Frequently Asked Questions About Japanese Knotweed

See below a list of most burning questions we frequently get asked by our clients. All of the queries have been answered by one of our experts. If you have any other questions or concerns about Japanese knotweed and the process of its eradication – fill in our our contact form and you will be provided with recommended solution to your case.

Can I sell my house with Japanese Knotweed?

Having Japanese Knotweed in your garden will make your house very difficult to sell, especially as people would prefer not to spend large sums of money to solve the problem. You will be advised to completely remove the native plant before attempting to sell your house.
It is important to remember:

  • There are specific questions relating to Japanese Knotweed and if answered incorrectly through the conveyancing process, a legal claim of misrepresentation could be brought against you.
  • It is a legal obligation for you, as the seller, to disclose the presence of Japanese Knotweed.
  • The key is to be as honest as possible with your Estate Agent and potential buyers.

Can Japanese Knotweed stop you getting a mortgage?

This is dependent upon the lender, as some will reject a mortgage application outright if you have Japanese Knotweed on your property. Some lenders will rely upon the surveyor’s report to determine whether it poses a significant risk to the property and the chances of selling it in the future.
However, it is important to remember:

  • Most lenders will demand a professional eradication, complete with a guarantee against the plant’s return, before making a mortgage offer.
  • The lender may also require the buyer to provide written confirmation that they are happy to proceed with the purchase of the house.

How fast does Japanese Knotweed grow?

It is described by the Environment Agency as ‘indisputably the UK’s most aggressive, destructive and invasive plant’.
Here are the facts:

  • It can grow 10cm a day (20cm at its most prolific).
  • In just 10 weeks its stem can reach 3-4 metres (that’s the height of at least two human beings).
  • Underground, the rhizomes – the mass of roots – are also growing and can spread up to 7 metres horizontally and 3 metres deep.

When is it Japanese Knotweed growing season?

  • Usually the plant starts growing in early spring and by June it can reach 3 metres in height.
  • It will carry on growing until autumn before dying off in the winter.
  • The growing season can be extended with a mild winter and a warm, damp summer.

How can I tell if I have Japanese Knotweed?

Japanese Knotweed is an attractive plant, despite its destructive potential. Its appearance changes depending on the season.
The key things to look out for are:

    • Fleshy, red tinged shoots.
    • Large heart or spade shaped leaves.
    • Leaves in a noticeable zig zag pattern along the stem
    • A hollow stem that resembles bamboo.
    • In spring, look out for fast growing, tall stems with lots of green leaves
    • In summer, you’ll be able to see clusters of cream flowers.
    • In winter, when the plant becomes dormant, the leaves die off and the stem remains upright.

See our more detailed guide to knotweed identification here.

How does Japanese Knotweed damage buildings?

  • As the rhizomes of the plant grow deep underground, its extensive roots can penetrate into the foundations and walls of the house.
  • It is not true the plant can grow through concrete, but it can force its way into the weak spots of buildings in its search for light and water and will grow through cracks, gradually prizing it apart. If you have a slight crack in your patio for example, it will be able to push its way through. If water can get through, so can Knotweed.
  • Underground, as the roots search for water, Japanese Knotweed can crack or block underground drains, which can also cause havoc with your house.

Can Japanese Knotweed cause subsidence?

Yes. Japanese Knotweed can cause subsidence, especially if your property has weak spots to begin with.

  • The extensive roots cause ground movement as they grow, causing soil to shift.
  • If Japanese Knotweed is within 7 metres of a habitable space and is causing damage to boundary walls, then it is a high risk situation and you should contact a reputable builder immediately.

What can be done to eliminate Japanese Knotweed?

The trouble with eliminating Japanese Knotweed yourself is that it grows so fast. Leaving just a root the size of a fingernail is enough for the native plant to start growing again.

  • An aggressive control and treatment regime is necessary to eradicate the plant completely.
  • If you try to dig out the plant, aim to get as much root out as possible and repeatedly destroy regrowth, as this will reduce the energy reserves in the root – however it may still take a few seasons to destroy it.
  • Do not strim or mow the plant as it will cause the plant to spread and grow around your garden and even into your next door neighbour’s garden, as the roots are highly regenerative.
    It is recommended that you use an herbicide weed killer to effectively get rid of the plant.

What herbicide kills Japanese Knotweed?

The most effective herbicide is Glyphosate as the plant’s leaves carry the herbicide right down to the roots. This will exhaust the root system and reduce the plant’s ability to regenerate itself. Don’t fall into the DIY trap – some herbicides actually induce root dormancy and make any subsequent treatment more difficult and possibly more costly.


Where can I dispose of Japanese Knotweed?

You should not consider disposing of Japanese Knotweed yourself.
It is very important to remember:

  • Under no circumstances can you dispose of Japanese Knotweed in your compost, recycling, or waste bins, due to its fast spreading and growing nature.
  • It is classed as ‘controlled waste’ under the Environmental Protection Act 1990 and requires disposal by a licensed waste carrier, who will ensure it is disposed of at a licensed landfill site.


Can I burn Japanese Knotweed?

Yes, but it is important to follow these steps:

  • If you are controlling the growth yourself, make sure you are cutting the stems and leaves off the plant.
  • Leave it out to dry (preferably on plastic material to prevent regrowth) and let it die, before burning it.
  • You must burn the plant on site in your garden and must not move it onto any other land, to riverbanks or the countryside, otherwise you could face fines and the offence in some cases carries a prison sentence.


Can touching Japanese Knotweed harm me?

No. Although it is very destructive, it is not poisonous or harmful to touch.
The only risk of dealing with and touching the plant, is that you could cause it to spread further just by pulling it out.

What to do if you find Japanese Knotweed in your garden?

Once you have identified Japanese Knotweed, it is important to do your utmost to control the invasive weed and make sure it doesn’t spread onto neighbouring properties. If you can’t control it, take photos and seek professional help as soon as possible.
It’s important to remember:

  • It is your legal responsibility to control and destroy the plant.
  • You could be at risk of fines and legal claims against you if it spreads to neighbouring property.

Can Japanese Knotweed be evident in the garden after a year?

Yes, you will probably still see the plant in your garden. Due to the size and regenerative quality of the underground roots, it takes up to three years to fully eradicate Japanese Knotweed. If you’re using herbicides, you should start to see deterioration in the plant. If not, seek professional help.

What’s the best time to treat Japanese Knotweed?

The best time to chemically treat Japanese Knotweed is in the summer. This is when the plant is flowering and the foliage is taking the most nutrients to the roots.

  • You can spray the treatment directly onto the foliage.
  • If you’re doing the treatment yourself, it is best to cut back the stems in the winter for easier access.

What’s the best way to get rid of Japanese Knotweed?

It is possible to control the plant yourself, but it is highly recommended that you hire a professional to ensure the plant will not grow back. A professional will survey your garden to determine the extent of the contamination and can create a management plan to ensure the effective removal and reduce the chances of regrowth.
Solutions and treatments which can be provided by a licenced Japanese Knotweed specialist are:

  • An herbicide application program.
  • Sifting and screening.
  • Root barriers.
  • Excavation (if the Knotweed needs removing immediately, usually on commercial sites).
  • Stem injections.By attempting DIY, you cannot be sure if you are getting the entire root up or causing a bigger problem by spreading it further.