are 6 – 15cm long, lance shaped, with sharply toothed edges and have a reddish mid-rib. Seed pods are produced from July to October. They are 1.5 – 3.5cm long and explode when touched.
Himalayan balsam roots are reddish and very shallow.
THREAT
This tall, annual plant spreads very quickly and rapidly dominates moist ground. In winter, when the plant dies back, it destabilises riverbanks.
Pollinators, such as bees, prefer to visit Himalayan balsam flowers over our native flowers because they produce a lot of nectar. This leaves native species unlikely to reproduce.
ACTION
Do not touch the plant if seed pods are already present, or they will explode and send seeds flying in all directions!
Small infestations of Himalayan balsam can be controlled by hand-pulling the whole plant, including roots, in April and any new growth in September; or by regular grazing, strimming or the application of herbicides.
For large, riverside infestations, a specialist invasive species control company should be consulted.
Giant Rhubarb
IDENTIFICATION
In summer, giant rhubarb (Gunnera tinctoria) has huge leaves up to 1.5m wide. The leaves are lobed and hairy underneath.
Prickly, reddish leaf stalks and flower heads grow from stems that can be 3.5m long.
From April to October, flowers and fruits appear. Flower heads (3 – 4 per plant) are cone-like, long structures with thousands of small flowers. Small orange/red fruits contain the seed.
Leaves and flowers die back from the beginning of October.
THREAT
Giant rhubarb forms large, dense stands along river banks, lakes, cliffs, roads and urban areas, blocking the sunlight from native species and badly affecting biodiversity.
Giant rhubarb grows so thickly that it often obstructs access to recreational areas and its huge leaves can block drains and streams and increase the risk of flooding.
When the plant dies back in winter, its decaying leaves give off a strong, rotting smell.
ACTION
Remove any flower heads as early as possible as most seeds produced will germinate.
Giant rhubarb can regrow from tiny fragments of over- or under-ground stems. Please consult a specialist before proceeding with a control program as incorrect action is likely to lead to spread.
New Zealand Pigmyweed
IDENTIFICATION
Small green weed with leaves (4mm – 2cm long) arranged opposite each other on rigid, round, fleshy stems.
New Zealand pigmyweed (Crassula helmsii), AKA Australian Swamp Stonecrop, can grow in three forms: on land, emergent (coming out of the water) and submerged (permanently under water).
The leaves have a sharp tip. Flowers with 4 white petals appear between Jul – Sep.
THREAT
New Zealand pigmyweed is a hardy plant that causes major problems in nature reserves and recreation areas by forming 100% cover and smothering and out-competing native flora for nutrients.
It spreads mainly by tiny plant fragments being transferred into new locations, which can then grow into a new plant. These fragments can be easily moved from place to place by boats, uncleaned equipment, or water-birds.
All aquatic weeds present the following threats:
They form thick, dense mats that prevent light from reaching native plants and can be mistaken for solid ground by children and animals.
The thick mats also prevent oxygen from entering the water, drastically reducing oxygen levels, which may kill fish and other aquatic life.
A large infestation greatly reduces access for recreational water activities.
Mats of aquatic weeds often result in a bad smell and reduced visibility in the water.
Dense mats can reduce the flow of water and increase the risk of flooding.
ACTION
Check, clean and disinfect all equipment, boots, wheels and anything that made contact with the water. This will ensure you don’t carry fragments to other possible habitats.
The control of large infestations of aquatic weeds will require the services of an appropriately certified invasive species control company.
Water Fern
IDENTIFICATION
Water fern (Azolla filiculoides), AKA Fairy Fern, is a very distinctive looking, small, floating, moss-like plant. The fern-like leaves are a bright green color with a thin red border. Water runs off the surface of the leaf.
The roots of water fern hang down into the water.
In winter or when exposed to stress, for example shade or low temperatures, whole leaves become red.
THREAT
The free-floating water fern grows extremely fast and forms dense, up to 20 cm thick mats on canals, ponds, slow-flowing rivers, ditches, and wetlands.
All aquatic weeds present the following threats:
They form thick, dense mats that prevent light from reaching native plants and can be mistaken for solid ground by children and animals.
The thick mats also prevent oxygen from entering the water, drastically reducing oxygen levels, which may kill fish and other aquatic life.
A large infestation greatly reduces access for recreational water activities.
Mats of aquatic weeds often result in a bad smell and reduced visibility in the water.
Dense mats can reduce the flow of water and increase the risk of flooding.
ACTION
Check, clean and disinfect all equipment, boots, wheels and anything that made contact with the water. This will ensure you don’t carry fragments to other possible habitats.
The control of large infestations of aquatic weeds will require the services of an appropriately certified invasive species control company.
Curly Waterweed
IDENTIFICATION
Curly waterweed, AKA African Curly Waterweed, grows underwater and is attached to the bottom by roots. Its leaves are arranged in spirals along the stem and are very curled. The stem is hollow and brittle, and can grow up to 6.5m long.
The leaves are green, with a yellowish tinge. In summer, the weed produces small flowers with 3 white or pink petals. Curly waterweed can be mistaken for Nuttall´s or Canadian waterweed but these have their leaves arranged in a circle (whorl) around the stem.
The leaves form a cluster at the top of the stem, giving the plant a distinctive look.
THREAT
Can form dense mats up to 2 – 3m thick.
Curly waterweed is very difficult to control, and its ability to form new plants from small fragments means that it is easily spread to new locations.
All aquatic weeds present the following threats:
They form thick, dense mats that prevent light from reaching native plants and can be mistaken for solid ground by children and animals.
The thick mats also prevent oxygen from entering the water, drastically reducing oxygen levels, which may kill fish and other aquatic life.
A large infestation greatly reduces access for recreational water activities.
Mats of aquatic weeds often result in a bad smell and reduced visibility in the water.
Dense mats can reduce the flow of water and increase the risk of flooding.
ACTION
Check, clean and disinfect all equipment, boots, wheels and anything that made contact with the water. This will ensure you don’t carry fragments to other possible habitats.
The control of large infestations of aquatic weeds will require the services of an appropriately certified invasive species control company.
Nuttall’s & Canadian Waterweeds
IDENTIFICATION
Nuttall´s (Elodea nuttallii) & Canadian (Elodea canadensis) waterweeds are underwater plants attached to bottom by roots. Their thin branching stems can grow 3m or more.
The bright green leaves are arranged around the stem in circles (whorls) of three leaves. The leaves of Nuttall’s waterweed are thin, less than 2mm wide, and slightly wider at the base. They curl back strongly towards the stem.
The flowers peek out of the water between May and October. They have three small white petals and are held on long, thread like stems.
Canadian waterweed is also a high-risk invasive species in Ireland. Although it can be confused with Nuttall’s waterweed and its flowers are similar, the leaves of Canadian waterweed are tongue-shaped, much broader in general (up to 4 mm) and not as curled as those of Nuttall’s waterweed.
THREAT
Nuttall’s and Canadian waterweed can tolerate low light conditions and are difficult to control due to their high growth rates.
All aquatic weeds present the following threats:
They form thick, dense mats that prevent light from reaching native plants and can be mistaken for solid ground by children and animals.
The thick mats also prevent oxygen from entering the water, drastically reducing oxygen levels, which may kill fish and other aquatic life.
A large infestation greatly reduces access for recreational water activities.
Mats of aquatic weeds often result in a bad smell and reduced visibility in the water.
Dense mats can reduce the flow of water and increase the risk of flooding.
ACTION
Check, clean and disinfect all equipment, boots, wheels and anything that made contact with the water. This will ensure you don’t carry fragments to other possible habitats.
The control of large infestations of aquatic weeds will require the services of an appropriately certified invasive species control company.
Parrot’s Feather
IDENTIFICATION
Parrot´s feather, AKA Milfoil, is a very distinctive looking plant that can grow up to 30cm above the water’s surface, and up to 2m below. It can also invade muddy banks. The above-water plant dies back in winter, but is found submerged throughout the year.
Leaves are arranged in circles (whorls) of 4 – 6 leaves, and are grey-green and feathery. The leaves above the water’s surface are stiffer than those below. Stems are brittle, with brown roots.
THREAT
Parrot’s feather (Myriophyllum aquaticum) can survive in a variety of conditions and can also grow onto wet mud banks.
All aquatic weeds present the following threats:
They form thick, dense mats that prevent light from reaching native plants and can be mistaken for solid ground by children and animals.
The thick mats also prevent oxygen from entering the water, drastically reducing oxygen levels, which may kill fish and other aquatic life.
A large infestation greatly reduces access for recreational water activities.
Mats of aquatic weeds often result in a bad smell and reduced visibility in the water.
Dense mats can reduce the flow of water and increase the risk of flooding.
ACTION
Check, clean and disinfect all equipment, boots, wheels and anything that made contact with the water. This will ensure you don’t carry fragments to other possible habitats.
The control of large infestations of aquatic weeds will require the services of an appropriately certified invasive species control company.
Zebra Mussel
IDENTIFICATION
Zebra mussels (Dreissena polymorpha) are small, freshwater mussel with a triangular shape, usually 2 – 5cm long. They often have yellowish and brown zigzag banding but colours can vary a lot.
Zebra mussels attach to hard surfaces by sticky threads. They have a flat underside such that when you put them onto a flat surface they lie steady, rather than rolling around.
Rarely seen alone, zebra mussels often occur in huge numbers.
THREAT
After one year, a female zebra mussel can produce up to 1 million eggs. The larvae are free swimming for several weeks, and then settle on any hard surface under water.
When they are present in large numbers zebra mussels filter the water so effectively that there is little food left for native aquatic animals.
Zebra mussels settle in such large numbers that they can clog intake pipes and completely cover underwater objects, including boat hulls and our native mussel species.
ACTION
Inspect and clean all equipment like boats, waders, engines etc. with high pressure hot water and/or an appropriate disinfectant before and after use in lakes and waterways. Unclean equipment can result in the spread of invasive species into different catchment areas.
Proper control of a zebra mussel infestation will require the services of an appropriately certified invasive species control company.
Asian Clam
IDENTIFICATION
Small, freshwater clams usually less than 2.5cm wide. Their shells are olive to brown with deep concentric rings. The colour can flake off, leaving white patches on the shell surface.
Juvenile Asian clams (Corbicula fluminea) are smaller again. Note that the inner clamshells range in colour from white to a deep purple.
THREAT
Asian clams can produce up to 70,000 young per year. They are hermaphrodites (both male and female) so they can reproduce even if only one enters a location.
In Ireland, Asian clams have been recorded in densities of over 10,000 per m2. In high concentrations, they can filter the water so efficiently that native fish and clams starve because of reduced phytoplankton concentrations.
ACTION
Inspect and clean all equipment like boats, waders, engines etc. with high pressure hot water and/or an appropriate disinfectant before and after use in lakes and waterways. Unclean equipment can result in the spread of invasive species into different catchment areas.
Proper control of an Asian clam infestation will require the services of an appropriately certified invasive species control company.