• This destructive weed could knock 10% off your home’s value

    A few weeks ago I listened to a British podcast about a fast-spreading invasive plant that is causing huge problems in some parts of the northern hemisphere. The plant that was being talked about is called Japanese Knotweed, and in Scotland and other areas of the UK, people are being denied mortgages as a result of knotweed infestations. This is because this plant can easily spread under walls, pavements and patios, and cause damage to buildings and roads.

    decorative picture of Fallopia japonica
    Japanese knotweed (Fallopia japonica) is causing havoc in the UK

    Not being familiar with this weed, I Googled ‘Japanese knotweed’ and found pictures of a weed that I’d seen in many home gardens in my state of Victoria, Australia. I thought, that’s not a particularly troublesome weed in my experience. It’s a small weed, easily prised out of the ground. What’s the problem? It turns out that in Australia, where I live, the common name ‘Japanese knotweed’ is used to refer to a rather benign plant, Persicaria capitata, (with aliases: knotweed, persicaria, pink-head knotweed, pinkhead knotweed, and pinkhead smartweed). Persicaria capitata can be controlled by hand weeding or foliar spray.

    descriptive photo of Persicaria capitata
    This is the ‘Japanese knotweed’ that we know in Australia (Persicaria capitata) – thankfully a rather benign weed and not to be confused with Fallopia japonica

    But whereas Persicaria capitata is small, the ‘Japanese knotweed’ that was the subject of the podcast is a giant plant that is unrelated to Persicaria in science and charasteric. Fallopia japonica is having a drastic effect on residential property sales in the UK, where it is present in nearly every 10 sq km. Legal firms are specialising in litigation surrounding Japanese knotweed infestations and landowners are being sued for allowing the plant to encroach onto neighbouring properties.

    You’d be forgiven for thinking that Fallopia japonica is quite an attractive plant with its mid-green, large, rough leaves and coral pink stems and petals. Leaves are up to 12 cm long and 10 cm wide and have a pointed tip. The hollow stems, which arise at intervals from rhizomes, form a zig-zag structure. Small, white flowers appear on the stem during summer. 

    Japanese knotweed
    (Fallopia japonica) can grow two metres a year

    Why is this plant so invasive?

    Fallopia japonica is one tough plant. Roots grow up to three metres deep and spread many metres from the parent plant. Young shoots grow as rapidly as 8 cm a day and rhizomes can grow two metres a year from a depth of six metres.  Plants are commonly 2-3 metres high and may reach 5 metres in height, and once it’s established, it displaces other plants, and becomes a dense thicket. Most of the foliage of this perennial plant dies back over the colder months, but the plant itself is very hard to eradicate and requires professional expertise.

    Fallopia japonica in Australia

    Not surprisingly, Fallopia japonica is a prohibited weed in Victoria. According to the Australian Government Department of the Environment and Energy, Fallopia japonica can be found in various places in Victoria and Tasmania, and a few isolated infestations have been recorded in Queensland and the Northern Territory. But it does seem that at this point in time, the presence of this plant is limited and under control in Australia. If you do recognise Japanese knotweed, anywhere in Australia, please contact the department of Primary Industries or its equivalent in your state, to help spare Australia from this highly destructive weed.

    Sources and related links

    Australian Government Department of the Environment and Energy

    Brisbane City Council

    Department of Primary Industries, Parks, Water and Environment (Tasmania) 2014

    Gardening Australia 2006