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Japanese knotweed and giant knotweed

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Why they are pest plants

Japanese knotweed and giant knotweed grow rapidly from an extensive rhizome system, forming dense, long-lived thickets. They exclude other desirable species and prevent native seedlings from establishing.

Japanese knotweed and giant knotweed are spread through the movement of rhizomes. They are very hardy plants and are difficult to eradicate. Originally from Japan, knotweed is a very serious invasive pest plant in parts of Europe. It grows primarily in disturbed areas, roadsides and river banks. Japanese knotweed and giant knotweed occur at sites in Taupiri, Karangahake (south Coromandel) and in south Waikato.

How to recognise these pest plants

Upright shrub-like herb perennials which grow to 3-4 m. They have reddish-purple shoots which appear early in spring but, as the canes grow, the leaves unfurl and the plants turn green.

Identifying features

  • Creamy white flowers in panicles appear from December through to March.
  • Glossy brown fruit, bluntly-angled nut about 3 mm long.
  • Leaves are broadly ovate, pointed at the tip, flattened at the base and the stalk is dark crimson.
  • Stems form zigzags, are reddish-purple when young, and hollow when mature.
  • Grows on roadsides, river-banks and waste places.

Responsibility for control

Waikato Regional Council is responsible for controlling this plant. Waikato Regional Council’s Regional Pest Management Strategy declares Japanese knotweed and giant knotweed total control pest plants for the entire Waikato region.

Japanese knotweed and giant knotweed are banned from sale, circulation, distribution and commercial display.

How to control Japanese knotweed and giant knotweed

Do not attempt to control Japanese or giant knotweed yourself.

If you see these pest plants on your property, call 0800 BIOSECURITY (0800 246 732) to report it to your local biosecurity pest plant contractor.

More information

For further advice contact your nearest biosecurity pest plant contractor on 0800 BIOSECURITY (0800 246 732).

For more information on what we do phone Waikato Regional Council's freephone 0800 800 401.

For more information on pest plants check out the Weedbusters website.

Related publications

  • View our Waikato Regional Pest Management Strategy
  • 'What makes a pest a pest? - A guide to Waikatos pest management future'. Pick up, download or order free from our offices.
  •  'Plant me instead - Plants to use in place of common pest plants'. Pick up or order for free from our offices.
  • 'National Pest Plant Accord Manual'. Pick up for $10 from our offices or order/ view it online for free.
  • 'Poisonous plants and fungi in New Zealand - A guide for parents, schools and child minders'. Pick up or order for $15 from our offices.

Disclaimer

Although this content has been prepared in good faith from a number of sources believed to be reliable, Waikato Regional Council does not give any warranty that all information contained is accurate or complete, or that advice given will be appropriate in all circumstances.