How to Treat/Remove Japanese Knotweed

Originally introduced to this county as an ornamental flower, Japanese knotweed is an incredibly voracious plant. Without the natural restraints found in the Japanese ecosystem, Japanese Knotweed can grow unhindered in the UK, to the extent that it can have significant detrimental effects to a property or building.

Potential Damage

Japanese Knotweed can grow up to 10cm in a single day, and a single shoot can grow up to three metres in length. Because of this rapacious growth, Japanese knotweed is famous for causing damage to buildings and substructures by targeting weak points and growing through them, thus exaggerating any cracks or seams.

A house which has an infestation of Japanese Knotweed in its garden

A house which has an infestation of Japanese Knotweed in its garden

Finding Japanese knotweed on a property is a significant cause for concern as it can easily grow through tarmac, building foundations, retaining wall structures and other parts of a building. This not only compromises the structural integrity of a building, but can also lead to plants invading the interior of a building. Ultimately, Japanese knotweed can have a very adverse effect on property values.

Additional Implications

Not only does knotweed present serious issues in terms of the maintenance of your home and the property value, but alongside these concerns is the fact that some mortgages companies will flatly refuse to give homeowners a mortgage should Japanese knotweed be found on the property.

And it’s not only mortgages that are affected; many local authorities also refuse any form of planning permission to infested areas. So knotweed can prevent you from having any kind of extension or further building work to your house.

What to Do?

As such, should Japanese knotweed be discovered on a building development site, you want to deal with the problem immediately. Although it may not seem too significant an issue, given its incredible rate of growth, things can escalate very quickly. All it takes is a couple of centimeters to be a problem, and even then, that little part can remain dormant within the soil for up to 20 years before it starts to really grow. Obviously the best bet is to take whatever measures necessary to remove the infestation, and to do this there are various steps to follow.

A sign at Kings Cross Railway Station, London – Japanese Knotweed has been identified for treatment

A sign at Kings Cross Railway Station, London – Japanese Knotweed has been identified for treatment

Japanese Knotweed Surveys

The first thing to do is call in an expert to make a survey of the site. They will be able to effectively assess the severity of the infestation and what removal treatments will be required.

Japanese Knotweed Removal

Once you have had an expert complete a survey, they can then suggest what method would be best in dealing with the problem, dependent on the density of the knotweed, its location and how much damage it has caused. Due to its aggressive nature, knotweed needs to not only have its visible foliage dealt with, but also its extensive root system underground. As such, drastic measures often need to be taken to ensure it is all completely removed.

A site where the weeds have all been treated, the orange fenced off areas show Japanese Knotweed which requires further treatment

A site where the weeds have all been treated, the orange fenced off areas show Japanese Knotweed which requires further treatment

Four primary methods of treatment/removal of Japanese knotweed removal are as follows:

Herbicide Spray

Herbicidal spray is the most common and cost effective means of dealing with Japanese knotweed. It involves exposing the plant to herbicide which kills it off. The herbicide is applied not only to the visible parts of the weed, but also to its root system so that it can’t grow back. This is the least invasive method of dealing with a problem, and is often the best method if the knotweed is found on a site already developed upon. However, it can take anywhere between 1-4 growing seasons to be completely effective so is quite a time-consuming solution.

Japanese Knotweed being treated with a herbicidal spray

Japanese Knotweed being treated with a herbicidal spray

Japanese Knotweed Barrier

A Japanese Knotweed Barrier is often used in conjunction with another treatment plan such as herbicidal spray, and acts to hinder the spread of the weed. These are dug into the ground up to three metres in depth, as the root systems of the plant have been known to grow to around two metres. These barriers will prevent the spread of any existing knotweed and isolate the infestation, making it much easier to deal with.

Excavation and Removal

The most effective and quickest means of dealing with a knotweed problem is by simply excavating the site and removing any trace of the weed so it won’t be a problem in future. It is possible therefore to use heavy machinery to dig up the site and offload all the soil to an appropriate disposal site (designated waste management area or licensed landfill site). This is ideal for new developments where there is no risk of damaging or disturbing pre-existing structures and can completely deal with the problem in under a week. However it is a costly solution.

Soil Screening

Although soil screening takes longer than complete excavation and removal, it is a much more cost effective and sustainable alternative to excavation. Soil screening involves gradual excavation of the site combined with the labour of an expert surveyor to manually remove all traces of the infestation without having to dispose of all of the soil around it.

Conclusion

Japanese knotweed needs to be dealt with as soon as it is discovered on a building development site. If the site is clear of pre-established structures, the best option can often be to excavate the site (either through complete removal or soil screening.) If the site being developed on has existing structures that might be damaged by this kind of work, a less intrusive method such as herbicide spray may be a better method.

 

Image credits: Wally GromMatt from Londondiamond geezerMarj Joly