Japanese walnut (Juglans ailantifolia)

Japanese walnut is a deciduous tree up to 15 m high. Its wide spreading foliage limits the growth of other plants.

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How does this weed affect you?

Japanese walnut is an environmental weed that grows in dense stands. It:

• releases a chemical that prevents other plants from growing under its canopy
• competes with native plants
• blocks waterways.

What does it look like?

Japanese walnut is a deciduous tree that can grow up to 15 m tall. It has spreading branches that produce a wide canopy up to 15 m. It can live for over 50 years.

Leaves:

Leaves are up to 90 cm long and made up of 4–8 pairs of opposite leaflets with a single leaflet at the end. Leaflets are:

• 6–18 cm long and 3–8 cm wide
• hairy underneath especially on the veins
• sparsely serrated along the edges.

Flowers:

Each tree has both male and female flowers.

Male flowers are:

• yellow-green
• in hanging clusters about 15 cm long.

Female flowers are:

• clustered in groups of 5–22 flowers
• on upright spikes, which are swollen near the base
• covered with purple hairs with obvious pinky-red to whitish green stigmas.

Fruit are:

• green ripening to rust-coloured
• round and contain hard, thick-shelled nuts
• 25–40 mm in diameter
• covered in a furry husk when young
• in clusters of up to 22 fruit.

Bark is:

• cracked
• grey.

Similar looking plants

Japanese walnut looks very similar to other walnuts:

• Black walnut (Juglans nigra), which usually have glossy rather than dull leaves and the female flowers are in groups of only 2–5 flowers rather than 5–20.
• Common walnut (Juglans regia), which is taller (up to 30 m), has less leaflets (1–9) and the leaflets do not have serrated edges. It only has 1-3 female flowers in each cluster.

 It also looks similar to these weed species:

• Tree of heaven (Ailanthus altissima), which is taller, has larger leaves that smell bad when crushed.
• Rhus tree (Toxicodendron succedaneum), which is a smaller tree and has smaller leaves that turn bright red in autumn.

Where is it found?

Japanese walnut has been grown as an ornamental tree.

It is a weed around Mt Irvine in the Blue Mountains in the Greater Sydney region. In 2021 plants were found north west of Port Macquarie on the North Coast and these plants are under an eradication program.

It is native to Japan. It has become a weed in New Zealand.

What type of environment does it grow in?

Japanese walnut can grow in a range of soils though it prefers moist well drained soils. It grows best in sunny sites and is frost tolerant. It can grow:

• along waterways
• in disturbed forests and shrublands
• in pastures
• along roadsides.

Maps and records

• Recorded presence of Japanese walnut during property inspections (Map: Biosecurity Information System - Weeds, 2017-2023)
  These records are made by authorised officers during property inspections under the Biosecurity Act 2015. Officers record the presence of priority weeds in their council area and provide this to the NSW Department of Primary Industries. Records reflect the presence of the weed on the date of inspection.
  
  

How does it spread?

By seeds

Seeds are spread by water, animals and by people dumping garden waste containing nuts.

References

Native Plant Trust USA (2021) Go Botany: Dichotomous Key, Juglandaceae, Juglans retrieved 12 April 2021 from: https://gobotany.nativeplanttrust.org/dkey/juglans/#c1,c2

nzflora. Weed Factsheet Juglans ailantifolia. Retrieved 12 April 2021 from:  http://www.nzflora.info/factsheet/Weed/Juglans-ailantifolia.html

PFAF Plants for a future. Juglans ailanthifolia - Carrière. Retrieved 12 April 2021 from: https://pfaf.org/user/Plant.aspx?LatinName=Juglans+ailanthifolia

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 12 April 2021 from: https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Juglans~ailantifolia 

Rietveld, W. J. (1983). Allelopathic effects of juglone on germination and growth of several herbaceous and woody species. Journal of Chemical Ecology, 9(2), 295-308.

Weedbusters New Zealand (2017). Weed Information Sheet Japanese Walnut Juglans ailantifolia. Retrieved 12 April 2021 from: https://www.weedbusters.org.nz/what-are-weeds/weed-list/japanese-walnut/pdf/?nocache=1

More information

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Control

If you suspect you have found Japanese walnut contact you local council weeds officer.

Hand weeding

Small plants can be hand pulled year-round.

Chemical control

Basal barking

Spray or paint the herbicide mixture around the base of each stem from ground level up to a height of at least 30 cm from the ground. Wet the bark to the point of runoff. Old rough bark will require more spray than smooth or young thin bark. Do not apply to wet stems as this can repel the diesel or kerosine. 

Cut stump method

Cut the stem and then thoroughly cover the cut stem with herbicide within 15 seconds.

Stem injection

Drill or make cuts into the sapwood and fill with herbicide within 15 seconds.

Herbicide options

See Using herbicides for more information.

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PERMIT 9907 Expires 31/03/2025
Fluroxypyr 200 g/L (Comet® 200 herbicide)
Rate: 35 mL per L diesel/kerosene
Comments: Basal bark
Withholding period: Do not graze failed crops and treated pastures or cut for stock feed for 7 days after application. See label for further information.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Fluroxypyr 333 g/L (Starane™ Advanced)
Rate: 21 mL per L diesel/kerosene
Comments: Basal bark
Withholding period: Do not graze failed crops and treated pastures or cut for stock food for 7 days after application. See label for more information.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Various products)
Rate: One part product to 1.5 parts water
Comments: Cut stump, drill, frill axe or injection
Withholding period: Nil.
Herbicide group: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L with Metsulfuron-methyl 600 g/kg (Various products)
Rate: 1:1.5 glyphosate to water + 1 g metsulfuron to 1 L water
Comments: Stem injection
Withholding period: Nil.
Herbicide group: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All pest plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
North Coast	Regional Recommended Measure* (for Regional Priority - Eradication) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
	*To see the Regional Strategic Weeds Management Plans containing demonstrated outcomes that fulfil the general biosecurity duty for this weed click here

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Reviewed 2023