Japanese Knotweed a real scourge

I HAD a call from a reader during the week complaining of Japanese Knotweed.

Frankly, I don’t know anyone who doesn’t complain about it once they see what destruction it can do, although one cannot be blamed for admiring its dark green, red-veined leaves and bouquets of lacy flowers growing on metre-tall stems.

Inability to identify the plant itself and ignorance of its destructive capability are what concern my reader. Why, he asks, do the county councils not launch an information campaign as with other noxious plants and trees? In his area, knotweed is annually consuming more and more ground.

In May, small, dark red shoots burst through the soil, fed by stored nutrients in the extensive rhizome system. In summer, the stems, resembling green, red-speckled bamboos, carry a host of gorgeous, creamy flower heads seductive to the eye (but not to insects; bees, hoverflies or butterflies rarely linger near them). When it dies back in winter, ugly, leafless stems remain. While The National Biodiversity Data Centre has been compiling some records of the presence of giant knotweed (growing to ten feet tall) ignorance is on the side of the common-or-garden variety (it was originally introduced as a garden plant). My reader tells me that his neighbours don’t know what it is and have no idea of what it will do. If they knew, they could make efforts to control it.

I recently saw a large field of an old estate dosed with herbicide and ploughed over, brought into use having lain fallow for decades. Goodbye the early purple orchids, the fleabane, buttercups, yellow bartsia and the rest, but goodbye the ragwort too. However, rhizomes from the knotweed patch, which had, in recent years, colonised over a half acre, were more widely spread during the bulldozing and levelling, and are now sprouting like a rash amidst the new grass. The shoots are racing ahead of the grass, of course: knotweed can grow an inch and a half per day, and ametre in just three weeks.

In towns here and in the UK knotweed has forced itself through the floors of houses, tennis courts, car parks and roadways. It has damaged building foundations, retaining walls, flood defence structures and archaeological sites, lifted gravestones in cemeteries, restricted access for anglers to river banks and subsumed the natural diversity and beauty of the landscape. Indigenous plants are suffocated, and the creatures that thrived on them are decimated.

In Britain, knotweed on your land seriously damages your chance of selling it. Lloyds, Barclays, Abbey National and Banco Santander turn down mortgage applications if Japanese knotweed is deemed to threaten a property. Owners require certificates to guarantee their property knotweed-free. UK law decrees that soil suspected of containing Japanese knotweed must be taken to an appropriate landfill and buried to a minimum depth of 5 metres.

Even then, total extermination is a pipe dream. County engineers have fought tooth-and-nail to eradicate the beast, without success. The ‘beast’ spreads outward at ten feet per annum. Rhizome systems may extend to 2m deep and 7m laterally from a parent plant. One centimetre of cut rhizome produces a new plant within 10 days.

So, what to do? Without the huge expense of burial or systematic chemical onslaught over many years, it cannot be eradicated. However, patches can be controlled chemically or by digging out and burning the rhizomes. Care is required. Stems can produce roots, even when submerged in water or dried. Rhizomes may remain dormant for as long as 20 years.

The new hope of salvation rests on a small bug native to Japan. ‘Knotweed specific’, it sucks the sap from the plant and, in time, debilitates it to death. It feeds on no other plant. Introduced to half a dozen closely guarded and monitored test sites in the UK, it will be observed for five years. If deemed effective and non-threatening to native vegetation, it will become available for introduction everywhere in the UK after that time.

We would then, no doubt, obtain some bug offspring and apply them here. Meanwhile, however, if the public is not educated in identification and control on a local level, valuable grassland and biodiversity may already have been subsumed.

In the interim, there may be some payback in eating it. It is, apparently, toothsome, a bit like rhubarb, and there are recipes on the internet.

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Saturday, May 8, 2021

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