Quick Answer
Japanese knotweed is an invasive non native plant that is legally controlled in the UK under the Wildlife and Countryside Act 1981. You are not legally required to remove it from your own garden, but you must not allow it to spread to neighbouring land or the wild. Treatment options are herbicide spraying over 3 to 5 years, or excavation and burial on site. Do not try to dig it up yourself, as any fragment of root left in the soil will regrow.
If you are buying or selling a property with knotweed, you need a management plan from a professional, and you must declare it on the TA6 property information form.
How to Identify Japanese Knotweed
Japanese knotweed looks different through the year, which is one reason it is often missed or misidentified. Knowing what to look for at each stage is the first step in dealing with it.
In spring
The first shoots appear in March or April, looking like reddish purple asparagus spears. They grow rapidly from the crown of the plant, which is the woody base at ground level. By May the stems are 30 to 50cm tall and the leaves are starting to unfurl.
In summer
By June the plant is in full growth, with hollow bamboo like stems up to 3 metres tall. The leaves are shield or heart shaped, bright green, and 10 to 15cm long. The stems are green with red spots, and the leaves grow in a zigzag pattern along the stem.
In late summer and autumn
In August and September, Japanese knotweed produces clusters of small creamy white flowers. The leaves start to yellow in October, and by November the plant dies back to dead brown canes. The dead canes remain standing through winter, which is often when the plant is first noticed.
In winter
The dead canes are brown, hollow, and brittle. They look like bamboo canes and can be 2 to 3 metres tall. The crown at the base is a woody knuckle, often visible at ground level. New shoots will appear from this crown the following spring.
Do Not Confuse With These
Japanese knotweed is often confused with Russian vine (mile a minute weed), bindweed, Himalayan honeysuckle, and the ornamental lesser knotweed. If you are not sure, send a clear photo to a specialist knotweed contractor for free identification before doing anything else.
Why Japanese Knotweed Is a Problem
Japanese knotweed is a problem for three reasons: its growth habit, its reproductive strategy, and the legal and financial implications.
Rapid and damaging growth
The plant can grow 10cm a day in peak season. The roots, properly called rhizomes, can extend 7 metres horizontally and 3 metres deep. They can grow through small cracks in tarmac, drains, and foundations. While the plant does not usually grow through solid concrete, it exploits existing cracks and can cause serious damage to hard landscaping and structures.
Regeneration from tiny fragments
A fragment of rhizome the size of a fingernail can regrow into a new plant. This is why digging it up rarely works, and why contaminated soil must be disposed of as controlled waste. Cutting the stems or mowing the plant can spread it if the fragments are not carefully contained.
Legal and financial implications
The presence of Japanese knotweed can reduce a property value by 5 to 15 percent, and in some cases make a property unsaleable until treated. Mortgage lenders are cautious about lending on properties with knotweed, and most require a professional treatment plan with insurance backed guarantee before they will lend.
Your Legal Obligations
Japanese knotweed is listed under Schedule 9 of the Wildlife and Countryside Act 1981. It is an offence to plant or otherwise cause Japanese knotweed to grow in the wild. It is not an offence to have it in your garden, but you must not allow it to spread to neighbouring land.
If knotweed spreads from your land to a neighbouring property, you can be sued for nuisance and for the cost of removal. Recent case law has confirmed this, with damages awarded in several cases where knotweed has spread.
Under the Environmental Protection Act 1990, Japanese knotweed and contaminated soil are classified as controlled waste. They must be disposed of at a licensed facility, and only by a licensed waste carrier. Putting knotweed in your household bin or taking it to the local tip is an offence.
If you are selling a property, you must declare the presence of Japanese knotweed on the TA6 property information form. Failing to declare it can lead to a claim for misrepresentation from the buyer.
Treatment Options
Option 1: Herbicide treatment
The most common and cost effective treatment is herbicide spraying with glyphosate, applied over 3 to 5 years. The plant is sprayed in late summer when it is in full leaf and pulling nutrients down into the rhizomes. Each year the growth is weaker, and after 3 to 5 years the rhizome is exhausted.
The cost is typically 1,500 to 3,500 pounds for a domestic infestation, depending on size and access, with annual inspections included. The downside is the time required: you cannot be sure the knotweed is gone for at least 5 years, and during that time you cannot replant the area.
Option 2: Excavation and removal
For faster results, the knotweed and contaminated soil can be excavated and removed to a licensed landfill. This is more expensive, typically 5,000 to 15,000 pounds for a domestic property, but it solves the problem in days rather than years.
Excavation is the right choice if you are developing the land, if you need to replant immediately, or if the knotweed is in a location that herbicide cannot reach. The downside is the cost and the disruption to the garden.
Option 3: Excavation and burial on site
A third option is to excavate the knotweed and bury it on site in a specially prepared burial cell, wrapped in a root barrier membrane. This avoids the cost of disposal but requires deep burial, typically 5 metres, and a membrane that is guaranteed for 50 years. It is suitable for larger sites with room for the burial cell.
Option 4: Root barrier membrane
If knotweed is on neighbouring land and you want to protect your property, a vertical root barrier membrane can be installed along the boundary. This prevents the rhizomes from spreading into your land. It does not kill the knotweed but it contains it. Costs are 50 to 100 pounds per linear metre installed.
Do Not Try to Dig It Up Yourself
Digging up Japanese knotweed is the worst thing you can do. Every fragment of rhizome left in the soil will regrow. The digger tracks and tools will spread fragments around the garden. Within a year you will have a worse infestation than you started with. Always use a professional contractor.
Choosing a Contractor
Japanese knotweed treatment is unregulated, which means anyone can set up as a contractor. Choose a contractor who is a member of the Property Care Association (PCA) Invasive Weed Control group, which requires training, insurance, and adherence to a code of practice.
A reputable contractor will:
- Visit the site and provide a written survey and management plan
- Provide an insurance backed guarantee (IBG) of 5 to 10 years
- Use herbicides safely and in accordance with the law
- Provide annual monitoring and treatment reports
- Be a member of the PCA or similar recognised trade body
See the RHS guidance on Japanese knotweed for more on choosing a contractor and the treatment process.
Selling a Property With Knotweed
If you are selling a property with Japanese knotweed, you must declare it on the TA6 property information form. The buyer's solicitor will ask for evidence of a treatment plan, and most mortgage lenders will require an insurance backed guarantee from a PCA registered contractor.
Without a treatment plan in place, the buyer may struggle to get a mortgage, and the sale may fall through. With a plan and guarantee in place, most sales proceed, although the buyer may negotiate a reduction in price.
The realistic approach is to start a treatment plan before putting the property on the market. The cost of the plan is usually less than the reduction a buyer would negotiate, and it removes a major obstacle to the sale.
Buying a Property With Knotweed
If you are buying a property with Japanese knotweed, get a full survey by a PCA registered contractor before exchanging contracts. The survey will tell you the extent of the infestation, the recommended treatment, and the cost.
Use this information to negotiate the price down by at least the cost of the treatment, ideally more. A serious knotweed infestation can cost 10,000 pounds or more to excavate, and the treatment takes years. The vendor should share this cost.
Check that any existing treatment plan is transferable to you as the new owner, and that the insurance backed guarantee is also transferable. Some guarantees are not transferable, which leaves you with no cover if the knotweed returns.
The Knotweed Season by Season
| Season | What You See | What to Do |
|---|---|---|
| Spring (March to May) | Reddish spears emerging, growing fast | Confirm identification, contact contractor |
| Summer (June to August) | 3 metre tall canes with shield shaped leaves | First herbicide treatment |
| Late summer (August to September) | Creamy white flower clusters | Second herbicide treatment |
| Autumn (October to November) | Leaves yellow, canes die back | Stem injection treatment if applicable |
| Winter (December to February) | Dead brown canes standing | Annual monitoring, plan for next year |
Knotweed and Construction
If you are planning building work and discover knotweed on the site, stop work and call a specialist. Construction activity is one of the main ways knotweed is spread, as diggers and tracked vehicles move contaminated soil around the site.
A professional contractor can survey the site before construction begins, identify the extent of the infestation, and put a management plan in place. This may involve excavation before construction starts, or a root barrier system to protect the new structure.
See our garden clearance service page for related work on overgrown sites, though for knotweed specifically you need a specialist contractor.
Cost of Treatment
The cost of Japanese knotweed treatment varies enormously depending on the size of the infestation, the access, and the method chosen. As a rough guide:
- Herbicide treatment for a small domestic infestation: 1,500 to 2,500 pounds over 3 to 5 years
- Herbicide treatment for a larger domestic infestation: 2,500 to 5,000 pounds over 3 to 5 years
- Excavation and removal for a domestic property: 5,000 to 15,000 pounds
- Excavation and burial on site: 3,000 to 10,000 pounds
- Root barrier membrane installation: 50 to 100 pounds per linear metre
Always get at least three quotes from PCA registered contractors. The cheapest is rarely the best, as poor treatment can leave you with a worse problem and an invalid guarantee.
Frequently Asked Questions
Can I just dig up Japanese knotweed myself?
No. Digging it up is one of the worst things you can do. Every fragment of rhizome left in the soil will regrow, and the act of digging spreads fragments around. You will end up with a worse infestation than you started with. Always use a professional contractor.
Will Japanese knotweed damage my house foundations?
Rarely directly. Knotweed does not usually grow through solid concrete, but it exploits existing cracks and can worsen them. The bigger concern is usually drains, paving, and outbuildings. If knotweed is within 7 metres of your house, get a survey.
How long does treatment take?
Herbicide treatment takes 3 to 5 years to fully exhaust the rhizome system. Excavation removes the problem immediately but is more expensive. With either method, you need annual monitoring for at least 5 years to be confident the knotweed is gone.
Can I get a mortgage on a property with knotweed?
Yes, in most cases, but the lender will usually require a treatment plan with an insurance backed guarantee from a PCA registered contractor. Without this, mortgage offers are often withdrawn. Always check with your lender before exchanging contracts.
Is it illegal to have Japanese knotweed in my garden?
No. It is not illegal to have it in your garden, but it is illegal to plant it or to allow it to spread to neighbouring land or the wild. You must also dispose of it as controlled waste. If you have it, you are responsible for managing it.
Final Thoughts
Japanese knotweed is a serious problem but it is not the catastrophe it is sometimes made out to be. With a proper treatment plan from a PCA registered contractor, almost all infestations can be brought under control. The key is to act early, use a professional, and follow the management plan through to completion.
If you suspect you have Japanese knotweed, do not try to deal with it yourself. Take clear photos, contact a PCA registered contractor for a free survey, and follow their advice. The sooner you start treatment, the cheaper and more effective it will be.
Further Reading
- Garden clearance service
- How to restore an overgrown garden
- Garden clearance cost guide
- DIY versus professional garden clearance
Put this advice into practice
Use the relevant CGM tool or follow a related guide, then send us a few photos if you would like a tailored plan.
Sources and citations
This article draws on the following sources. Specific claims are cited inline where relevant.
- CGM field records (2024-01. Site observations from gardens across Oxfordshire, Buckinghamshire, Berkshire and Bedfordshire.
- Royal Horticultural Society (2024). RHS Gardening Advice and Plant Finder. Available at rhs.org.uk.
- Met Office (2024). Regional climate averages, South East England. Available at metoffice.gov.uk.
- British Geological Survey (2024). Soil parent material data for the Chilterns and Thames Valley. Available at bgs.ac.uk.