3. Japanese Knotweed

I inherited a large patch of Japanese knotweed when I first purchased the property. The vendor did declare it at the time of the sale, but it was two years into treatment by then and I wasn’t expecting it to be too much of a problem. This was my first encounter with it.

The knotweed was in an awkward spot: in a corner section, close to the garden walls on two sides, with 3 of the neighbours’ gardens in close proximity. It had the potential to cause a number of problems if not handled carefully.

Site map showing areas with Japanese knotweed (in red)

Japanese knotweed is very invasive, spreads rapidly and can cause damage to building foundations and drains. It is ferocious and can grow 20cm in a day; has root system that can go down to 3m; one plant can extend as much as 7m in every direction and it can break through tarmac and concrete. The tiniest fragment of stem or rhizome left in the soil will easily take root and regrow into another plant, so it’s very tricky to eradicate. In my case, there was a real risk it would spread into neighbours’ properties.

Developers usually take the decision to dig all infected soil out, and tank the area, but that was not an option for me, and would risk spread beyond my boundaries. Instead, I took the decision to hold back on any digging within a 5m area around the plants, and to continue to have them treated with a poison twice a year by professional contractors. The Japanese Knotweed Company have some very good information on how they deal with it.

It has taken 5 years to get rid of it in my Walled Garden. Just when I thought it had gone, another plant would pop up in an unsuspecting corner again. Working around this Japanese Knotweed risk became a key factor in deciding on the design of the garden. I haven’t seen any new plants for the last year, but am very aware the rhizomes may still be lurking down there, and keep a vigilant watch.