The root of all evil? No – now Japanese knotweed can be good for your garden

·2-min read
Japanese knotweed grows on the banks of the river Wye - Michael Roberts
Japanese knotweed grows on the banks of the river Wye - Michael Roberts

Scientists have finally found a way to make something positive of Japanese knotweed, which invades gardens and damages homes.

The plant, native to Asia, was initially brought to the UK because of its pretty flowers – but its fast-growing roots are strong enough to pierce concrete, bricks and mortar.

Now Environet, an invasive plant specialist which excavates and removes knotweed infestations, has patented a method to turn the roots into a form of charcoal called biochar.

Biochar has a honeycomb-like structure that can be infused with liquid fertiliser and ploughed back into the soil to enrich the land.

The process of transforming the roots revolves around pyrolysis – burning in the absence of oxygen – and Nic Seal, the Environet managing director, spent years fine-tuning it.

Knotweed roots transformed into a form of charcoal called biochar
Knotweed roots transformed into a form of charcoal called biochar

"Japanese knotweed rhizome is very difficult to burn because it is incredibly smelly and incredibly smoky and that is because you are burning off resins," he told The Telegraph.

"The method that we've devised to try and minimise the emissions comes down to the temperature. If you don't get it hot enough you basically get a load of smoke and a load of smell – but if you get it hot enough, to around 600°C, you get a completely clear haze."

The key is getting it hot enough to burn cleanly but prevent it getting so intense that the carbon molecules break down and form carbon dioxide.

Mr Seal said that, for every tonne of dried Japanese knotweed fed into the process – around half of which is water – roughly 200 kg of biochar will be produced.

"It also sequesters [captures and stores] 150-200kg of carbon into the soil – that's got to be good," he added. One tonne of Japanese knotweed fed into the process effectively sucks up to 720 kg of CO2 out of the atmosphere, equivalent to negating the emissions of driving 3,000 miles in a Ford Fiesta.

Mr Seal said the process also works for other troublesome invasive plants like bamboo and giant hogweed, and he hoped to pyrolyse all the plant waste he excavates from clients' gardens. By the end of next year he hopes the biochar production process, which takes less than 24 hours, will make his business carbon neutral.

"Japanese knotweed may be the villain of the plant world – but for the first time it has the potential to do some good," he said.

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