What is Japanese Knotweed?
In the least favourable terms possible, Japanese knotweed is an invasive perennial weed. Which means two things. One, it out-competes native plant species, and two, it might die back during winter, but it doesn’t go away completely. It’s classified as one of the most common invasive types of knotweed in the UK, and it’s notoriously difficult to get rid of. It can withstand extreme temperatures, happily growing on the sides of volcanoes and in sub-zero temperatures alike, and it has no natural pests to curb its spread across the UK. It’s pretty much indestructible, and you should get it removed or treated by a trained professional. Otherwise even a piece of the plant the size of a thumbnail could lead to a whole new plant growing.
If you want a fun fact to go with those slightly daunting ones, Japanese knotweed is edible and apparently tests similar to rhubarb!
It’s most easily identified by a distinctive zigzagging lateral stem structure and shovel–shaped leaves, along with white flowers that bloom in the summer. The reason Japanese knotweed is so difficult to deal with is because it grows incredibly quickly, sprouting tall, dense stems that resemble bamboo. These stems can grow to over 2 metres tall, and sprout thousands of lateral stem structures to create an umbrella that blocks out any sunlight. It’s been in the UK since the mid-1800s, when it was a popular plant for landscaping, and has been causing problems ever since.
Why Does Japanese Knotweed Cause Problems?
Killing native plant life isn’t the only problem that Japanese knotweed can cause. It’s well known to also cause damage to structures and property, which is why it’s such a concern in the residential property industry.
As we mentioned above, Japanese knotweed is a very hardy, almost impossible to eradicate plant, which can rapidly overgrow areas if not managed properly. This can affect hard landscaping, disrupting lightweight structures, drainage systems and garden walls in the process. In rare situations it can even cause direct structural damage to buildings.
Of course, there are other plants that can cause problems and damage to buildings, like buddleia or ivy, and they can be difficult to manage too. But because Japanese knotweed is resistant to pretty much everything (including fire, acid, traditional herbicides and even petrol), you need to involve professionals to remove it, and this can get very expensive.
Ground that’s affected by Japanese knotweed can’t be easily developed without taking special measures, which usually involve the removal and disposal of the plant and any contaminated soil by appropriately licensed operators, which will, again, increase development costs. There are also issues if a neighbouring property has Japanese knotweed, as it tends to spread between properties and management is difficult. This has even led to legal action in the past!
So you can see how the presence of Japanese knotweed, or even the stigma of it being on the property before can affect amenity, saleability or ultimately the total value of a property. The big difficulty we as surveyors face is that whilst it can be easy to recognise it sometimes, it can be easily missed. Particularly in the winter months, or if the plant has been deliberately removed or even concealed.
What Should I Do If I Have Japanese Knotweed?
While it’s not illegal for you to have Japanese knotweed on your property, you can get into legal trouble if it spreads beyond your boundaries, or if it affects adjoining properties. If your survey finds Japanese knotweed, further investigation should be done by an accredited member of a recognised trade body (currently the Property Care Association or INNSA) These professionals will produce a management plan to remediate the problem with aherbicide treatment, chemical control or remove it completely. The option they recommend will depend on a number of things, including cost.
Chemical control is the application of a specific type of herbicide over a period of several growing seasons. In our experience it’s an effective and economical treatment, but it does usually require a minimum of 4 years treatment and monitoring before a completion certificate can be issued.
On top of that, only properly licensed organisations may remove Japanese knotweed from a property, and they must take it to an appropriately licensed waste facility, which can be expensive. Due to how deep the spreading rhizomes (the bit that makes the plant grow) go, a significant amount of soil will also need to be removed or displaced to install a root barrier and stop the spread. So you’re not just getting rid of the knotweed that’s visible – you’re removing everything in an area of between 2m-3m beyond the nearest visible piece of knotweed.
What If the Property I’m Buying Has Japanese Knotweed?
The good news is, the seller is required to state whether Japanese knotweed is, or has been, present on the property using the TA6 form used for conveyancing. Whether you’re a buyer or a seller, it’s also worth being pro-active and checking the property yourself for Japanese knotweed. Disputes over the identity of the plant, the failure to disclose its presence or the lack of a management plan can all result in delays to the purchase process, increased costs and even claims for damages after the sale is complete.
So, if you’re buying a property, we always recommend that you commission a building survey. This will include an inspection of the grounds. On several occasions we have found knotweed on, or near properties. We can then make an assessment based on the current RICS guidance, and provide reasoned advice on the next steps.
However, the survey isn’t a detailed check of each plant, and for large properties or those with overgrown or unclear boundaries this might be difficult to do. In fact, there are many reasons a competent surveyor might miss Japanese knotweed in a survey, for example if it’s been cut down, concealed or if dormant rhizomes lie buried below a cleared site. So a specialist survey may be necessary.
If you’re considering a property and are worried about issues that could affect your purchase, we are here to help. Just give the team at Harrison Clarke a call on 02381 550 051 to speak to one of our experienced surveyors today.
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