The Lower Mangapiko Streamcare group, formed in 2006, is working with local landowners, Waikato Regional Council and Waipā District Council to stop the spread of the invasive tree species Japanese walnut (Juglans ailantifolia).
The invasive tree has spread near Mangapiko and Mangaohoi Streams on public and private land.

Japanese walnut is a weed because the tree is long lived (50 years plus), it's a canopy tree, so shades out other more desirable plants from underneath, and produces many, long-lived seed.

It can also tolerate hot and cold conditions, wet and dry conditions and isn't shaded out, like some weeds can be.

It is also likely to be allelopathic, which means it produces biochemicals that can inhibit the germination, growth, survival and reproduction of other plants in its vicinity.

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Japanese walnut (Juglans ailantifolia) male catkin.
Japanese walnut (Juglans ailantifolia) male catkin.

At a recent meeting of the Lower Mangapiko Streamcare group, the idea was raised to ask members of the public who live in and around Te Awamutu, Pirongia and along any streams, to help in tackling this weed. Japanese Walnut trees occasionally drop nuts that are then widely spread via waterways.

As Japanese walnut forms tall stands, it can also topple and block waterways, and the seed is favoured by rats.

The removal of any Japanese walnut when young is much easier and less expensive than when the trees get larger.

The easiest way to get rid of it is to hand pull small plants – this can be done all year round, and mulch afterwards.

Other ways to control this weed, that can be done at any time of the year, is to cut through the trunk and paint the stump: cut trunk near to the ground, and swab freshly cut stump with metsulfuron-methyl 600g/kg (5g/L), or a 50/50 mix of Round Up (Glyphosate) and water.

Pest - Juglans ailantifolia; common name, the Japanese Walnut, is a species of walnut native to Japan and Sakhalin.
Pest - Juglans ailantifolia; common name, the Japanese Walnut, is a species of walnut native to Japan and Sakhalin.

Bore and fill: make 1 hole every 100 mm around the trunk and fill each hole with 5g metsulfuron-methyl 600g/kg. Otherwise, you can spray small plants with metsulfuron-methyl 600g/kg (5g/10L), but this is only effective in the summer.


Photos of Japanese walnut for identification can be found on the Weedbusters site: weedbusters.org.nz and for further information see the WRC website: waikatoregion.govt.nz/services/regional-services/plant-and-animal-pests/plant-pests/