Biosecurity Spotters logo

Japanese knotweed (Fallopia japonica)

Japanese knotweed is naturalised in New South Wales, Victoria and Tasmania and can grow in a range of Tasmanian environments. Its rapid growth and persistent rhizomes can form dense stands that displace native vegetation and damage infrastructure.

Key facts

Type: Weed, herb.

Native to: East Asia, including Japan, China, Korea and Taiwan.

Established in: NSW, VIC, TAS.

Other names: Asiatic knotweed, Reynoutria japonica, Polygonum cuspidatum.

Best time to act: Look for plants from spring through autumn when shoots and leaves are visible, and time treatment during active growth in accordance with the herbicide label.

Identification

  • A fast-growing perennial that can reach about 5 metres tall.
  • Stems are stiff, hollow and bamboo-like, often purplish red when young, and remain upright after the leaves die back.
  • Leaves are large, with a squared-off base and a pointed tip.
  • Small white flowers grow in slender spikes in summer.
  • Coarse, spreading rhizomes can produce new shoots; the fruit is a papery, three-angled covering around a shiny black seed.
  • Look-alike: It can be confused with other knotweeds, including giant knotweed and hybrids; seek expert identification where needed.

Where it grows

In Australia it occurs in riparian areas and disturbed sites, including near settlements and in waste areas.

Why it matters

  • Dense stands shade and can displace other vegetation, including native plants in riparian communities.
  • Its deciduous habit can leave infested ground vulnerable to winter erosion.
  • Dead stems and litter can accumulate, reduce plant diversity and impede water flow, potentially increasing flood risk.
  • Rhizomes can damage roads, footpaths, buildings and sewage pipes.

How it spreads

  • Rhizomes and stem fragments can grow into new plants.
  • Fragments can be carried downstream, while contaminated soil can spread propagules when it is moved.
  • Seed production is possible when male and female plants are present.

Control options

  • For isolated plants, carefully remove the whole root system and collect all stem and rhizome fragments; avoid disturbing or moving contaminated soil.
  • Use only herbicides registered for the target plant and site, following the product label and any applicable state or territory requirements; seek qualified advice for established infestations.
  • Plan for diligent follow-up over multiple seasons, as long-lived rhizomes can produce regrowth.
  • Contact the relevant state or territory biosecurity agency for current advice, reporting requirements and approved control options.

Hygiene and washdown

  • Clean soil and plant fragments from machinery, vehicles, boots and tools before leaving an infested site, and contain the cleanings so fragments cannot spread.
  • Do not move contaminated soil or dispose of knotweed fragments where they could take root or enter waterways.
  • Inspect and clean vehicles, plant and footwear before leaving an infested area, paying attention to tyres, radiators, belly plates, tracks and floor mats.
  • Wash down at a designated bay where runoff is contained and can be monitored for seedlings.
  • Record each washdown so the client has evidence of clean entry and exit.

Classification by state

  • Australia (national): No national listing. Managed under state and territory law.
  • Queensland: Not prohibited or restricted. Not listed as prohibited or restricted matter in Queensland. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Not a priority weed. Not a state or regional priority weed in NSW. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • Victoria: State prohibited weed. State prohibited weeds are the highest priority: the Victorian Government is responsible for eradicating them, so report any sighting to Agriculture Victoria.
  • South Australia: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • Western Australia: Declared pest (C1), prohibited. C1 Exclusion: to be kept out of WA, or out of the parts of WA where it is not established. Report any sighting to DPIRD. It must not be brought into WA (prohibited organism).
  • Tasmania: Declared weed. Property managers are required by law to control declared weeds, and their import and sale are prohibited.
  • Northern Territory: Not declared. Not on the NT A to Z weed list, and Weeds Australia does not list it as declared in the NT. Check the NT declared weeds list before moving or planting it.
  • Australian Capital Territory: Not declared. Not declared under the Biosecurity Act 2023 (ACT). A general biosecurity duty still applies.

Legal obligations

Every state and territory places a duty on landholders and people working on land to prevent the spread of invasive species, such as the general biosecurity obligation in Queensland and the general biosecurity duty in New South Wales. Declarations and control requirements differ between states and local government areas, so check the current official list before work starts.

  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria
  • Tasmania: Biosecurity Act 2019 (Tas), administered by Biosecurity Tasmania

Sources