How Japanese knotweed presents an opportunity for carbon capture

27th January 2022

Nic Seal explains how the UK’s Japanese knotweed problem could present an opportunity for carbon capture

Every year, the Japanese knotweed removal industry generates thousands of tonnes of plant waste, which is consigned to landfill. Here, it regrows or decays, giving off gases such as methane. We need a new way of dealing with it – ideally one that is not just zero-carbon but carbon-negative, reducing atmospheric carbon dioxide (CO2).

Japanese knotweed is rife in the UK and costs an estimated £166m to treat each year. It’s not a notifiable plant, and it isn’t illegal to have it growing on your land, but it is illegal to let it spread through inaction – encroachment-related legal cases are on the rise. The Environment Agency calls knotweed the UK’s “most aggressive, invasive and destructive plant” due to its regenerative capabilities, arising from an extensive underground rhizome system that can regrow from a piece as small as a fingernail.

At Environet, a Japanese knotweed specialist, we recognise that our business activities create waste and emissions to land, air and water, which we seek to mitigate as best we can. We don’t like adding excessive amounts of dangerous herbicides to the environment, as they can find their way to watercourses. As far back as 2008 we thought it was crazy to dig up knotweed-infested soils from one site only to dump it in a landfill site many miles away. We wanted a zero-waste solution, so our research and development team developed an eco-friendly screening method, whereby the rhizome is sifted and separated from the soil, allowing the cleaned soil to be reused. This avoids the environmental and financial costs of consigning vast quantities of otherwise good soil to landfill and importing clean fill.

However, with the knotweed removal industry still generating thousands of tonnes of plant waste every year, our industry needs to go further. Developing a solution for dealing with the waste we generate is the next step – and all the better if we can also deploy the plant’s powerful CO2-scavenging powers in the fight against climate change.

Waste not

A couple of years ago, we started developing a method of converting excavated knotweed rhizome into biochar. This involves heating the organic matter in the absence of oxygen – pyrolysis – thus removing naturally-occurring tars to leave carbon in the form of charcoal. The charcoal, which benefits from a honeycomb-like structure, can be charged with additives such as liquid organic fertilisers to create a soil amendment, improving soil structure and locking carbon away for hundreds, if not thousands, of years.

We recently secured a patent for the apparatus and method used in this solution, which could eliminate the need for landfill disposal and eventually be used to deal with all of the UK’s Japanese knotweed waste, as well as that produced by other invasive plants such as bamboo and giant hogweed. This would not only save clients considerable sums of money, but also create a useful byproduct that could be supplied to the agricultural and horticultural sectors and ensure that the carbon captured by invasive plants is never released back into the atmosphere.

The next stage of our research will concern delivering economies of scale, including how to process large quantities of knotweed waste quickly and efficiently while managing emissions produced by the pyrolysis process, as well as sourcing markets for the biochar.

We hope to be converting all excavated plant waste we produce into biochar by the end of 2022 as part of our pledge to become a carbon-neutral business – and, in doing so, reduce our industry’s reliance on landfill and the associated costs incurred by those dealing with this highly invasive plant. In harnessing its CO2-scavenging abilities, Japanese knotweed could finally do some good!

Image Credit | Shutterstock

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