Japanese knotweed

Is Japanese Knotweed an urban myth or your worst nightmare asks Debi Holland.

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If you’ve heard the horror stories about Japanese knotweed you might want to know how to identify it and whether it’s really as bad as everyone makes out?

Japanese knotweed or Fallopia japonica is a very vigorous herbaceous perennial that spreads via deep rhizomes (underground stems). Its bamboo-like hollow canes can reach three metres high and grow 10cm a day in the summer, smothering surrounding plant growth.

Japanese knotweed smothered riverbanks at Frangy. Image: Debi Holland

Native to East Asia, this resilient plant was introduced to the UK by the Victorians in the 19th century as an ornamental plant and later used to feed cattle. Its fast-growing nature was embraced to stabilise areas prone to erosion like railway embankments.

Infestations are suppressed in Asia by local pests, diseases and fungi; unfortunately this is not the case in Europe. With no natural predators the strong stems are invasive and destructive. With potential to grow through concrete, it can raise alarm bells if you want to sell your house near infected land.

It can take years to eradicate Japanese knotweed and so it has acquired a bad reputation. Rhizomes can creep seven metres horizontally and two metres deep, optimising success by releasing allopathic chemicals in the soil, which hamper other plants germinating, stifling biodiversity.

What does Japanese knotweed look like?

Young leaves of Japanese knotweed. Image: Debi Holland

Identification can be challenging and you need to get it right. Zig-zag red stems host large green heart-shape leaves on red-flecked canes. Clusters of dainty creamy-white flowers sit on upright racemes during summer and autumn. It’s often mistaken for lilac, Himalayan honeysuckle or the pungent heart-shaped houttuynia.
In spring the underground rhizomes throw up rapidly developing pink shoots, forming asparagus-like spears, dying back in winter to repeat its lifecycle.

Favouring waste ground and riverbanks, it runs rife throughout Europe. At great cost!

  • Millions were spent ridding the Olympic Park of Japanese knotweed in preparation for the London 2012 Games.
  • The river Wye at Tintern, Monmouthshire boasts an impressive display along its banks. The amber leaves are beautiful in autumn and create riverbanks of gold before the plant’s winter hibernation.
  • France shares our problem with extensive growth throughout the country. Areas such as the Rhône-Alpes have infestations along the Usses river around Frangy, Ugine and Alberville; its attractive summer flowers are a spectacle but at the expense of other vegetation flourishing.
  • The City of Annecy in southeastern France held an invasive species exhibition in 2016 which highlighted some of the country’s most troublesome culprits and had the novel idea of placing examples of the excused in cages; very apt. If only that would contain it!

But it is not all bad news. In Japan the plant is foraged as a wild edible vegetable; high in vitamins and antioxidants. The sweet rhizomes make good crumbles and taste like rhubarb! Known as ‘Itadori’ (remove pain) it is used in traditional medicine as an anti-inflammatory and laxative as well treating numerous heart and digestion ailments.

What do I do if I discover Japanese knotweed?

  • Contact the Environment Agency for help and advice.
  • Sightings can be logged on Plant Tracker app.
  • It is not an offence to have Japanese knotweed on your land.
  • Do not let it spread onto neighbouring properties or the wild. Failure to prevent spread can result in a heavy fine or ASBO!

How do I control Japanese Knotweed?

Japanese Knotweed foliage and flowers at Tintern. Image: Debi Holland

Seek professional advice: Invasive Non-Native Specialists Association hold a database of registered specialists.

  • Treat with glyphosate-based weed killer.
  • Regrowth inevitable but plant weaken by digging out and burning when canes dry.
  • Classified as controlled waste; dispose of at an authorised landfill site. Do not compost!

The International Union for Conservation of Nature list Japanese knotweed as one of the world’s most invasive species. So apply caution but in the right environment it could also be admired.

Biological controls are being trialled. Aphalara itadori or Japanese knotweed psyllid is a jumping plant louse from Japan introduced to south Wales and England. The sap-sucking insect feeds on Japanese knotweed. Hopefully mother nature will be able to restrict spread naturally. Time will tell.

Links

Environment Agency

Plant Tracker app

Invasive Non-Native Specialists Association

The International Union for Conservation of Nature (IUCN)

Debi Holland

About Debi Holland

Debi runs her own gardening business in the South West. She has an RHS diploma in Horticulture; studied at Bristol Botanic Gardens and Cannington Walled Garden and was a volunteer Harvester at the National Trust Tyntesfield Estate. She is obsessed with plants and wildlife and loves to visit gardens and seek out plants in their natural habitat. Debi is an avid propagator and seed sower of ornamentals and edibles and a passionate photographer and writer. Her gardening diary can be found at www.debihollandgardening.com
@DHgardening
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