A knotty issue

It’s time for an eco-solution for Japanese knotweed waste, says Nicolas Seal

When I started dealing with Japanese knotweed removal over 20 years ago, herbicide was the preferred method of treatment. However, environmental pressures, the hazards associated with chemical use and the need for guaranteed instant removal by developers and homeowners selling property, have meant that excavation has grown hugely in popularity in recent years. 

This type of removal takes a matter of days or weeks, as opposed to herbicide treatment which needs to be applied over two or three growing seasons; and even then there’s a risk the plant is induced into dormancy rather than being completely killed. 

Japanese knotweed has been in the UK for around 180 years and in that time it’s spread pretty successfully. Research suggests that around 5% of residential properties are affected, posing problems for homeowners who are now required to declare it when selling, even if the infestation has been treated.  Lenders also require treatment plans with insurance-backed guarantees be in place before they will lend on an affected property, meaning an entire industry has grown up around Japanese knotweed treatment. 

But where knotweed is being excavated, what’s happening to the plant waste? The sad truth, in most circumstances, is that it’s being re-buried in landfill sites. It doesn’t make sense to dig it up from one location, transport it miles and dump it in another. Thousands of tonnes of such waste are being landfilled unnecessarily each year, a total waste of valuable resources.

In response, in 2008 I invented a method and machinery for removing Japanese knotweed root and rhizome from the soil on commercial sites. Using an excavator, infested soils are loaded into the Xtract™ machine, which separates the rhizome from the soil, leaving the soil ready for immediate re-use on site and the separated rhizome, representing typically 1% of the original volume, needing to be disposed of. The method saves massive quantities of otherwise perfectly good soil from being sent to landfill, with considerable financial and environmental savings. 

We’ve developed the method for smaller residential properties, following the same principles. In simple terms it’s called weeding, but don’t underestimate the challenge. The root and rhizome system can be extensive, spreading laterally from the visible above ground growth, often to a depth of 2m or more. And the roots and rhizomes are very brittle; leave a small piece and it can grow.

So, once we have the roots and rhizomes out of the ground, what happens? Is there a beneficial use for the Japanese knotweed rhizome and cane waste?

A lot of research into this subject has shown that the creation of biochar has the potential to solve the Japanese knotweed waste problem. Biochar is created by heating organic matter in the absence of oxygen, otherwise known as pyrolysis. This not only kills the roots and rhizomes, but also removes the naturally occurring tars, to leave carbon in the form of charcoal. The charcoal has a honeycomb-like structure with remarkable characteristics, which can be charged up with additives such as liquid organic fertilisers to make an excellent soil amendment.

Not only does biochar substantially improve soil structure, but more importantly, its carbon is locked-up in the soil for hundreds if not thousands of years. When Japanese knotweed is growing, it is very efficient at scavenging carbon dioxide from the atmosphere, forming carbon-based plant material and releasing oxygen back into the atmosphere. Where carbon is retained in the plant material or in the soil, the process can be considered carbon negative, reducing atmospheric carbon dioxide levels and ultimately helping in the fight against climate change.

Following field trials at our research and development site in Send, Surrey, we’ve recently filed a patent application for a method and apparatus for converting invasive weeds, including Japanese knotweed, into biochar and are extremely excited with our results. Improvements need to be made to make the process even more efficient and scalable, but ultimately I believe all knotweed material could be converted from being a biologically active pest into carbon gold. 

Putting it back into the ground to improve soil structure, and locking-up carbon in the ground for hundreds if not thousands of years must be better than filling up landfills with infested soil, where, if it doesn’t regrow, probably will rot, creating damaging landfill gases.

Our next phase of research is to develop the process to create cost-effective biochar that is proven to improve soil structures and sustain new (non-knotweed!) plant life, for the benefit of all.   


Nicolas Seal is an environmental scientist, founder and MD of environet UK, www.environetuk.com

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