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Japanese walnut (Juglans ailantifolia)

Japanese walnut is an environmental weed in NSW, found around Mt Irvine in the Blue Mountains and near Port Macquarie. It can form dense stands, suppress other plants and block waterways.

Key facts

Type: Weed, tree.

Native to: Japan.

Established in: NSW.

Best time to act: Small plants can be hand-pulled year-round.

Identification

  • A deciduous tree that can grow up to 15 m tall, with spreading branches and a canopy up to 15 m wide.
  • Leaves can be up to 90 cm long, with 4–8 pairs of leaflets and one leaflet at the end; the leaflets are hairy underneath and sparsely serrated.
  • Male flowers are yellow-green and hang in clusters; female flowers grow in clusters on upright spikes and have purple hairs and pinky-red to whitish green stigmas.
  • Fruit are round, green ripening to rust-coloured, 25–40 mm across, and contain hard, thick-shelled nuts; young fruit have a furry husk.
  • The bark is grey and cracked.
  • Look-alike: Black walnut (Juglans nigra) usually has glossy rather than dull leaves, and female flowers in groups of 2–5 rather than 5–22; common walnut (Juglans regia) can grow to 30 m, has 1–9 leaflets without serrated edges, and 1–3 female flowers per cluster.
  • Look-alike: Tree of heaven (Ailanthus altissima) is taller and has larger leaves that smell bad when crushed; Rhus tree (Toxicodendron succedaneum) is smaller, with smaller leaves that turn bright red in autumn.

Where it grows

In NSW it is a weed around Mt Irvine in the Blue Mountains and has been found north-west of Port Macquarie, and it can grow along waterways, in disturbed forests and shrublands, pastures and roadsides.

Why it matters

  • It grows in dense stands and releases a chemical that prevents other plants growing beneath its canopy.
  • It competes with native plants.
  • It can block waterways.

How it spreads

  • Seeds are spread by water and animals.
  • People can spread seeds by dumping garden waste containing nuts.

Control options

  • Small plants can be hand-pulled year-round.
  • Chemical control methods listed are basal barking, cut stump and stem injection; use registered herbicides and follow product labels and permit conditions.
  • Contact your local council weeds officer if you suspect you have found Japanese walnut.

Hygiene and washdown

  • Avoid dumping or moving garden waste containing nuts, as seeds can spread this way.
  • 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: Regional priority in 1 of 11 regions. Regional priority weed in North Coast. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • Victoria: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • South Australia: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • Western Australia: Permitted, not declared. Listed as a permitted organism in WA, so it can be brought in, but it is not a declared pest.
  • 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