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Property Surveying Blog
Written by Beau Davies

Japanese Knotweed

Unsurprisingly, mortgage lenders have become cautious with properties that are affected by Japanese knotweed. It is estimated that it has resulted in a loss of value to the UK property market by up to £20 billion!

Classified as an invasive non-native plant, the fast-growing plant can reach a height of two meters and spread underground through its roots. It has bamboo-like stems and inhibits waste ground, road verges, gardens, railway lines and so on.

Japanese Knotweed

The damage of Japanese Knotweed

At its origin, Japanese knotweed is unsurprisingly native to japan. In the 19th Century, it was introduced to the UK as an ornamental plant thanks to its ability to grow rapidly and form a thick screen.

These days, since its issues have become apparent, under the Wildlife and Countryside Act 1981, it is a criminal offence to plant it or cause it to grow. The plant itself poses such a risk as it not only makes lenders incredibly nervous, in extreme cases it can find its way through cracks in concrete and cavity walls, resulting and causing damage to homes.

Other places that it likes to grow in are, drains and other buried services, patios, driveways, retraining walls, lightweight outbuildings with poor foundations, conservatories and gardens.

Japanese Knotweed

Treatment of Japanese Knotweed

The eradication of this stubborn plant can be a real challenge, and as the Royal Horticultural Society stated, it requires ‘steely determination’.

There are four treatment options available:

Excavation of the Plant and its Roots

This involves deep and extensive excavation on site. The waste is then priced per ton to dispose of the weed, this method is quick but by no means cheap. Expect to see this when a large well-funded developer wants to rid the problem quickly.

Depending on the location of the excavation, this approach can also bring into play party wall procedures.

On-Site Burial

The plant will be covered with a minimum of 5 meters of overburden rock and soil. This requires qualified professional advice and is not always effective. It can also significantly throw off the level of the site, so in our experience is rarely used.

Encapsulation with Membranes

Affected soil can be treated by installing a barrier membrane to the roots. If done effectively, this can stop cross-boundary spreading and while it doesn’t address the issue, it does contain it. This would then need to be combined with chemical control or excavation. You’d expect to see this approach to stop a potential claim from a neighbouring property owner.

Chemical Control

Chemical Control

Chemical control is probably the most cost-efficient option and also tends to be the most widely used. While it is effective, it is extremely time-consuming as the process can last for years, requires ongoing attention and visits. This can be a bit of a pain if the plant is located in rear gardens as it means you’ll need to arrange access periodically for at least 12 – 24 months, so expect a few late days to work!

While this is only a plant, don’t underestimate it! It can cause havoc to even the most well-prepared homeowner. Over the years, we’ve seen this plant on a fair few inspections. If you’d like some further advice, please feel free to get in touch today.

Remember don’t panic, you’ll soon be saying sayonara of さようなら to this pesty plant!