Juglans ailantifolia

Japanese walnut

Family: Juglandaceae

Origin: Japan

Japanese Walnut leaves and immature fruit.
Large spreading tree growing up to 15m tall. Leaves are very large, growing up to 60cm long.
Photo credit: Weedbusters

Regional Pest Management Plan (RPMP) status

  • Whole region — Sustained control
  • National Pest Plant Accord Species

View more about the RPMP statuses

General description

Wide-spreading, deciduous tree < 15 m tall. Leaves are < 60 cm long with long petioles. Male catkins are c. 15 cm long. Flowers are red/pink/purple and borne in October – November. Fruit is obovoid and < 4 cm long.

What you need to know

To help protect our environment:

  • You must not breed, distribute, release or sell Japanese walnut. As Japanese walnut is a National Pest Plant Accord species, these restrictions apply within the Auckland region and across the whole of New Zealand.
  • You must not plant Japanese walnut within the Auckland region, unless you are transferring an existing plant on your land to another location within the boundaries of the same property.
  • You must destroy any Japanese walnut on land that you occupy if it has been planted in breach of the above rules and you are directed to do so by an authorised person.


Riparian margins, wetlands, roadsides, scrub, disturbed forest, grasslands, drainage channels.


Seeds dispersed by water and possibly rodents. Human-mediated dispersal through deliberate plantings.

Impact on environment

Forms dense stands, excluding native vegetation. Allelopathic.


Site Management

Follow up treated areas 3 times per year. Encourage natural regeneration of native plants or replant treated areas where possible after 2-3 treatments to establish dense ground cover and minimise reinvasion.

Recommended approaches

Physical control

Method: Dig out.

Plant parts requiring disposal: Seeds.

Disposal options: Remove to greenwaste or landfill


Biocontrol is currently not available for this species.

Community agrichemical control recommendations

No qualifications: Cut stump and paste freshly cut base of stems with metsulfuron gel.

Basic Growsafe certified: Cut stump and spray freshly cut base with 5g metsulfuron-methyl per 1 L of water.

Certified Handler/Experienced agrichemical user: Drill and inject trees with 5g metsulfuron-methyl per 1L of water if safe to do so. Drill 18mm holes (tangentially angled downwards) in a spiral up the trunk. For 50mm stems drill one hole. For 100mm stems drill two holes. For larger stems drill holes 150mm apart. Foliar spray seedlings with 5g metsulfuron-methyl per 10L of water and 20ml penetrant.

Safety notes

Large trees must not be drilled that are closer than 1.5 times the height of the tree from paths, walkways and property.

Trees over 4 metres in height should be treated and then removed by a qualified arborist.

Caution: When using any herbicide or pesticide please read the label thoroughly to ensure that all instructions and safety requirements are followed.

Close up of Japanese Walnut fruit.
Flowers are red/ pink/ purple and appear in October. Male catkins also appear with pollen to pollinate the female flowers.
Photo credit: Weedbusters
Japanese Walnut leaves with large cluster of fruit.
Fruit can float and can be distributed by water. Grows in dense stands out-competing native trees.
Photo credit: Weedbusters
Japanese Walnut flowers.
Flowers are pollinated by wind and insects. Deciduous tree that loses leaves in winter.
Photo credit: Weedbusters