Growth Habit: 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. 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.
Type of Plant: Japanese knotweed is a fast growing, hollow-stemmed and semi-woody clump forming perennial plant that forms dense leafy thickets. Plants are commonly 2-3 metres high and may reach 5 metres in height.
Flowers: Japanese knotweed flowers from December to February. Flowers are small and white and borne on slender branched spikes.
Fruit/Seed: Fruit is a three-angled papery sheath that covers a single shiny black seed.
Dispersal: Japanese Knotweed does spread by seed in Japan. So far in Tasmania it has only spread vegetatively in the following ways:
- Stem: New Japanese Knotweed plants can grow from the green stems nodes in water or in soil.
- Crown: This part of the stem is able to survive composting and drying. If you want to dispose of using either the method of drying or composting, cut the stems right above the crown. It is able to spread by producing new canes once it contacts soil or water.
- Rhizome: Even the tiniest piece of rhizome can grow into a plant. Breaking up of the rhizome into small pieces stimulates it to produce small buds; these then grow to form each new plant. Never accept top soil that has not been checked for this underground stems, you may spread them in your vicinity without your knowledge.
Distribution: 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.
Status: Japanese knotweed is an environmental weed. Japanese knotweed is a declared weed in Tasmania under the Tasmanian Weed Management Act 1999. The importation, sale and distribution of Japanese knotweed are prohibited in Tasmania.
- Ability to spread from stem, crown or rhizome makes Japanese Knotweed very difficult to control.
- Japanese knotweed affects ecosystems by outcompeting native vegetation and limiting species diversity, and is therefore widely attributed to causing reduced species diversity in both flora and fauna.
- Early summer the bamboo-like stems emerge from rhizomes deep underground to shoot to over 2.1m (7ft), suppressing all other plant growth.
- 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, a particular problem along waterways.
- The rhizomes (underground stems) of Japanese knotweed can spread under walls, pavements and patios and cause damage to roads and buildings
- Can reduce the capacity of river channels by forming blockages and dams causing floods, as well as blocking vital flood infrastructure such as sluices, drains and ditches.
- At its most prolific, Japanese Knotweed can grow up to 20cm per day. The roots can grow 3 metres deep into the ground and spreads 7 metres in all directions, which can lead to structural problems within properties.
- 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.
- 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.
- Wash-down equipment and personnel to reduce the chance of spreading Japanese knotweed. See the Washdown Guidelines for Weed and Disease Control for detailed information.
- 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. 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.
- 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.
Visit DPIPWE website for more information.