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Giant knotweed (Fallopia sachalinensis)

Giant knotweed is a tall, rhizome-forming perennial that can form dense stands in moist, disturbed places. Where it occurs in Australia, it can crowd out other plants and spread when contaminated soil or plant fragments are moved.

Key facts

Type: Weed, herb.

Native to: Japan, Sakhalin and the Kuril Islands.

Established in: ACT, VIC.

Other names: Sakhalin knotweed, Sacaline, Polygonum sachalinense, Reynoutria sachalinensis.

Best time to act: Survey during the growing season and treat actively growing plants in late summer or autumn, before the stems die back.

Identification

  • It grows as a tall, upright herbaceous perennial, with stout, hollow, bamboo-like stems that die back in winter.
  • The leaves are very large, broadly oval to heart-shaped, with a pointed tip and a heart-shaped base.
  • Small cream to greenish-white flowers appear in branched clusters, usually in late summer or autumn.
  • The plant grows from an extensive underground rhizome system, which can produce new shoots some distance from the main clump.
  • Look-alike: Japanese knotweed (Fallopia japonica) looks similar but usually has smaller leaves with a squared-off or nearly straight base, rather than the distinctly heart-shaped base of giant knotweed.

Where it grows

Where it occurs in Australia, look for it in moist, fertile, disturbed ground such as gardens, roadsides, stream banks and drainage lines.

Why it matters

  • Dense growth can shade and displace native plants and other vegetation.
  • Stands can restrict access along waterways and other affected sites.
  • Winter dieback can leave ground exposed, potentially contributing to erosion on banks where vegetation cover is lost.

How it spreads

  • The main means of spread is by rhizome fragments, which can grow into new plants.
  • Plant fragments can be carried in moved soil, on machinery or by water.
  • Seeds can also contribute to spread.

Control options

  • For small plants, carefully dig out the entire rhizome system and collect all fragments; do not compost the material.
  • Use only herbicides registered for the relevant site and purpose, and follow the product label and any local requirements; seek advice from a qualified weed-control professional if needed.
  • Avoid slashing or mowing, which can spread fragments, and dispose of cut or excavated material safely.
  • Check treated areas regularly and repeat control as needed because regrowth can arise from surviving rhizomes; contact the state or territory biosecurity agency for local advice where relevant.

Hygiene and washdown

  • Remove soil and plant fragments from boots, tools, vehicles and machinery before leaving an infested site, and dispose of the material safely.
  • Avoid moving soil or plant material from affected areas, especially near 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: Not listed. Not found in the Western Australian Organism List. Check with DPIRD before moving or keeping it.
  • Tasmania: Not declared. Not a declared weed in Tasmania. Other laws may still apply.
  • Northern Territory: Check NT list. There is no NT weed profile for it. Check the NT declared weeds list for its current status.
  • 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.

  • Australian Capital Territory: Biosecurity Act 2023 (ACT), administered by ACT Biosecurity
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria

Sources