What you need to know about Japanese knotweed?

You may have heard about Japanese knotweed in numerous media reports over the years, characterising it as a growing threat to our homes. But how worried should you be as a homeowner? Perhaps cautious is a better word.

Japanese knotweed was introduced to the UK almost 200 years ago, as a decorative addition to the growing trend for spectacular gardens. It’s a herbaceous perennial, which means it dies back each winter before re-emerging in the spring. Crucially, Japanese knotweed is an invasive, extremely tough plant which can cause serious damage to property.

How to identify Japanese knotweed

Knotweed is distinctive in appearance, and can be identified by these features:

  • Reddish-purple shoots when it first breaks ground in spring.
  • A thick, bamboo-like purple-flecked stem.
  • Heart-shaped green leaves  growing in a zig-zag pattern along the stem .
  • Creamy-white flowers which appear in late summer/early autumn .

What's the problem with Japanese knotweed?

The real problem is threefold:

  • The speed at which it grows.
  • The damage it can cause to property.
  • The difficulty of getting rid of it.

Knotweed will usually start growing in early spring and the plants can reach a height of three metres by June, far outgrowing any native plants. However, it’s not what you see above ground that’s the real problem. The plant’s roots go down so deep that they can reach the same length as the height of the plant above ground and can spread out by up to seven metres.

Japanese knotweed is also incredibly strong, often growing through tarmac and sometimes even concrete. That’s why the roots are a real danger to drainage systems, house foundations and walls. 

Japanese knotweed is considered a potential risk to a home if found within seven metres of it. And some mortgage lenders may refuse a loan if knotweed is identified in a property survey.

The government deems knotweed such a menace to buildings that it requires people do all they can to combat it and like here at Legal & General most buildings insurance policies won’t cover the damage it causes.

That leaves the final problem – Japanese knotweed is very difficult to get rid of and tackling it can cost a tidy sum.

How to get rid of Japanese knotweed 

Getting rid of Japanese knotweed permanently is easier said than done.

There are two main methods, plus a speculative third option:

  • Digging it out
  • Chemical treatment
  • Bugs

Digging out
While it’s possible to dig out knotweed, it won’t be a quick or easy operation due to the depth and spread of its root system. It may take a number of seasons to see whether you’ve been completely successful or whether it re-emerges from the soil.  

Then there’s the problem of the disposal of what you dig out. Knotweed can be officially classified as ‘controlled waste’ under the 1990 Environmental Protection Act and can only be disposed of at licensed landfill sites. It’s illegal to dispose of it in normal household waste or garden waste.

You could also dry it out and incinerate it yourself, provided your local council allows for it. If you pursue this option, you'll have to make sure every bit is burned before being buried, as anything left alive may start growing again the following year.

Chemical treatment
A glyphosate-based weed-killer will do the job, but could take three years to be fully effective. You might prefer to get professionals in, who will have access to more powerful weed-killers.  

Bugs
In Japan, the plant is kept under control by a type of insect called psyllids, which eat it. Trials are presently underway with these insects in the UK, to see whether they could be part of the solution to the problem. 

Both disposing of dug-out knotweed and professional chemical treatment can prove expensive, though possibly less than the potential loss of property value and rebuilding work that could be necessary if the weed is left untreated.

It’s also worth to know that while it isn’t illegal to have knotweed in your garden, it's controlled under the Wildlife and Countryside Act 1981. An amendment to the Anti-Social Behaviour, Crime and Policing Act 2014 also states that you mustn’t plant knotweed yourself or cause it to grow in the wild. The penalties for violating these regulations run from heavy fines to a possible prison sentence.   

On the whole, it’s best to avoid buying a property that already has the problem and get a professional survey done if you have any suspicions.

Always be cautious when undertaking any task that you're not fully familiar with as you could injure yourself or those around you, seek professional help where required.

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