Japanese knotweed
| Common name: | Japanese knotweed |
| Scientific name: | Fallopia japonica |
| Management programme: | Organism of interest |
Report this pest!
If you have seen or suspect the presence of this pest, we need to know. Please report it.
Japanese knotweed, also known as Asiatic knotweed, grows rapidly from an extensive fleshy underground root system, forming dense thickets that can live for a long time. They exclude other desirable species and prevent native seedlings from establishing.
Knotweed is a very hardy plant and difficult to control. Originally from Japan, knotweed is a very serious invasive pest plant in many parts of New Zealand. It grows to form dense thickets that expand rapidly. It can choke waterways, as the thickets completely replace any other vegetation.
- Size: A perennial shrub that forms dense thickets with numerous cane-like stems up to 2 metres high. The stems are hollow when mature and have a zig-zag appearance between each leaf node. The stems are hairless with a reddish tinge.
- Leaves: Oval and pointed, alternating, and up to 25 cm long.
- Flowers/seeds: Small and white, and are clustered along short branches between December and April, followed by small (2.5–3 mm long) angled glossy brown seeds.
- Root system: They propagate from rhizomes (underground shoots) as well as from seeds.
- Habitat: Includes shrublands, urban areas, landfills and dumping areas, and waterways.
Japanese knotweed is easiest to spot when its small white flowers are out in summer and autumn (December–April).
There are no rules for landowners in Otago regarding Japanese knotweed. Under the Otago Regional Pest Management Plan (2019–2029), it is only classified as an organism of interest, with sites being recorded for possible control in the future.
Landowners/occupiers are encouraged to report Japanese knotweed on their properties or sightings using the reporting tool at orc.govt.nz/reportpests
Physical control:
- Dig out small patches and dispose at landfill or burn.
Chemical control:
- Cut the stump near the ground and paste year-round using a suitable herbicide gel containing either glyphosate, metsulfuron-methyl or triclopyr.
- Inject the stem year-round with metsulfuron-methyl.
- Spray foliage when the plant is actively growing from spring to autumn with a penetrant plus glyphosate, metsulfuron-methyl or triclopyr.
- Spray in spring, but a follow-up may be required in February or March.
- Check for regrowth every three months for at least two years and re-treat as necessary.
Caution
Japanese knotweed can spread by soil movement, garden waste and equipment such as diggers and farm machinery. When disturbed, it can break up easily into small fragments that can easily regrow. Take special care not to disturb it or transport it to new sites.
When using any herbicide or pesticide PLEASE READ THE LABEL THOROUGHLY to ensure that all instructions and safety requirements are followed.
Disclaimer: Mention of product trade names does not endorse these products nor imply criticism of similar products not mentioned. The Otago Regional Council does not give any warranty that the information is accurate or complete or that it is suitable for all circumstances.
November 2019
PDF | 5 MB
The Otago Regional Pest Management Plan focuses on dealing with harmful pests that impact our environment, economy, and communities. It identifies species like rabbits, wallabies, gorse, and wilding conifers that cause problems on our land. Guided by the Biosecurity Act 1993, the plan works with stakeholders to set rules and controls for managing pests. Through consultation, it aims to protect our region's unique ecosystems and species from the damage caused by these invasive organisms.
August 2025
PDF | 1 MB
Japanese knotweed, also known as Asiatic knotweed, grows rapidly from an extensive fleshy underground root system, forming dense thickets that can live for a long time.
