KNOTWEED: ‘Authorities decided to ignore this issue for years, because of the costs in dealing with it’

Liam Flynn standing amongst Japanese knotweed in Millstreet, Co Cork. Picture: Eddie O'Hare

Sean Ó Riordan talks to Liam O’Flynn who says there are tens of thousands of knotweed plants in the country

THOUGH Government and local authorities had been aware of the knotweed problem for years, they ignored it until quite recently.

That’s the view of Liam Flynn, who has been fighting knotweed for years, and who has seen it spread massively in the last three to four years.

“The authorities have been very slow to act and only decided to do something following concerns raised by some local councillors and other people who were conscious of what was happening on the ground,’’ he said.

Mr Flynn, a farm manager and naturalist in Millstreet, Co Cork, believes there are tens of thousands of infested sites all across the country and it will cost millions to bring the pest under control, never mind eradicate it.

“The reason authorities were so slow to act was probably because of the costs involved in dealing with it. In the UK, there are several companies that specifically control knotweed. Costs start at about £10,000 per site of one-quarter acre,’’ he said.

Japanese knotweed is now on the list of the 100 most invasive species in the world.

“Knotweed is not destructive in its native environment, but in our environment it can cause chaos. Our main concern is its effect on buildings and ground surfaces,’’ he noted.

“Left untreated, it will penetrate the foundations of buildings and walls and its vigorous growth will cause affected areas to subside.

‘’It can penetrate the smallest opening above, or below, ground and will cause walls and kerbs to tilt, or crack. It can emerge through six inches of tarmac. It also crowds out all other competition from other shrubs and plants in its environment.”

Going on his experience of treating sites, he has witnessed knotweed spread in a linear manner along roadsides and riverbanks. He has seen it spread unintentionally from a site, which was being cleared, to another site, due to fragments being moved on the wheels of, possibly, a JCB. He has also seen it spread under a road from one site to another.

‘’Once it takes root, it just takes off and the most common method of infestation is through contaminated soil being moved from one place to another,’’ Mr Flynn explained.

“The plant has a rhizome (stem-like) root which is extremely aggressive and can travel up to seven metres horizontally underground in a year. It can grow up to four metres vertically each growing season.’’

KNOTWEED: ‘Authorities decided to ignore this issue for years, because of the costs in dealing with it’

A tiny fragment of a rhizome, or stem, immersed in water, can produce a viable plant within seven days.

The worst thing anyone can do is to cut back, or trim, the knotweed as such action only spreads the weed further, he cautioned.

Neither should it be composted and, even if it is buried, it can come alive after being dormant for many years.

While he continues to treat sites around Millstreet, Mr Flynn is acutely aware of the plant and seems to find it almost everywhere he goes.

For instance, he has recently seen it on both sides of a stream near the Catholic church, in Bantry, Co. Cork. He also spotted numerous infested sites along a 1.5km stretch of road on the boundaries of Killarney National Park, at Muckross.

The plant doesn’t have any natural predators in Ireland. In its natural, Asian environment, it is affected by a fungus and the insect, psyllid.

Here, Mr Flynn strongly maintained, the only real method of control is through chemical spraying into the hollow stems of the cut plant for up to three years.

“This seems to eradicate it, but vigilance is needed,’’ he said.

‘’Soil can also be screened to remove the rhizomes, but Irish soils are not really suitable for this method. Steam injection of affected areas also works, but may not be viable at all sites.’’

Steam injection was used to clear the Olympic Games complex, in London, of knotweed, at a cost of €80m.

His modus operandi is to spray the plant in May, July and again in September after it starts to die back. The work is repeated over three years, at least .

‘’You must spray into the tube of the plant. It can’t be hit or miss. People must be totally diligent and professional in the way they approach the job,’’ Mr Flynn emphasised.

There are strict laws to help stop the spread of knotweed in the UK, with several financial penalties for allowing it spread. For instance, people won’t get mortgages, or planning permission for a house, if the plant is growing next to the site.

As a matter of urgency, he believes the government should start a public information campaign to make people aware of the problem and to help them identify knotweed. People should also be advised not to touch, or cut, the plant and be informed of treatment methods. As well as that, he felt, local authorities should be given responsibility, with proper funding, to deal effectively with the problem.

KNOTWEED: ‘Authorities decided to ignore this issue for years, because of the costs in dealing with it’

Meanwhile, his practical advice to people is to know the source of any topsoil they buy and to be vigilant if they live near waste ground, or streams. And they should always thoroughly clean boots, tools and tyres of soil before the leave any contaminated area.

In the absence of a national eradication policy and a co-ordinated approach by various state agencies, local authorities are struggling to cope.

Cork County Council has recently formed a group to assess the scale of infestation.

Last week, it emerged an €8.7m social housing project, in Clonakilty, has been halted until knotweed is cleared from the site.

People in Glounthaune, on the outskirts of Cork city, recently received a grant from the council to control the problem locally, but a council spokesman said it does not have the resources to give grants to all areas experiencing infestation.

The council does not currently have a budget to deal with knotweed, but that may change as the newly-formed group plans an action programme.

Local authorities have no direct enforcement role in relation to invasive alien species. The National Parks and Wildlife Service is the primary regulatory and enforcement authority, but it has very limited resources and does not generally investigate reports involving the growth of invasive species on private land.

“All landowners, including local authorities, have legal obligations to control the spread of Japanese knotweed on their properties as far as is possible,” said a Cork County Council spokesman.

“Thus, the problem is of particular relevance to operational departments such as roads, recreation and amenity and any section of the council engaged in physical works on the roadside, in parks or on other council-owned land.’’ If a member of the public has knotweed growing on their land it is their responsibility to treat and eradicate it.

Kerry County Council, which has a €100,000 budget for knotweed control this year, has produced signs, leaflets, run courses and workshops and has been involved in pilot eradication schemes with the NRA, Transport Infrastructure Ireland and other partners.

Signs continue to be placed along roadsides where knotweed is found, including the Killarney area where the national park, already infested with another invasive species, rhododendron, is threatened by knotweed and other invasive species.

This must also be dealt with in planning applications in Kerry, and the construction of a new community hospital in Kenmare, for example, was delayed until knotweed was eradicated from the site.

Fianna Fáil Cllr John Joe Culloty urged the council to include information about knotweed with all other information being send to landowners in the county.

A council spokesman said staff would continue to place signs along roadsides where knotweed is found asking landowners not to cut the plant. Reminder letters to landowners to cut hedges in the autumn would also contain information on knotweed.

No national eradication plan for one of Europe’s most aggressive invasive species

Donal Hickey

KNOTWEED: ‘Authorities decided to ignore this issue for years, because of the costs in dealing with it’

The Government has no national eradication programme for Japanese knotweed, one of Europe’s most aggressive invasive plant species.

Tackling it is expensive and a national programme would cost tens of millions of euro to implement. But, a crisis situation is fast approaching, with the prolific plant out of control in many areas and delaying some major construction projects on infested sites.

Environmentalists have been calling for urgent action at national level to at least control the spread of knotweed, but the Department of Arts, Heritage and the Gaeltacht has pointed out control of such invasive species is a matter for landowners. A department spokesperson said it was carrying out considerable work on control of such species in national parks and nature reserves, including, for instance, work over many years to deal with the rhododendron threat in Killarney National Park.

“However, this department does not have the resources required to extend such work into urban areas or indeed the wider countryside and is not in a position to provide dedicated funds for such work to other bodies such as local authorities,” the spokesperson added.

“There is no national eradication programme for Japanese knotweed currently and this department is not involved in any eradication or control measures in respect of this species at the moment.”

According to Fintan Kelly, natural environment officer, An Taisce, not enough is being done at EU and national levels to tackle the issue.

“Action has been stifled by a lack of funding and safeguards against the introduction and spread of invasive species have been prevented from being put into force by vested interests,’’ he claimed.

“For example, new potentially invasive species are being targeted as a possible opportunity in both our aquaculture and forestry sectors, with little consideration being given to the potential negative impacts on Irish ecosystems or the knock on effects on society of exacerbating critical levels of biodiversity loss.”

Climate change will further complicate the situation as new, potentially invasive species will be able to establish themselves in Ireland, he warned. “We need to see a response that is consistent with the environmental and economic threat posed by invasive species. Raising awareness alone is not sufficient,” Mr Kelly said.

The Department of Arts, Heritage and the Gaeltacht, through the National Parks and Wildlife Service (NPWS), is responsible for the enforcement of the Wildlife Acts and the European Communities (Birds and Natural Habitats) Regulations 2011, both of which prohibit the spreading of invasive species, including plants and animals.

The European Communities (Birds & Natural Habitats) Regulations 2011 restrict the importation, distribution, sale or release of a number of species of plants and animals considered to be the most harmful.

Limited action is being taken by different government departments, agencies and local authorities in relation to invasive species including knotweed. Initiatives are also underway by bodies such as Transport Infrastructure Ireland, Inland Fisheries Ireland and some city and county councils.

Provisions in a EU regulation on invasive species, which deal with bringing into the member countries, keeping, breeding, transporting or placing on the market species included on the ‘Union list’, came into force on August 3. A core provision of the regulation is a list of invasive alien species of EU concern (the ‘Union list’). The first “Union list” list of 37 species of plants and animals was finalised last December.

Nine of the species are already found in Ireland _ the grey squirrel, Muntjac deer, Chinese mitten crab, red-eared terrapin/slider, ruddy duck, curly waterweed, American skunk cabbage, floating pennywort (Northern Ireland) and parrot’s feather.

According to the Dept of Arts, Heritage and the Gaeltacht, Japanese knotweed was not included on the list as it is already very widespread in the EU and therefore is likely to remain a matter for individual member states.

The edible plant that comes with a health warning

KNOTWEED: ‘Authorities decided to ignore this issue for years, because of the costs in dealing with it’

Japanese knotweed is an edible plant, with a “lemony” or rhubarb-type taste, used in tarts and jams and as a flavouring for some cooked meats and sweets.

Young shoots are said to be the best part of the plant to eat, when they are about 15cm-20cm tall. The shoots can be consumed raw or cooked. It has also been used as fodder for farm animals in its native environment.

Anyone using knotweed in the kitchen has to be careful about how they dispose of any leftovers to ensure they do not cause the plant to spread.

All of which comes with a health warning for anyone going out to pick knotweed for consumption, make sure it hasn’t been sprayed with herbicides.

Herbalists have used it for centuries to treat gastric problems. It has also been used as a laxative and for arthritis, rheumatism and related diseases.

The knotweed is also regarded as a source of vitamins A and C and contains the chemical, resveratrol, which is reputed by some researchers to have properties that prevent heart disease, cancer and Alzheimer’s.

However, the Harvard Medical School warns there’s a long way to go before confirmation of its disease prevention qualities can be absolutely confirmed, as research is still at an early stage.

Very few studies on humans have been carried out to measure long-term health or survival, while the side effects of resveratrol are unknown.

How to identify Japanese knotweed

Generally reddish/brown in colour, the plant has a hollow stem.

The leaves come out light brown, developing into heart-shaped green leaves on zig-zag stems.

The stems emerge bamboo-like and grow to about a metre in height before leaves, which have a brown appearance with green stripes, begin to appear.

The stem grows to about three metres before flowers appear in August.

The flowers are long and white, but don’t develop seed heads in our climate as it is thought to be too cold for pollination.

The plant is usually found on waste ground, beside streams, on roadside ditches and in places where soil has been brought in.

Japanese knotweed has become an economic and an ecological threat to Ireland

Chris O’Keeffe using a stem injector gun to apply round-up directly into the plant stem. Picture: Larry Cummins
Chris O’Keeffe using a stem injector gun to apply round-up directly into the plant stem. Picture: Larry Cummins

FIRST introduced as an ornamental plant from Japan, in the early 19th century, knotweed (Fallopia japonica) has spread massively and is now found extensively in Ireland. It thrives along rivers, roadsides and waste ground.

Also native to China and south-east Asia, it has spread widely in the last two centuries. It is one of the most pervasive invasive species and is a serious pest in many parts of Europe and the US.

In Ireland, controlling knotweed, not to mention eradication, is a major challenge for the government, local authorities and the building industry as it can seriously damage houses by undermining foundations and going through walls.

It can penetrate surfaces as hard as concrete and tarmac, as well as threatening native plants and animals by forming dense thickets.

Once established, it can be hard to control. Along rivers, it also damages flood defence structures, can lead to bank erosion.

Red/purple shoots, which can look like asparagus, appear in early spring. The mature canes are like bamboo and have a characteristic pattern of purple speckles. The plant can grow to over three metres in height.

Flowering occurs in late July and August, with small, creamy white flowers. During the winter, the leaves die back and reveal orange/brown, woody erect stems. The root-like stems can extend up to seven metres from the parent plant and up to three metres in depth.

Only female Japanese knotweed plants have been recorded to date in Ireland and the UK, according to Invasive Species Ireland (ISI). The plant is, therefore, spread from site to site through the deliberate or accidental movement of roots, or stems. It can, for instance, be easily moved

Some studies suggest a tiny fragment of knotweed can result in new growth. Therefore, control depends on preventing its spread. People are advised not to cut knotweed and herbicides are used in its control.

Chemicals are generally those containing glyphosate as the herbicide can remain in the environment for 24 to 48 hours after application. According to ISI, the best time for application is mid to late September, or before the foliage starts to die back.

An early season spray maybe required in the first year to assist with access for the late season spray.

It is illegal to dump Japanese knotweed waste in the countryside, or to plant or spread knotweed. It is also illegal to dispose of knotweed at a landfill without informing the site authorities that the waste is knotweed.

To move soil containing knotweed requires a licence from the National Parks and Wildlife Service (NPWS).

Plant material should not be composted as this is ineffective and may result in further spread. Plants should be treated in the same season as they are identified. Try not to let stands of Japanese knotweed become established as the species is very difficult to control. If it is a recent introduction it is best to tackle it quickly to prevent roots from fully establishing.

Treatment often needs to be repeated until no regrowth is observed over several years for eradication to be achieved.

Repeated herbicide treatments over several years are normally recommended for complete control of Japanese knotweed. Continued monitoring of the treated areas should be carried out to ensure no new shoots appear.

Under ISI best practice guidelines, people burying dead Japanese knotweed material, or disposing of it off-site for deep burial should only use glyphosate formulations. Other persistent herbicides may not be allowed for deep burial under various waste regulations and due to a potential risk of pollution of groundwater.

It is recommended not to deep bury knotweed on-site within 10 metres of the site boundary as a precaution.

Also, when planning works with knotweed, people are urged to have built-in biosecurity measures in their management plan.

The globalisation of trade and an increase in the movement of goods and people has enabled the spread of non-native species around the world. Invasive species are a major driver of biodiversity loss and extinctions worldwide.

It is estimated that invasive species cost the EU at least €12bn per year over the past 20 years.

Infestation has caused delays in major construction projects and Dr Vincent O’Malley, head of environmental policy and compliance, Transport Infrastructure Ireland (TII), stressed knotweed and other invasive species had implications for all major and minor road works.

Species needed to be removed from locations prior to work taking place, he said.

TII has pilot projects in Galway and Kerry and the aim is to slow, or contain, the spread of knotweed on the national roads.

About 10 new species become established in the EU annually and the EU has recognised non-native invasive species as one of the causes of damage to natural life and second only to habitat destruction. Of all the species, Japanese knotweed has the highest impact.

There are also severe impacts on the economy. In 2013, the cost to the Irish economy was estimated at €266m.

Worldwide, such species have contributed to 40% of the animal extinctions in the last 400 years and, some scientists believe, invasive species are a more significant cause of biodiversity loss than climate change.

Cutting back knotweed can just make the problem worse

Hundreds of metres of Japanese Knotweed growing along the old N25 to Rosslare at Glounthaune
Hundreds of metres of Japanese Knotweed growing along the old N25 to Rosslare at Glounthaune

People trimming and cutting back hedges should not cut Japanese knotweed, as the plant is spread by fragments which easily take root.

That’s the advice from Colette O’Flynn, invasive species officer, National Biodiversity Data Centre, who pointed out the plant is usually spread inadvertently by people.

‘’By doing things like tiding up their gardens, or cutting back foliage to improve access to a river bank, people can make the problem worse,’’ she explained.

There is no national strategy for controlling Japanese knotweed and the responsibility rests with landowners. Some local authorities have eradication programmes on roadsides, parks and other properties they own.

In Ireland, there are 48 high impact invasive species, including the knotweed, giant hogweed, the grey squirrel, American mink, New Zealand flatworm and the zebra mussel, all of which threaten native species.

There are another 79 medium impact invasive species which need to be monitored. These include a creeping plant known as traveller’s joy, butterfly bush and sliders, or turtles, which can grow to be quite large and which can live for up to 30 years.

Ms O’Flynn welcomed an increase in public awareness of the problem, in recent years, but a lot of people still need to be informed and reached on the issue, she stressed.

Knotweed is tackled through the use of the glyphosate herbicide, similar to round-up, and the key is to kill the underground stems (rhizomes).

The most effective time to apply the herbicide is when the plants are beginning to die back, usually in September, and the process has to be regularly repeated for a number of years, Ms O’Flynn said.

Japanese knotweed has no natural enemies in this side of the world, but scientists in the UK have been looking at ways of possibly introducing a bug, psyllid, which comes from its native range.

Using the bug for biological control could be an alternative to the use of herbicides, but Ms O’ Flynn said that should be ‘’an absolute last resort’’ in tackling invasive species.

An obvious risk would be the possible introduction of yet another species which could be detrimental to the ecology and to native species.

The tests in the UK, which are being carried out in infested sites and in controlled conditions, have not been promising thus far.

Thousands of psyllids (Aphalara itadori) were reared in laboratories and brought from their native habitat in Japan where they suck the sap from Japanese knotweed and keep it under control along with a range of other bugs.

Since 2003, scientists from CABI, a non-profit organisation that provides scientific expertise, have been working with the psyllids, releasing the bugs on numerous trial sites.

It is believed to be the first time a foreign insect has introduced in the EU to tackle a weed. The results, however, are described as mixed.

Scientists have confirmed with earlier lab tests that the bugs are safe and do not have a negative effect on other plants.

But, there’s a difficulty in maintaining a sufficient number of adult psyllids at the trial sites, mainly due to cold winter weather. Some of the bugs are surviving the winter though not in the numbers necessary to establish a population which will keep on growing and have an effect on the knotweed.

If the psyllids take hold on knotweed, it may take up to 10 years for effects to be seen on the growth of the plant, scientists say. The hope is that several communities of psyllids could take hold and then spread across the UK to tackle knotweed.

The idea behind the project is for the psyllids to feed on the sap and make the knotweed weaker. That should make the chemicals more effective on the plant should they be needed.

Common ‘alien invader’ plant species in Ireland

Giant Knotweed

A relative of Japanese knotweed, Giant Knotweed grows much taller, up to four metres high, and has large, heart-shaped leaves. Similar to Japanese knotweed, this plant produces woody, knotted, bamboo-like stems.

It is highly invasive, spreading from green plant and root fragments. Plants originating from buried fragments can grow through damaged walls and concrete. It is less widely distributed in Ireland than Japanese or Bohemian knotweed.

Originating from the Japanese island, Sakhalin, it was first introduced as an ornamental garden plant but has thrived in the wild here. In spring, it regenerates rapidly, as much as seven to 10 centimetres per day. In summer, small clusters of flowers are produced.

This highly aggressive invader breeds easily with Japanese knotweed and the hybrid is known as Bohemian knotweed, also an invasive species.


Introduced in 18th century and native to Spain and Portugal. Its lilac flower stands out in May and is often admired. Does well in acid, wooded areas of Cork, Wicklow and Kerry. There have been ongoing eradication campaigns for decades. Getting rid of the ‘rhodo is seen as one of the biggest challenges in Killarney National Park where it threatens ancient oak and yew woodlands. Despite an eradication programme over the past 35 years, it continues to grow in the park.

Can grow up to eight metres tall. It can prevent the growth of other plants over which its leaves and branches form a canopy. Has a fungus which attacks other plants. Can be tackled by uprooting, chemically treating stumps or by continuing herbicide application. Hard to kill.

Giant Rhubarb

Of Chilean provenance, but not related to the garden rhubarb. Does best in milder, wetter areas which explains why it is more common in western and seaboard counties. Can be seen growing prolifically in Achill and Connemara, for example. Its massive leaves smother other plants and it has coarse prickles.

Control is feasible through chemical means that require follow-up action.

Giant Hogweed

Native to south-west Asia and known for vigorous, early season growth. Found on waste ground, woodland edges and river banks. Dies back in winter leaving soil bare and vulnerable to erosion. Chemical treatment but, as seed can remain viable for up to 15 years, ongoing control is necessary. Highly toxic and dangerous to humans, it can cause burn-like inflammation on skin.

Indian or Himalayan Balsam

A member of the Busy Lizzy family, it originated in the Himalayan region and introduced here as a garden plant in the 19th century. Spread swiftly along waterways and wet places. Dies in the autumn leaving ground at risk of erosion. Chemical control readily achieved, but may need ongoing control for years.


From the Cape Region, South Africa, it has become naturalised in many parts of Ireland. Found especially in the south-west where it is common along road banks and hedgerows and by rivers and lakes. Chemical control can be difficult, but can be achieved with regular, follow-up action.

Winter Heliotrope

Native to north Africa, it is found here on roadside hedges, woodland edges and waste ground. Removal only practical on a limited scale through a combination of excavation and herbicides. Can spread from small root fragments.

Buddleia (butterfly bush)

Believed to have come from China, it is a common garden plant and its flowers attract butterflies. Found on waste ground in the environs of cities, but countrywide distribution. Does well on poor soil and grows on walls, rocky outcrops and roadsides. Controlled through digging out roots and chemical application. But can regrow vigorously from the stump.

More on this topic

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Cork Council wants powers to access private land as owners ‘failing to’ eradicate knotweedCork Council wants powers to access private land as owners ‘failing to’ eradicate knotweed


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