Knotweed News

They think it’s all over…it isn’t

Author: Mike Clough

Date Posted: Wednesday 16th August 2017

I have spent a lot of my life warning people about the dangers of Japanese knotweed. I have also spent a lot of time telling people not to panic about Japanese knotweed. Like many other things in life, there is a fine line between having something to worry about and there being nothing to get in a blather about.

Many people say to me…. ‘Will Japanese knotweed grow through concrete??’. The correct answer should be – ‘no’ – …. ‘in well laid, properly finished properties with decent depth of foundations Japanese knotweed will not damage the building structurally.’

But – I then find myself saying…. ‘Then again…. if you do build on top of actively growing knotweed you will find that with poorly laid concrete or inadequate depth of building foundation – this can result in the plant showing its ugly head.’

Once you have explained the problems and the potential for damage that Japanese knotweed has then most people GET what it’s all about. However, some people just don’t understand how plants grow.

If I were to say that Japanese knotweed is capable of growing 7 metres in all directions, which is a phrase you will often hear repeated, then your average client will imagine JK rampantly growing in all directions – at speed – whilst devouring everything in its path.

Now in certain circumstances this can happen – open ground/free draining/no obstructions etc. etc. However…if you were to have a concrete block on one side of the knotweed growth and open ground on the other side – then the knotweed will not force its way through concrete just for the hell of it ….it will take the easiest route and grow into the open ground.

Clients and often surveyors will take the ‘7 metres in all directions per season’ – as a strict rule. If a property has a knotweed problem at the bottom of the garden (say 21 metres away) the surveyor measures the distance – divides it by 7 (answer 3) – and then says you will have Japanese knotweed in your front room in 3 years’ time (or whatever the answer from the sum is)…

It’s just not as simple as that though.

If you can – then you should ask an experienced Japanese knotweed consultant for the risk factors within any newly found stand of the plant. With working with the plant for several years, you can generally tell which plants will cause a problem and which will be less troublesome.

Occasionally Japanese knotweed will just ‘sit there’ and do nothing – no growth, no spread…nothing but come into leaf, flower and then die back. It may do nothing…for years – but then perhaps having been disturbed by nearby construction work – it will suddenly shoot off in all directions…

It’s difficult to explain this to clients who just want a ‘black and white’ answer…

I’m also having problems explaining the after-care management of Japanese knotweed. Most clients get that you have to have an aftercare package – but once you have a situation where there is no visible growth they… ‘think it’s all over’…and don’t want to pay you to come back any more.

This is when the problems can start to come back…

Be careful when managing your land that’s been infested with Japanese knotweed. You need to monitor and check for a period of ten years. Try and establish a new sward onto the area that you’ve cleared – try and get the naturally occurring species to re-establish. This may mean the use of seed mixes or taking turves from nearby ground.

Leaving bare ground where you have treated an invasive non-native species just leaves ground available to the next invader to come along.

Be pro-active.

It’s not over till the fat bloke says it is.


Mike C


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Mike Clough

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