Japanese Knotweed

(Fallopia japonica)

Japanese knotweed leaves

What is Japanese knotweed?

Japanese knotweed is an environmental weed.
Japanese knotweed is a declared weed in Tasmania under the Tasmanian Biosecurity Act 2019 and associated Regulations. The importation, sale and distribution of Japanese knotweed are prohibited in Tasmania.

How to identify Japanese knotweed

Japanese knotweed is a fast growing, hollow-stemmed and semi-woody perennial plant that forms dense leafy thickets. Plants are commonly 2-3 metres high and may reach 5 metres in height.
Japanese knotweed stems are stiff, hollow and bamboo-like, becoming tough, woody and speckled with age. The stems arise at intervals from rhizomes or underground stems. Leaves measure up to 12 cm long and 10 cm wide and have a pointed tip.
Japanese knotweed flowers from December to February. Flowers are small and white and borne on slender branched spikes. Fruit is a three-angled papery sheath that covers a single shiny black seed. Most of the foliage dies back over autumn.
Roots are coarse, perennial rhizomes which are yellow when cut. They grow up to three metres deep and spread many metres from the parent plant.

Japanese knotweed leaves & infestation, © Chris Moore.

Japanese knotweed in Tasmania

Japanese knotweed occurs in the north and south of Tasmania, mostly in garden situations. Japanese knotweed has not been recorded in the north-west of Tasmania.
Japanese knotweed is a significant weed in riparian (river-side) areas, disturbed coastal habitats, wetlands and roadsides, where it displaces other vegetation by shading and root competition. Japanese knotweed dies back over winter, leaving bare soils open to erosion.
The rhizomes (underground stems) of Japanese knotweed can spread under walls, pavements and patios and cause damage to roads and buildings.

Legal status of Japanese knotweed in your area

The legal responsibilities of landholders and other stakeholders in dealing with Japanese knotweed are laid out in the Japanese Knotwood Statutory Management Plan.
Use Table 1 (Zone A municipalities) in the Japanese Knotwood Statutory Management Plan to find out if this weed occurs in your municipality.

What you need to do

If you locate Japanese knotweed anywhere in Tasmania, or if you find a plant that you think could be Japanese knotweed, immediately contact Biosecurity Tasmania on 03 6165 3777 to report this weed.

Detailed management and control guidelines for Japanese knotweed can be found in the Japanese Knotweed Control Guide. Refer also to Herbicides for Japanese Knotweed Control. For further information see Weed Links and Resources.

See also

Japanese Knotweed Control Guide

Get in early - large infestations can be very difficult to eradicate;
Remove and carefully dispose of all rhizome material: small broken pieces of Japanese knotweed left in the ground or in the removed soil can grow into new plants;
Check an infestation regularly over several years in case of regrowth from remnants of the root system; and
Commit to follow up control measures each year for up to 3 years to eradicate the plant.

Don't

Don't leave new infestations to establish - get in early and eradicate;
Don't let it seed;
Don't rely on one attempt at eradication - the plant will grow back from the persistent rhizome;
Don't leave fragments of rhizome in the soil: they will regrow into new plants;
Don't dump removed material or removed soil, as this can spread the plant to new areas; and
Don't compost or mulch root material as rhizome fragments can reshoot.

Spread of Japanese knotweed

In Tasmania Japanese knotweed appears to have only reproduced asexually by regeneration of rhizome fragments. However, spread by seed also occurs in the plant's native range in Japan.
Japanese knotweed rhizomes are able to regrow from small fragments. Rhizome fragments can be spread in contaminated soil and garden waste. Plants have been known to regenerate from fragments buried up to 1 metre deep.
The long lived rhizomes can spread at a rate of 2 metres a year, leading to rapid expansion of an existing colony.
See the Washdown Guidelines for Weed and Disease Control for detailed information on how to wash-down equipment and personnel to reduce the chance of spreading Japanese knotweed.

Physical removal

Small plants can be pulled by hand or dug up. Make sure that all root and rhizome material is removed. The use of a sieve to get all root material out is recommended.
Larger clumps can be controlled through repetitive cutting of the stems near the soil surface and immediate application of herbicide (see Chemical Control, below). Several cuttings and herbicide applications will be needed to kill the plant. Cutting back on its own is not enough as the rhizome will survive.
The dug-up plant and root material must be disposed of carefully to avoid spreading the weed. All material should be removed and dried out and then burnt in a safe manner.

Chemical control

There are herbicides registered for foliar and 'cut and paint' application to Japanese knotweed in Tasmania. See Herbicides for Japanese Knotweed Control for more information.
Repeat applications are required within the growing season to capture all new growth. If sprayed with a foliar herbicide you will need to cut back material after approximately three to four weeks to see new shoots and get access to them.

Herbicides for Japanese Knotweed Control

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