By most accounts, Japanese Knotweed is a remarkable plant. It can grow some 10cm a day, has a root depth of over 3m and can spread at a rate of knots whilst destroying pretty much anything in its path.
Japanese Knotweed can break foundations, penetrate concrete, tarmac and other super-tough materials. Once established on a piece of land, it can cause serious issues and one thing is for sure – it will not go away on its own.
There is a great deal of misinformation and confusion surrounding Japanese Knotweed. If you find it on your property, then it doesn’t mean that your house will have to be knocked down. However, there are serious implications to not tackling a knotweed problem, both for builders – commercial and self-build, and homeowners.
In this article we’ll cover how much you can expect to pay to remove Japanese Knotweed and other common questions around Japanese Knotweed.
Cost of Removing Japanese Knotweed
Other than the method of Knotweed removal in question, the primary factor in costing Knotweed removal is the size of the area. Japanese Knotweed spreads quickly but if you catch it quickly enough, it can be confined to small spaces.
For reference, an average-sized garden is about 150m² in the UK, according to the ONS.
By compiling quotes from 5 specialist knotweed removers and 7 secondary sources, we discovered these average costs for Japanese Knotweed removal in the UK.
|Type of Removal||20m² - 40m² area||40m² - 60m² area||60m² - 80m² area||80m² - 100m² area|
|Herbicide Treatment||£500 to £850 over 5 years||£700 to £950 over 5 years||£1,000 to £1,400 over 5 years||£1,500 to £2,500 over 5 years||£2,200 to £5,000 over 5 years|
|Sifting and Screening||£1,000 to £1,500||£1,500 to £2,250||£2,250 to £3,250||£3,250 to £4,250||£4,250 to £5,500|
|Excavation and Root Barriers||£1,000 to £1,600||£1,500 to £2,350||£2,250 to £3,350||£3,250 to £4,550||£4,250 to £5,800|
|Complete Excavation with On-Site Disposal||£2,000 to £4,000||£4,000 to £6,000||£6,000 to £8,500||£8,500 to £11,500||£11,500 to £16,000|
|Complete Excavation with Off-Site Disposal||£3,500 to £8,000||£5,500 to £11,000||£8,000 to £14,000||£12,000 to £18,000||£15,000 to £30,000+|
Available Methods of Removing Japanese Knotweed
There are several methods for removing Japanese Knotweed. These range from more basic herbicide treatments to manage the problem to full excavations.
Here is a breakdown of the most common Japanese Knotweed removal methods:
Herbicide Treatment – Cheapest – £1000 to £2500 for a typically-sized infested garden
Japanese Knotweed cannot be killed using standard garden weedkillers. Herbicide needs to be administered at regular intervals where a trained specialist will spray and inject the Knotweed. This eventually tires and kills the plant.
However, this is considered more of a management solution rather than a complete removal solution – the Knotweed will likely return. Costs for herbicide treatment cover a 5-year period on average.
Sifting and Screening – Lower Middle – £3000 to £5000 for a typically-sized infested garden
Sifting and screening involves digging up the suspected area, sifting the soil and screening for Japanese Knotweed material. Rhizomes will be selected and removed, allowing the remainder of the soil to be replaced at the site.
The contaminated soil and material will be burned or removed from the site. Since some rhizomes are likely to remain in the ground, this isn’t always completely effective, but should seriously stunt the growth of Knotweed or kill it off altogether.
Excavation and Root Barriers – Upper Middle – £3000 to £6000 for a typically-sized infested garden
Sifting and screening can be combined with a wider excavation of the contaminated soil, including the emplacement of root barriers that stunt the spread of Knotweed. This is a near-permanent solution if the excavation is effective at removing the rhizome.
Full Excavation with On-Site or Off-Site Burial/Disposal or ‘Dig and Dump’ – Most Expensive – £8000 to £1200 for a typically-sized infested garden
A full excavation will remove the entire contaminated area and either bury the soil on-site at a depth of over 5 metres, or off-site. The material may also be incinerated. This is the most comprehensive but expensive option, it should totally remove all contaminated soil from the land. Knotweed must be disposed of to strict protocol to avoid breaking the law – you can’t just dump it in any old place.
The best option for Knotweed removal depends on the following factors:
- Access to the site
- Geographical location
- Ground/soil type and conditions
- Knotweed maturity
- Rhizome depth and lateral spread
- Site characteristics
If you’re unsure of what type of removal and disposal you need, consult with a surveyor who can conduct an in-depth knotweed survey. Independent surveyors can provide professional advice that can be forwarded to the knotweed removals firm.
One key point is that any removal technique can be backed by an insurance-backed guarantee (IBG). This guarantees that the Japanese Knotweed will not return for a set period, usually between 2 and 10 years. An IBG-backed removal is usually required for mortgage lenders.
What Does Japanese Knotweed Look Like?
If you suspect Japanese Knotweed, check the plant for the following characteristics:
- A hollow stem, similar to bamboo but more fleshy
- Fleshy, reddish shoots
- Fast-growing, tall stems and large leaves
- Cream flowers in summer
- Stem remains in winter but leaves drop
- Large (15cm+ wide) heart or spade-shaped leaves
- Stems have a zig-zag pattern
- Reddish veins in the leaf
In spring, Japanese Knotweed will spread very quickly. If a small clump of suspected Knotweed starts to proliferate and expand then this is a big warning sign.
The Law on Japanese Knotweed
In the UK, Japanese knotweed is classed as a controlled plant under the Wildlife and Countryside Act 1981 section 114 (2) (WCA 1981). It is not illegal to have Japanese Knotweed on your property, but you can be prosecuted for allowing it to spread.
According to surveys, only 49% of those who are aware of Japanese Knotweed know that they are legally responsible for controlling it. The penalty is not exactly harsh – it’s considered an antisocial act and thus can result in an ASBO.
The Anti-Social Behaviour, Crime and Policing Act 2014 states that failure to control Japanese Knotweed where it negatively impacts those in the community can result in a Community Protection Notice. There are also local powers granted under the Town & Country Planning Act 1990 section 215 that allows councils to force homeowners to clear up Japanese Knotweed. Non-compliance results in a fine.
Buying a Home With Japanese Knotweed
Prior to purchasing a home, Japanese Knotweed should be discovered by a surveyor – this is why independent surveyors like RICS Chartered Surveyors are advantageous when purchasing a home.
If a property owner knows about the Knotweed and fails to disclaim it then the buyer may be able to sue for damages, but this kind of case is difficult to prove unless there is some evidence that suggests the seller knew about the Knotweed problem but failed to disclose it. If you hire a surveyor and they fail to pick up Knotweed, which is later exposed as a problem, then you may be able to claim against their insurance.
For the most part, it’s important to exercise caveat emptor or ‘let the buyer beware’ when purchasing a home in the UK. As many as 5% of homes in the UK have some type of Knotweed growing on the premises – it’s always worthwhile checking, both yourself and with the assistance of a surveyor.
Japanese Knotweed and Mortgages
If Japanese Knotweed is presented in a survey then securing a mortgage could be more difficult. This generally depends on the category of Knotweed infestation. There are 4 categories of Knotweed infestation which are described by RICS.
Managing Director for Online Mortgage Advisor, Muglesto, said: “The main consideration for sellers (and buyers) is distance. Japanese knotweed infestations are separated into 4 categories: Knotweed Category 1 if it’s on a neighbouring property, and Category 4 if it’s within 7 meters and/or actually causing damage to the property.”
Your ability to secure a mortgage will likely only be affected if the Knotweed is close to your home. Otherwise, you might need to commit to removing the Knotweed as soon as you move in – the removal firm can provide you with an insurance-backed guarantee that your Knotweed will not return for a guaranteed period.
Selling a Home With Japanese Knotweed
If you find or are otherwise aware of Japanese Knotweed on your home or land, then failure to disclose this may result in action being taken against you.
Locating Japanese Knotweed that is closer than 7 metres to your home is more likely to cause issues – this is when RICS Surveyors will likely recommend further checks before putting the home up for sale.
Japanese Knotweed can drastically affect the value of a home, but this is rare. One extreme example is this couple in Broxbourne, Hertfordshire, who experienced a Japanese Knotweed infestation so bad that plants were sprouting through the floorboards.
This eroded the value of their house from around £300,000 to just £50,000. In such situations, insurance will rarely payout on the damage and cost of removing the Knotweed. This is why stopping Knotweed in its tracks is crucial if you do discover a Category 1 or 2 Knotweed infestation.
Japanese Knotweed and Buildings
The presence of Japanese Knotweed is a major concern for commercial builders and self-builders. It is essential to carry out a full Japanese Knotweed survey on any land prior to purchasing the plot for building.
Specialist surveys will also excavate the land to a depth of some 3 metres to screen for Knotweed rhizomes (roots).
Overall, Japanese Knotweed is becoming easier to treat. It is not the death sentence for a home that some believe it to be, but it’s also far from harmless. Tackling a Knotweed problem early is essential.
Chartered Surveyor Philip Santo FRICS,, said “RICS shares concerns that many people believe Japanese Knotweed poses a much greater risk than it really does. Since RICS issued guidance in 2012, the situation for buyers and sellers has greatly improved. For most affected properties there is now access to mortgage finance once an approved Japanese knotweed management plan is in place.”
Can you Remove Japanese Knotweed DIY?
It is possible, but many underestimate how difficult it is to kill off Knotweed for any reasonable period of time. Also, digging Knotweed and dumping it illegally is a criminal offence – it’s a controlled plant and needs to be disposed of according to correct legal protocol.
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