JAPANESE KNOTWEED - FREQUENTLY ASKED QUESTIONS in Ireland 

While these questions and answers relate to Japanese knotweed, they are also applicable to the regulated invasive species Giant knotweed (Fallopia sachalinensis), Bohemian knotweed (Fallopia x bohemica) and Himalayan knotweed (Persicaria wallichii).

 

 

What does Japanese knotweed look like? 

Japanese knotweed is a green shrub with bamboo-like hollow stems that grow to 3 meters in height and forms dense stands during the summer. In Spring, small red buds appear that develop into ‘asparagus tip’ shaped stems. As it grows, it has a green stem with red/purple flecks. Leaves are arranged in a zig-zag pattern on the stem. Clusters of small creamy white flowers emerge in late summer. In winter, the plant dies back leaving dead brown cane-like stems still standing or lying on the ground. Often, there is still evidence of browned flowers on stems. Underground, it has an extensive rhizome (which is essentially an underground stem) system commonly with an orange centre that snaps easily like a carrot when bent. 

Online Identification resources 

• National Biodiversity Data Centre

• GB Non-native Species Secretariat  

• Invasive Species Ireland

• JKC

 

Japanese knotweed (Fallopia japonica), Giant knotweed (Fallopia sachalinensis) and Bohemian knotweed (Fallopia x bohemica) are similar plants with slight differences in leaf shape and size and plant height. Himalayan knotweed (Persicaria wallichii) is often confused with the invasive Fallopia species but it has long lance-like leaves. 

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Can Japanese knotweed or the other invasive knotweed species be confused with any other plant in Ireland?

Yes. Due to its bamboo like stems and dense growth it is sometimes confused with bamboo plants or Red dog osier (Cornus sericea). These other plants are however rarely encountered outside of areas where they have been planted and the shape of the leaf and zig-zag leaf arrangement on the stems will distinguish Japanese knotweed from these species.

 

Himalayan knotweed (Persicaria wallichii) can sometimes be confused with Himalayan balsam (Impatiens glandulifera) as they have similar leaf shape. However Himalayan balsam has toothed leaf edges (like a knife), its stem is very fragile and easily broken and its flowers are very different being larger and ‘bonnet’ shaped and mostly pink to purple. 

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What harm does Japanese knotweed do to our wildlife?

Because of its ability to grow quickly into tall dense stands, it shades out native plant species and can dominate an area. This also has knock-on effects for other wildlife depending on those plants. As Japanese knotweed dies back in the winter and the native plants are gone, it can leave areas particularly river banks, exposed to erosion from heavy rain and flooding. 

 

Why is Japanese knotweed so invasive? 

Because of its ability to rapidly grow tall and densely in a variety of habitats and spread by rhizomes and small pieces of plant fragment, it can quickly establish and dominate in new areas. It can most easily begin to grow in disturbed areas. There are also no predators, parasites or microorganisms in Ireland that significantly impact on its growth, this gives it a competitive advantage over other species. 

 

Can Japanese knotweed grow-up through roads and foundations of buildings? 

Yes it can, but this tends to be where there is some impairment or weakness in the structure such as a crack or a thin covering of surface material. 

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Is Japanese knotweed a regulated species in Ireland? 

Yes, under Regulation 49(2) any person who plants, disperses, allows or causes to disperse, spreads or otherwise causes to grow Japanese knotweed or any of the other invasive plants listed in the Third Schedule of the European Communities (Birds and Natural Habitats) Regulations, 2011 (S.I. No. 477 of 2011) shall be guilty of an offence. Furthermore, Sections 52(7) and (8) of the Wildlife Act, 1976, as amended,1 make it an offence to plant or otherwise cause to grow in a wild state exotic species of plants. 

 

I have Japanese knotweed on my land, do I have to get rid of it? 

Yes, you should take action to control and eradicate this plant from your site. By doing nothing it could be considered an offence by ‘allowing’ it to be dispersed or spread. It also makes sense to try and control and eradicate the plant from the site sooner rather than later as the longer it is left to expand and spread, the more resources will be required to deal with it.

 

Japanese knotweed is growing near my house, should I be concerned? 

In most instances, Japanese knotweed and the other invasive knotweeds are growing far enough away from a house for it not to be an imminent threat but one that should be dealt with before it spreads closer to your home. If you can see Japanese knotweed growing above ground and within about 10 meters of your house then you should seek expert advice on assessing the risks to your home.

 

While the rhizomes (underground part of the plant) are said to grow out to about 7 meters horizontally and 3 meters vertically, most often this tends to be much less. However, spread of rhizomes beyond 7 meters has been documented where an easy route such as along ducting is available. The latest guidelines from RICS  advises a minimum of 3 meter safe zone when excavating Japanese knotweed.

 

There is an infestation of Japanese knotweed on a neighbouring property and it is spreading into my land, who is responsible to get rid of it?

The landowner is responsible to ensure that they are not causing or allowing it to be dispersed or spread and should take action to control it. It is an offence for anyone to cause or allow it to be dispersed or spread. 

 

Will having Japanese knotweed in a garden prevent selling a property or getting a mortgage?

While there are no known cases of mortgage applications being denied due to the presence of Japanese knotweed on site (as has occurred in Northern Ireland and England), having Japanese knotweed on the site can and has deterred some from purchasing a property. However, seeking professional advice on the actual level of threat should be sought. 

 

Can anyone spray herbicide on Japanese knotweed? 

No. Under the Sustainable Use of Pesticides Directive there are restrictions on use and application of herbicides by professional and amateur users. For further information see: http://www.pcs.agriculture.gov.ie/sud/

1 Section 52(7) and (8) of the Wildlife Act, 1976, as inserted by 56(d) of the Wildlife (Amendment) Act, 2000.
2 Regulation 49(2) of the European Communities (Birds and Natural Habitats) Regulations, 2011 (S.I. No. 477 of 2011). 

 

 

I’ve sprayed Japanese knotweed with glyphosate for the past 3 consecutive years and this summer year there is no visible re-growth, is the plant dead?

Whilst there is no obvious above ground growth of the plant, the below ground rhizomes might still be living (viable). If you do not need to disturb the site where the plant is, you should continue to monitor the area for several years and treat any re-growth. 

 

There’s a patch of Japanese knotweed growing up through my lawn, is it okay to mow it?

No. By mowing Japanese knotweed you may be creating plants fragments that could then be spread to other areas of your garden where it could grow. 

There is Japanese knotweed growing in my community, who should I tell? 

If possible, notify the land owner of the presence of the plant and its potential impact. If the plant is on public property you should inform the city or county council. Also submit the sighting details to the National Biodiversity Data Centre noting where it was seen and include a photograph if possible. 

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See: http://www.biodiversityireland.ie/projects/invasive-species/submit-sightings/

 

When I submit a sighting of Japanese knotweed to the National Biodiversity Data Centre, where does that information go?

Once the sighting has been verified, it will be made available online through Biodiversity Maps for all to access. At the end of every year, all sightings of invasive species are sent to each city and county council for their areas. 

 

I have Japanese knotweed on my land and I want to know how to get rid of it, who should I ask?

If you need information in addition to what is available through the below online links, you may wish to ask a consultant or company that have expertise and a track record in treating Japanese knotweed. 

 

Where do I dispose of Japanese knotweed off site? 

Japanese knotweed can be disposed at a deep-fill licenced landfill site. It is advised that you first check with the landfill if they can and will accept the waste. They may require notice to prepare to accept the waste. 

 

Do I need to apply for a licence to move Japanese knotweed material or infested soil or spoil off site?

Yes. A licence is required to remove and dispose of Japanese Knotweed and contractors employed to carry out this work should be aware of this requirement. If you have any queries in relation to the transport of Japanese Knotweed please contact the Licencing Unit of National Parks and Wildlife Service at email: wildlifelicence@ahg.gov.ie or Tel: (01) 888 3298 or (01) 888 3275. 

 

What should I NOT do? 

• -  Do NOT cut and discard any part of the plant on the ground, this could cause it to grow and spread 

• -  Do NOT mow, strim or hedge-cut Japanese knotweed. This could cause it to spread and grow from broken plant fragments. 

• -  Do NOT dig it out of the ground and break-up the rhizome system unless it is part of controlled deep excavation works 

• -  Do not compost cut knotweeds as they may grow from this 

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