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Japanese Knotweed FAQs

Why is Japanese knotweed a problem?

Japanese knotweed can spread from tiny fragments – a piece of root as small as 0.7g has the potential to grow into a new plant. In theory, most parts of the plant above ground could also potentially regenerate, but the risk is much higher with the parts of the plant closest to the ground.

Japanese knotweed is very hard to kill. Herbicide treatments can take up to five years. The plant has the potential to remain dormant for twenty years, and to grow back when the roots (called the “rhizome”) are disturbed.

Japanese knotweed grows very fast – in peak times, Japanese knotweed can grow up to ten centimetres per day!

Japanese knotweed is also covered by legislation which makes it an offence to dispose of the plant inappropriately.

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Is it illegal to have Japanese knotweed on my land?

It is not illegal to have Japanese knotweed on your land and you do not have a legal duty to notify anyone that you have Japanese knotweed on your land, although you must not misrepresent this, for example when answering the question about Japanese knotweed on the TA6 Property Information form.

However, there are laws which cover the spread and transport of Japanese knotweed, and without taking action it is possible that you may commit an offence, or be liable for action in the civil courts (you could be sued).

It is an offence under the Wildlife and Countryside Act 1981 to plant or cause Japanese knotweed to grow in the wild. Prosecutions are very rare, however, and JKSL are not aware of any charges brought under this legislation for allowing Japanese knotweed to spread into a domestic property.

Japanese knotweed (and soil or other material containing Japanese knotweed) is considered “controlled waste” under the Environmental Protection Act 1990. This means that if you move Japanese knotweed off your land there are various legal duties in how knotweed waste is managed. Controlled waste can only be taken to licensed landfill – this is expensive, the material needs to be booked in with the landfill and there are limited sites which will accept Japanese knotweed materials.

The Invasive Alien Species (Enforcement and Permitting) Order 2019 does not apply to Japanese knotweed at the time of writing (January 2020).

Do I have to treat Japanese knotweed on my land?

You do not have any legal responsibility to treat Japanese knotweed – however (as outlined above), there are laws which cover what you must, can and cannot do with Japanese knotweed material.

In addition, there have been cases in the civil court where neighbours have been successfully sued for damages for allowing Japanese knotweed to affect adjacent properties. The defendant’s lack of action to control the problem has been cited in some of these cases.

Local authorities have the power to issue a “community protection notice” (CPN) to compel you to treat Japanese knotweed if it can be shown that you are causing an impact on “local amenity” through your failure to treat or manage the plant.

A Bristol company was prosecuted and fined £18,000 plus costs in 2018 for failing to comply with a CPN which ordered the control of Japanese knotweed on their land. The company was also ordered to secure a management plan from a specialist company within a month of the judgement.

How much damage does Japanese knotweed cause?

Japanese knotweed is not the most damaging of plant species – but it can grow through tarmac and through small gaps or weaknesses in paving, concrete and other surfaces.

Where construction works are carried out in areas where Japanese knotweed is present, we have seen cases where the plant grows up through floorboards, or between gaps between the old and new construction, resulting in Japanese knotweed plants growing inside a house or commercial building. When this occurs, treatment becomes significantly more complex.

JKSL have seen numerous cases of Japanese knotweed causing damage to hard surfaces, paving, retaining walls, garages, sheds and other structures.

Does Japanese knotweed grow through concrete?

No – Japanese knotweed is good at finding and exploiting weaknesses in the built environment – and is known to grow through cracks, joins, weaknesses or damaged concrete – although it will generally take the easiest route to a growth opportunity.

What should I expect from Japanese knotweed?

Japanese knotweed plants die back each year in the winter. This usually happens at the end of October but it varies from site to site and will happen earlier the further north you are.

Japanese knotweed’s root system stays alive underground all year round. The canes are not like trees – once they have died back, they don’t get new leaves.

The rhizome (root system) can spread several metres away from the plant, and as the herbicide treatment progresses, it is possible that some new growth will occur in the areas on top of the rhizome. For this reason, it is particularly important that domestic clients communicate with their neighbours and keep a check on whether there is Japanese knotweed on adjacent land.

Is Japanese knotweed poisonous?

Japanese knotweed is not poisonous to humans, but we don’t recommend that people or pets should consume any part of the plant, particularly on sites where herbicide treatment has been carried out, because the plant may contain harmful chemicals.

Some products derived from Japanese knotweed (resveratrol, jams and other foodstuffs) are available on the market but we strongly recommend against any foraged or home-made products containing Japanese knotweed due to potential herbicide contamination.

What other plants can cause problems?

Many other plants can cause problems on your land. Trees, particularly sycamore, can be damaging, and horsetail and other plants can grow through tarmac and other surfaces.

Many plants can cause problems in landscaped areas, overgrowing planted areas and out-competing native species; in many cases this will be an issue that a (landscape) gardener can resolve for you.

There are a number of other plants which are covered by laws including the Wildlife and Countryside Act 1981 and the Invasive Alien Species Order 2019 (and others); these plants should be dealt with by a specialist with knowledge of the plants and the relevant waste legislation. Plants which cause particular problems include giant hogweed, Himalayan balsam and buddleia.

JKSL will respond to enquiries regarding any plant species.

Why is Giant hogweed a problem?

Giant hogweed spreads by seed; it grows very tall (up to 5m) and causes some of the problems which other plants cause – although it is rarely seen to cause structural damage.

The main issue with giant hogweed, is that if your skin touches the plant or its sap, you can get serious burns if you are exposed to UV light (including sunlight).

Giant hogweed material contains chemicals which break down the compounds that protect the skin and this effect can last for a long time – in some cases for a period of years. It is very important to take steps to prevent people and animals from coming in to contact with giant hogweed, and you should act immediately if you think you have come into skin contact with giant hogweed.

What should I expect from Japanese knotweed treatment?

You should receive a full plan for the works that JKSL will be carrying out, including the approximate dates for the visit(s). We will attend as close to these times as we can.

When we attend site to apply herbicide, this is normally done using a backpack sprayer, but is sometimes done with an injection gun or leaf wiping equipment, particularly where there are other plants nearby and where there is only a small amount of Japanese knotweed on site.

You should see the plant die back within a few weeks of treatment – and you may see some other plants around the knotweed dying back too (although we will try to avoid killing other plants unless this is agreed with you).

The knotweed plants might not re-grow for some time after a visit, but this is difficult to predict accurately, so it is important that we continue to attend the site in line with the knotweed treatment plan that has been put in place – even though you may not see Japanese knotweed plants growing.

If we agree it with you as part of our contract, we will clear and chip the dead canes over winter – but this is an optional extra and a paid-for service. It is very rare that we will ever remove the canes from site, because they need to be taken to a licensed landfill, who charge by the tonne – so this will be very expensive compared to an ordinary gardener’s services.

When will you carry out the treatments?

Depending on what plants are present on your site and the treatment method or herbicide that is used, we could attend at different times of the year. For most plants, herbicide applications need to be made before the plant has chance to make seeds – so for Himalayan balsam, for example, initial treatments should normally be completed before the end of June.

Japanese knotweed lives for many years and stores nutrients in its root system; but it does not spread by seed. The ideal time to apply herbicide is later in the year – usually from mid-August until just before the plant starts to die back for the winter.

Early in the treatment, an extra spray may be carried out earlier in the year in order to prevent the plants from over-growing. Details of your treatment plan and costs will be provided before commencement and will form part of the contract agreement.

How does chemical treatment work?

In most cases, JKSL’s herbicide treatments work by killing the root system of the plant. We spray (or inject) the surface growth, but the herbicide moves through the plant’s system and interferes with the plant’s natural processes and cause it to die off. With Japanese knotweed, multiple applications are required in order to fully kill off the root system.

All of the herbicides that we use will damage or kill any shrubs, flowers or trees that they come into contact with, so unfortunately anything that is in the same area as the Japanese knotweed is at risk of dying. Some of the herbicides that we use will also kill grass.

JKSL can sometimes use different treatment methods to minimise the damage to other plants (such as stem injection) – ask us if this is suitable for your site.

What should I expect to see after herbicide treatment?

Herbicide will take effect in different timescales for different plants. For Japanese knotweed, it can take as long as 14-21 days for the full effects to occur; although you may see the leaves wilt and drop sooner than that.

Other, less hardy plants (particularly Himalayan balsam) may be affected in a much shorter timescale – sometimes dying back within the space of a few days.

The knotweed has grown back – what is wrong?

This is not necessarily a problem. In the first years of treatment, it is common for Japanese knotweed to grow back, even soon after a treatment. We expect to see growth at any point during the treatment plan – potentially for up to five years.

It is important not to disturb this growth and you shouldn’t cut it down or apply other herbicides to it – it should be left undisturbed as much as possible until our next visit.

What happens if there is still re-growth on completion of the warranty?

This situation is rare, but depends on a number of factors, including the reasons for the re-growth.

Where there is a guarantee or insurance-backed guarantee in place, JKSL will continue to attend to treat the re-growth for the duration of the guarantee, subject to the terms and conditions of the guarantee. It is important to note that in most cases, where you or a contractor have done works in the contaminated areas without JKSL’s involvement, guarantees may be void.

If the re-growth has been caused by something outside our control (like a flood or fly-tipping) then you will need to speak to us about how we can continue with the treatment.

Where there is off-site growth that we haven’t been able to treat, we will need to talk about how we can continue to manage your site – but ideally, we would like to treat the off-site areas so that we can kill the whole infestation (this should be discussed at the earliest opportunity, to ensure the best results).

The knotweed canes are dead or haven’t grown back – is the plant dead?

The root system of Japanese knotweed is what needs to be killed – and this can stay dormant for a long period of time. It is important that we continue to attend for the rest of the planned visits, to make sure that as many effective treatments as possible can be carried out.

JKSL may consider signing the site off early if there is a period of two full years with no growth on site – but this is not guaranteed and the decision would have to be signed off by the JKSL project manager who is looking after your project.

Japanese knotweed plants are growing now – why should I wait for treatment?

The ideal time to treat Japanese knotweed is toward the end of the year, when the plant is taking nutrients down into its root system in preparation for winter. Late-season treatments take advantage of this process in order to kill the root system most effectively.

Treating too early can result in the surface growth dying back, but the treatment having no major effect on the root system. Treatment at the wrong time is likely to have little effect on the plant and can actually negatively affect the long-term success of the treatment plan.

Wherever possible, you should follow our recommendations about treatment times and leave the plants alone until the treatment has been carried out.

What should I be aware of when you are carrying out treatment?

You should be aware of the schedule that we provide for the treatment plan – this should be followed as closely as possible, and we will need to access the contaminated areas in order to complete the treatment. If we can’t carry out the treatments as planned, this may affect the long-term success of the treatment plan (and potentially could impact on any guarantee).

We will send you documentation about what we will do and the safety measures you will need to put in place. In general, this will mean staying out of the contaminated areas while we do the work – we also recommend that people and pets stay out of the area for the rest of the day if possible.

You should leave the Japanese knotweed alone, and you shouldn’t cut it back or apply any herbicide in-between our visits – this could prevent JKSL’s applications from being successful.

Is the herbicide that you spray harmful?

We only use approved professional herbicide products, which are extensively safety-tested. However, these are purpose-designed to be harmful to plants.

In some cases, the products only affect plants, and should have no negative effect on humans; in other cases, the herbicide also has documented health risks to humans. If we are applying herbicide on your property, you should assume that it is potentially harmful, and you should keep pets, children (or any other creatures) away from the treatment area at least until the herbicide has dried, and preferably until the end of the day.

Where there are health risks, we take measures to prevent accidental exposure.

We can supply you with information about the products we use, and advise you on the methods we will use to spray safely.

Why do you wear so much protective equipment?

JKSL take the health and safety of our employees very seriously – as do the UK and EU authorities and the companies which manufacture and produce the products we use.

There is extensive guidance for the use of the products, and this guidance must be followed by law. JKSL operatives wear the protective equipment that is specified for the use of the products, and in some cases, wear additional protective equipment where our risk assessment requires it, or in order to prevent potential contamination with herbicide.

The teams who carry out herbicide applications are applying herbicide day-in-day-out for around six months of the year, so we take precautions to make sure that they are not regularly exposed to anything which has the potential to damage their health if it builds up in their system.

You might see (or hear about) other contractors applying herbicide, particularly glyphosate, without using the same protective measures that JKSL use – however, this is not appropriate for the works that our operatives do, and we make sure to protect our employees, clients and members of the public so far as reasonably practicable.

Doesn’t glyphosate cause cancer?

A report from one international agency (IARC) suggests that glyphosate may cause cancer. This report places glyphosate in the same category as eating red meat, drinking hot beverages (above 65°C), night shift work, working as a barber and emissions from frying.

It is also worth noting that the category above glyphosate (confirmed carcinogens) includes outdoor air pollution and diesel engine exhaust, alcoholic beverages, UV tanning beds, indoor coal fires, wood dust and Chinese-style salted fish.

Reports from a variety of other sources, including extensive research undertaken recently in America, disagree, and state that glyphosate does not cause cancer. Other sources include the independent scientists who determine the safety of herbicide products for the EU (and who have shown effective leadership on safety by banning a number of other harmful products in recent years).

Recent research does suggest that some products on the US market have the potential to cause cancer – due to other ingredients in the products. While JKSL remain on alert for any changes to best working practice, we are confident that the measures we have been taking over many years of application have been safe and fully in-line with legal requirements and best practice.

Do I need to stay away from the area when the herbicide is applied?

Yes. You should stay away from any herbicide application.

JKSL’s safety controls include keeping people and animals out of the areas where we are applying herbicide. If you are present when we carry out the application, you should stay out of the area until the application is complete and at any time if you are instructed to do so by our Operatives.

Although some herbicides are safe as soon as they have dried on the leaf, we recommend that people and animals are kept out of the area for the rest of the working day.

Can I eat fruit and vegetables from the area?

We would strongly recommend that you do not eat any plants which have grown in or around areas that have been treated with herbicide.

Although some of the products that we use are classed as non-harmful, we cannot recommend that it is safe to eat fruit and vegetables from the areas where herbicide treatment has been applied. The products which we use are not approved for use on crops of any kind, so neither JKSL nor the suppliers of the products can vouch for their safety.

JKSL can supply detailed information about the products we use, including Safety Data Sheets and contact details for the manufacturer.

Can I remove / move the canes?

JKSL recommend that if the canes are cut too close to the ground, then the material has the potential to re-grow. When JKSL cut Japanese knotweed canes on site, we leave the stems in place to a height of 100-150mm to reduce the potential for cross contamination.

You can’t remove any Japanese knotweed material (including dead canes) from site without complying with the relevant waste regulations. You can’t put them in your green bin and most municipal waste facilities will not accept Japanese knotweed waste.

You should avoid putting Japanese knotweed material in your compost, as it could potentially re-grow and Japanese knotweed canes do not break down very quickly.

It may be suitable to incinerate Japanese knotweed material on-site – however, you must ensure that you comply with any clean air regulations or local bye-laws, and you should consult JKSL before doing this sort of work to ensure that the potential for cross-contamination is kept to a minimum.

Can I mow the grass?

It is safe to mow the grass in areas where there is no Japanese knotweed – but it’s important to make sure that there is no knotweed present, because even very small fragments of Japanese knotweed can re-grow (including in compost) and no Japanese knotweed is allowed in green bins.

In addition, if an aminopyralid herbicide is used on your site (e.g. ICADE or Garlon Ultra), this herbicide can be absorbed by the grass and the herbicide remains active for a long period. This means that the grass cuttings themselves can act as herbicide if they are used as compost or if they come into contact with other plants. For this reason, aminopyralid treatments are generally not suitable for domestic gardens.

When can I start planting?

Anything you plant before the end of the herbicide regime will potentially be killed by future applications. We recommend putting ant plants that you want to keep in movable pots so that they can be moved out of the way during herbicide application.

You can potentially plant these once JKSL have signed off the treatment as complete – but you should be aware that in some cases, disturbing Japanese knotweed’s root system can cause the plant to re-grow, and you should still take care when digging in the area. You should ask JKSL for advice before doing this.

Japanese Knotweed Solutions also offer a number of services which may assist you, including tree work, planting, seeding, and turfing.

When can I start building?

If you have had a mechanical remediation (if we have dug the plants up and removed them), then it is likely that we have discussed your development plans with you, and our proposal should include a timescale for building. It is important to contact us if your plans change.

If we haven’t already discussed the timescale for any development on your site, then you should contact your surveyor as soon as possible to discuss this, because any building works in contaminated areas have the potential to spread plants to new areas of site, and can also result in problems with plants growing where they are not wanted, potentially causing damage to the new construction and resulting in higher long-term costs for remediation.

Can’t you just put “x” on it? (where “x” is not a UK-approved pesticide)


JKSL only use approved professional pesticide products licensed for use in the UK. Legally, anyone who takes money in exchange for applying pesticides on your property must only use pesticides which are approved for professional use, and they must have a relevant certification to do this.

JKSL is also responsible enough not to put a list here of all the things that people over the years have come to us thinking it might be a good idea to throw on their garden (Hint: it’s really not a good idea).

I have some herbicide in my shed – should I apply it between your visits?

This will not help the treatment plan for a couple of reasons: we don’t want to spray too much herbicide on your site (the amounts we can apply are limited by law) and we don’t want to “overdose” the plant with herbicide; for Japanese knotweed, this could put it into a dormant state, meaning that the plant could stay alive underground after our treatment plan has finished.

JKSL also don’t want to over-treat the plant early in the year, as this can result in problems including bonsai growth. Also, if someone else applies herbicide before we visit in the late summer, then this could prevent us from getting an effective application at the right time.

Especially because of the risk of cross-contamination, it’s best to avoid doing anything to the plants if possible – just leave it to us.

Can you sell me some of the chemical you use?

Can you sell me some of the chemical you use?


JKSL will not sell any of the herbicide products that we use.

Professional herbicides are only allowed to be sold for use by qualified operatives, and must be sold with a qualified advisor on hand to provide information and advice about the product. Although JKSL have qualified pesticides advisors on staff, we are not a pesticide distributor.

If anyone offers to sell you professional herbicides, you should report it to your local Trading Standards officer (through your local authority) and also to the Chemical Regulations Directorate using HSE’s online form. If you are unable to complete the online enquiry form, then you can phone 0300 003 1647 during office hours – 8:30 am to 5:00pm, Monday to Friday, Thursday 10:00 am to 5:00 pm, and a call handler will complete the form for you.

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