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Hybrid knotweed (Fallopia bohemica)

Hybrid knotweed is an invasive plant formed when Japanese knotweed and giant knotweed cross, and it can produce viable seed. In Australia, knotweeds are occasionally cultivated and rarely naturalised, but infestations can form dense stands and damage waterways, riverbanks and built structures.

Key facts

Type: Weed, shrub.

Native to: A hybrid of Japanese knotweed and giant knotweed, whose native ranges are in temperate East Asia..

Established in: NSW.

Other names: Bohemian knotweed, Fallopia × bohemica, Reynoutria × bohemica.

Best time to act: Inspect from early spring as shoots emerge, and target actively growing plants before they die back in late autumn, with follow-up checks through subsequent seasons.

Identification

  • It is a perennial shrub with robust, bamboo-like stems and extensive woody rhizomes just below the soil surface.
  • The leaves are generally intermediate in size and shape between the parent species, about 5–30 cm long, with scattered short hairs on the underside.
  • Small whitish, greenish or cream flowers grow in dense clusters in the upper leaf forks.
  • It can form dense thickets during the growing season and die back in winter.
  • Look-alike: Japanese knotweed has smaller leaves, about 3–10 cm long, and usually lacks visible hairs on the underside; giant knotweed has much larger leaves, about 20–40 cm long, with scattered elongated hairs along the underside margins and veins.
  • Look-alike: The three knotweeds can be difficult to distinguish, so have suspected plants checked by a herbarium or qualified botanist.

Where it grows

In Australia it grows in disturbed, often moist places, such as roadsides and riverbanks, and is cultivated in cooler parts of south-eastern Australia.

Why it matters

  • Dense stands can displace other vegetation and leave riverbanks and disturbed ground susceptible to erosion when stems die back in winter.
  • Dead stems can accumulate in rivers and streams, obstruct channels and increase flood risk.
  • Infestations can restrict access to waterways and interfere with irrigation channels.
  • Underground stems and emerging shoots can damage paving, bitumen roadways and other structures.

How it spreads

  • Rhizomes spread the plant, and even a small fragment can regenerate.
  • The hybrid can produce viable seed, which may be dispersed by wind or water.
  • Contaminated soil or plant fragments moved by people, vehicles or machinery can spread knotweed to new sites.

Control options

  • Report suspected infestations to the relevant state or territory biosecurity agency and seek identification advice before disturbing the site.
  • Hand-pull only young, small plants when the soil is moist; remove roots and runners carefully, and contain all plant material because fragments can regrow.
  • Cutting, mowing or grazing can suppress new growth, but repeated follow-up is needed because plants can regrow from rhizomes.
  • Where suitable, use only herbicides registered for the target plant and site, strictly following the product label and any local requirements; seek advice from a qualified weed-control professional.

Hygiene and washdown

  • Keep soil and plant fragments contained, and clean soil from boots, tools, vehicles and machinery before leaving an infested site; dispose of the material where it cannot spread.
  • Avoid moving contaminated soil or plant waste, and do not wash debris into drains or 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.

  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development

Sources