Article 8 – Japanese Knotweed
Thus far in this series, we have dwelled on the typically slow moving, passive influences on a building’s structure such as rising damp and wet rot. Now, we look at an active, living, fast growing influence which has gained some considerable notoriety in the last few years – Japanese Knotweed.
Identifying Japanese Knotweed
Also known as Fallopia Japonica, the plant is a fast growing perennial with tall, dense stems which die back in the winter and re-grow from deep-rooted rhizomes (underground stems) in the early summer.
In summer time, the plant can grow up to 2.1m tall, with shovel shaped leaves up to 14cm in length. The stems are bamboo-like in appearance and typically a reddy hue.
What’s the problem?
In Japan, the plant is found on the slopes of volcanoes – where the ash and hot climate keep the plants small. When transferred into the temperature British climate, however, it is able to grow unabated.
At its fastest growth rate, Knotweed can grow at up to 20cm (8 inches) a day. It has no natural predators and the ferocity of its expansion is such that native plants are outcompeted and consumed.
Difficult to believe? Check out this video:
With property in mind, Knotweed can grow through tarmac and concrete – which means that growth near a building can be devastating to the structure. One family in 2013, for example, were told that it would be cheaper to knock down and rebuild their £300,000 home, rather than have the extensive Japanese Knotweed growth treated and eradicated.
The estimated cost to the UK economy of treating Knotweed and in devalued homes is now £166m a year.
Non-chemical solutions
Digging
This is a rather more hit and miss solution than is perhaps sensible in most instances, bearing in mind the very serious consequences to property value, condition and mortgage prospects of identifying knotweed in a garden.
Nevertheless, digging is possible. Bear in mind:
Rhizomes penetrate deeply, meaning the re-growth is likely. This will need to be destroyed again, potentially every season for several seasons until the energy reserves in the rhizomes are exhausted.
Japanese Knotweed is ‘controlled waste’ under the Environmental Protection Act 1990 and requires disposal at licensed landfill sites only. Ensure that any specialist knotweed contractors you consider using are licensed to remove and dispose of the weed safely.
Absolutely do not include the weed in your household or green waste collection scheme disposal.
Psyllid (bug)
A more recent solution, a plant sucking bug with a penchant for knotweed has been introduced as a biological control. It is not available to gardeners at the moment, but should spread naturally within a five to ten year period.
Chemical solutions
Glyphosate-based weed killers may be effective when applied to the cut canes and the foliage, which will pass it through the plant.
The best time for spraying is in the late summer at the flowering stage, though access can be difficult as the weed will be tallest at this stage.
A gardener undertaking this themselves may need several seasons to obtain full results. A contractor may be able to cut this period, with access to stronger chemicals.
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