Dormant Japanese Knotweed: Your Questions Answered

By Paolo Martini on 30th April 2020 (updated: 30th April 2020) in News

Japanese knotweed is a rhizomatous plant, meaning that it has a modified stem system that grows underground (much like a tree’s roots), which can render it dormant for extended periods, making it practically invisible to the naked eye from above the system. Unlike a tree’s roots, however, each rhizome is capable of producing new nodes and sending shoots up to the surface to create a new plant.

How does Japanese knotweed remain dormant?

Japanese knotweed remains dormant thanks to its rhizome system, which is both an efficient means of reproduction and survival. Throughout the spring, new shoots quickly develop into mature plants (up to 3 metres tall), whose broad, shield-shaped leaves capture energy from the sun and send nutrients down to the rhizome for storage. During autumn and winter, when there is not enough light or warmth for the plant to survive above ground, the plants die off and the rhizome network remains dormant until the following spring when new shoots can be sent back to the surface.

This rhizomatous system is not unique to the plant, many other common weeds and flowers in the UK have developed the same feature as a powerful tool to thrive, without relying solely on their traditional form of reproduction. Japanese knotweed’s rhizomatous nature, and its ability to remain dormant, are the reasons why it can be such a heavily resource-consuming plant to eradicate.

How long can Japanese knotweed stay dormant?

Japanese knotweed can stay dormant for as long as 20 years, according to the Environment Agency’s Knotweed Code of Practice. Although this document has now been labelled as out of date by the Government, this number matches up with what other Japanese knotweed treatment agencies have reported. Unfortunately, there has been no scientific study to back up this anecdotal evidence yet.

Can Japanese knotweed become dormant after being treated?

Japanese knotweed can be forced into a dormant state with the use of glyphosate overdosing or residual herbicides. Unqualified and unaccredited firms may use these methods to deliver a short-term result at a cheaper price to property owners who want the plant quickly eradicated. Unfortunately, the use of these techniques can be damaging to the environment and ineffective in the long term. Such treatment causes the plant to die back for a season or two, whilst the rhizome system remains dormant underground recovering from the chemical attack. When the right conditions present themselves, the Japanese knotweed will return from dormancy and continue to grow as before.

Professional, PCA-accredited Japanese knotweed removal firms will use legal glyphosate treatments within the recommended doses and time periods. They will not promise a short-term fix to an infestation and will be honest about the length of time that will take to get rid of the plant (which can vary depending on the severity of the infestation). PCA-accredited Japanese knotweed treatment plans can take several years to complete, as firms must return to the site to apply the chemicals after each season to ensure that any new growth is kept to a minimum until the plant is permanently eradicated. This is why trusted Japanese knotweed specialists provide insurance-backed guarantees so that the property owner can trust that the infestation will be irrevocably terminated.

How far can Japanese knotweed spread whilst dormant?

Japanese knotweed will not spread whilst it is dormant, either above ground or underground. The plant does not receive any nutrients from above the ground whilst it is in a state of dormancy, leaving it with no energy to spread further. However, it is possible for the plant to be inadvertently spread if the ground it is sitting in is disturbed. If rhizomes are disturbed by a gardener or heavy machinery, it is more likely that the plant will come out of dormancy to grow and spread once more.

How can you tell if a property has dormant Japanese knotweed?

You can tell if a property has dormant Japanese knotweed by inspecting the grounds for evidence of dead stems. When the plant dies back during the late autumn and winter months, its stems turn brown and brittle but remain above ground, which acts as a protective layer for the rhizome system against the damaging effects of frost. As the leaves fall off the plant during this period, the stems turn from their usual green speckled-purple to a brown-orange colour. These canes can often remain standing throughout the winter months, whilst the plant remains dormant and are the best indicator that a property has dormant Japanese knotweed present.

Can you build on land with dormant Japanese knotweed?

It is possible to build on land with dormant Japanese knotweed, however before doing so certain precautions must be taken to ensure that the plant is not inadvertently spread throughout the development site or, worse still, off the site altogether. Doing so can result in a hefty fine or even custodial sentences for those responsible. For this reason, building developers will need to put a management plan in place to ensure that the spread of the plant is controlled, and the plant will not return from dormancy after construction has been completed to affect others at a later date. There are a number of Japanese knotweed removal techniques open to developers, depending on their budget and the size of their site.

Do surveyors check for dormant Japanese knotweed?

RICS surveyors do not specifically check for dormant Japanese knotweed unless they have been given a reason to. In the RICS guidance, there is a brief mention of the plant’s ability to remain dormant (especially after treatment), however, its appearance whilst in this state is not noted.

The extent to which surveyors should be found negligent for missing Japanese knotweed has been the subject of some contention, usually, they are only found to be so if they could have reasonably found the plant within the course of their survey. Surveys can come in different forms, for example, the RICS note that a mortgage evaluation might not focus enough on the land surrounding the property to make the discovery of Japanese knotweed possible.

Is it worth buying a house with dormant Japanese knotweed?

Buying a house affected by Japanese knotweed (even in its dormant state) is not recommended unless a PCA-accredited, insurance-backed treatment plan has been put in place first. Even if the plant does not pose a threat to any structure, many mortgage lenders will not deal with properties that have the plant present, regardless of if a treatment plan is in place or not. Although some lenders have relaxed their stance on the invasive plant over the years, many still have a zero-tolerance policy concerning properties affected by Japanese knotweed.

Can I sell a house with dormant Japanese knotweed?

It is possible to sell a house with dormant Japanese knotweed, however, it is not advisable to do so without stating clearly that the property is affected by the plant in the TA6 Property Information Form. Even though it might be tempting to make the assumption that the plant has been permanently eradicated, unless you have an insurance-backed guarantee saying as much, the Japanese knotweed is technically still affecting the property and will adversely impact the value of your home. Choosing to gamble that the plant will remain dormant, and selling your property without mentioning it, could lead to you being sued later on.

What happens if I am missold a house with dormant Japanese knotweed?

If you are missold a house or property with dormant Japanese knotweed then you may be able to sue the seller for misrepresenting the property. In most cases, the property owner will have to fill out a TA6 Property Information Form declaring whether Japanese knotweed is present on the land or not (they also have the option to respond with ‘Not Known’). If you can prove that their answer on this form was a lie and that they reasonably should have known about the Japanese knotweed, then you should be able to claim back the discrepancy in the price that you paid for the property, as well as the costs of removing the plant.

If you’ve discovered Japanese knotweed on your property after being told otherwise, or have found that a neighbour has allowed the plant to spread onto your land, we may be able to help you claim back the costs of treatment. Call us today on 0151 242 9050 or send us an enquiry using the contact form, to see if we can help you with your Japanese knotweed problem.

References

Knotweed Code of Practice, Environment Agency

Japanese Knotweed and Residential Property, RICS

Paolo Martini

Paolo Martini is the lead solicitor for Knotweed Help and has over 30 years of experience in the field of Civil Litigation and is an expert on the legal issues faced by individuals dealing with Japanese knotweed on their land. Now considered one of the country's leading litigators in Japanese knotweed law he works alongside the country’s top barristers and experts. His in-depth legal experience and connections to the Japanese knotweed removal industry make him uniquely suited for handling your case.