Japanese knotweed and construction sites – herbicide treatment or physical removal?

It’s a question we’re often asked, and there is a very clear answer. 

If your planned works are likely to result in ground disturbance do NOT rely on herbicide alone. The entire rhizome system should be removed, because the chances are that not all of the rhizome system will be killed by herbicide, the consequence being that your groundworks will spread the knotweed around the site, and possibly off-site too.

Herbicide treatment, if carried out correctly, can result in eradication, although here is the catch, it’s not possible to verify with 100% certainty. Unfortunately all too often we find so called experts not treating it correctly, and instead of killing it they simply put it into temporary dormancy. You may get no re-growth for several years giving the false expectation that the knotweed has been eradicated. I’m sorry to say, that having dug out thousands of cubic metres of soil containing knotweed we know that in the majority of cases a complete kill using herbicide is just not likely.

Re-growth often occurs after the ground has been disturbed – it basically awakens the plant. That is why many knotweed companies state than any ground disturbance following herbicide treatment will invalidate their guarantee.  Beware any knotweed company that recommends just herbicide treatment on a site that is to be developed, it will prove an expensive mistake. 

The rhizome system needs to be completely removed – no half way measures digging down just 500mm and relying on horizontal root barriers. All of the infested soil needs to come out, especially if you are building over it. 

What does one do with the excavated soil and rhizome? 

There are several options available to developers:

1.    Muck the lot away to landfill, and pay the premium for it – a very expensive option.
2.    Stockpile it in a non-critical part of the site and treat with herbicide for years to come. 
3.    Bury it on site, containing it with root barriers, but this isn’t a method we’d recommend.
4.    Process the knotweed infested soils using our Xtract™ method to separate the rhizome from the soil on site. It’ll save you a lot of money, allow you to recover and re-use 99% of the soil volume, and avoid the need for many lorry loads in and out of site. Completed in a matter of days, supported by 10 year insurance backed guarantees underwritten at Lloyd’s.  See Xtract™ in action here.

For more information on our Japanese knotweed removal strategies, call us on 01932 868 700. 

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