what is japanese knotweed

Japanese knotweed (Fallopia japonica) or Japanese bamboo is a tall herbaceous perennial plant with bamboo like stems which can grow up to 10cm a day.

It was brought to Britain from Japan as an ornamental garden plant in the mid-nineteenth century. However, over time it has become widespread in a range of habitats, particularly roadsides, riverbanks and derelict land where it causes serious problems by displacing native flora and causing structural damage.

Due to its vigorous nature and the damage it causes it is one of only two terrestrial plants listed by the UK Wildlife and Countryside Act as illegal to cause it to grow in the wild.

An estimate in 2010 put the cost of Japanese knotweed to the British economy at £165 million (Williams et al.) and would cost £1.56 billion were control to be attempted countrywide. These control methods which rely mainly on chemicals have been deemed unsustainable by many and so a longer-term solution to the problem is required.

It often grows in dense thickets. The mature stems are speckled purple; with young stems growing in zig-zags. Leaves are shield-shaped, alternate along the stem and can grow up to 15cm (6 inches) long. They are often a distinctive, uneven pale yellowy-green colour when mature. Dead stems persist throughout the winter.

It is common in urban areas, particularly on waste land, railways, road sides and river banks.

Infestation is widespread and common thoughout the whole of the UK.

By far it outcompetes native plants and once established is extremely difficult to eradicate. It can cause major structural damage to roads and houses, even growing through asphalt and concrete, and in some instances the presence can lead to mortgage applications being refused.

Japanese knotweed spreads rapidly in the wild,  solely by vegetitive means from either fragments of rhizome (root) or stem.

Please do not strim, shred or attempt to compost this plant!

You may not know

  • Japanese Knotweed shoots are capable of growing through tarmac and concrete.
  • A piece of rhizome less than 0.7g, which is smaller than a fingernail, is capable of growing into a new plant and starting a new infestation.
  • The plant is so resilient it is even immune to burning and can rise from the ashes to grow once again.
  • Japanese Knotweed can grow up to 40mm per day during late spring/early summer.
  • It is estimated that there is at least one infestation of the plant in every 10km2 in the UK.
  • The biggest cause of the Japanese Knotweed spread is fly-tipping.
  • Japanese Knotweed has been known to spread after travelling from machinery used on building sites.
  • The plant stores nutrients in a maze of roots underground thus enabling it to hibernate in the winter months.
  • Dead Japanese Knotweed stalks and stems can take up to 2 years to fully decompose.