Dealing with Japanese Knotweed

We are reasonably keen gardeners, especially so when we were younger, and we recognised fallopia japonica when we bought our current house in Mellor, Lancashire in 1989. At that time it was known to be a rapidly spreading and invasive pest, a legacy of the Victorian plant hunters, that does not have any natural predators in the UK. It is an attractive plant with reddish stems and leaves with a distinctive shade of green.

By Ancatdubh43 at English Wikipedia [Public domain], via Wikimedia Commons

Section 14(2) of the Wildlife and Countryside Act 1981 states that:

“if any person plants or otherwise causes to grow in the wild any plant which is included in Part 2 of Schedule 9, he shall be guilty of an offence”.

Japanese knotweed is a Schedule 9 listed plant, as are Himalayan balsam, rhododendron, giant hogweed and numerous other plants. As a result anyone allowing it to spread from their land onto others is guilty of an offence.

The Environmental Protection Act 1990 requires that it must be disposed of at a disposal facility licensed to deal with it. If allowed to grow unchecked Knotweed, which propagates rhizomes (think root ginger) can travel long distances below ground at some depth and can damage roads, foundations and walls.

Perhaps surprisingly, it took time for mortgagees and the RICS to press the issue seriously, the first edition of the RICS guide relating to Knotweed and residential property only having come out in May 2012. We had successfully discouraged our Knotweed by nipping out early shoots and burning them but it still re-surfaced in a the south-western corner of our garden each year. In view of our intention the sell the house, I had been following articles and correspondence and when the guide came out I contacted Phlorum, a specialist consultant and treatment contractor with an office in Manchester. We instructed them to carry out chemical treatment which travels down into the rhizomes, backed up by a ten year guarantee. By these measures we hoped to prevent a potential problem during a sale. The cost was a little under £2,000 with three annual re-inspections to confirm the success of the treatment. The alternative is deep excavation to remove contaminated soil. This is costly and can add substantially to the cost of developing sites that are affected.

Only when emptying a large plastic drum of soil and water for disposal, when tidying the garden did I recall that prior to burning the Knotweed shoots, I had at one time put them into water to rot down. Given the pernicious survival capacity of the plant, I decided that I could not assume that this process would necessarily have been successful. I concluded that I should dry the soil residue and sieve out any organic material for burning. I dried the soil in a paved, sheltered spot and sieved it twice. The residue was dried in a plastic box in the airing cupboard and burnt in the garden incinerator, an elaborate precaution, perhaps, but this plant is one of the great survivors and I suspect that following a nuclear holocaust couch grass, bindweed and knotweed would soon emerge triumphant.