Quick Facts

Quick facts

• Japanese Knotweed (Fallopia japonica) is an ornamental tall perennial shrub that grows to 1–3 m high with upright, robust, bamboo-like annual stems.
• It can spread easily from even a very small piece of its robust rhizome.
• It is a major weed in Europe, North America and other regions with cool climates.
• It is closely related to Giant Knotweed, and these two species hybridise readily.
• It is occasionally cultivated in south-eastern Australia and has the potential to become a major weed.

What Does It Look Like?

What is it?

Japanese Knotweed (Fallopia japonica) is a perennial shrub that grows to 1–3 m high, with extensive robust woody rhizomes (horizontal roots just under the soil surface). Its annual stems are somewhat bamboo-like, greenish and with purple-brown streaks, with the ultimate leaf-bearing branches in a zig-zag like pattern. It forms a dense thicket each year before dying back in winter. Its leaves are slightly heart-shaped, egg-shaped or triangular, 3–12 cm long and 2–10 cm wide. The leaves have dark green upper surfaces and paler green undersides and their veins are often reddish in colour. Separate male and female flowers grow on different plants.

The flowers are small (about 2.5–3 mm across) and whitish, greenish or cream. They are borne in dense, elongated and often branched, clusters (10–12.5 cm long) in the upper leaf forks. The male flower clusters are generally upright, while the female ones often have a drooping appearance. The individual flowers have five 'petals' that are 2–8 mm long.

The fruit consists of a small dark brown or blackish seed (2–4 mm by 2 mm) enclosed in a three-winged structure that is 6–10 mm long (Spencer 1997; Wilson 2000; Navie 2004; Wilson 2007).

This species hybridises readily with the related species Giant Knotweed (F. sachalinensis), and this hybrid is referred to as Fallopia x bohemica. The hybrid can produce viable seed (Barney et al. 2006; Bailey 2003; Hollingsworth & Bailey 2000).

For further information and assistance with identification of Japanese Knotweed contact the herbarium in your state or territory.

Flower colour

White, Green

Growth form (weed type/habit)

Shrub

Where it currently grows? Preferred habitat

Japanese Knotweed is a colonising species in its native range, growing on open roadsides, river banks and managed pasture, and even colonising lava fields (Hollingsworth & Bailey 2000; Gillespie & Faithfull 2004). In Australia it grows in similarly disturbed sites, often in moist situations (Ainsworth & Weiss 2002; Gillespie & Faithfull 2004).

Are there similar species?

Japanese Knotweed looks similar to the related Giant Knotweed, Fallopia sachalinensis, which is a more robust species with larger but relatively narrower leaves with a shortly acute apex, and with greenish to whitish flowers. If grown together, these two species can produce a hybrid, known as Hybrid Knotweed (F. x bohemica), which looks intermediate in its features. They can be told apart by differences in leaf shape and size and whether the undersurface of the leaf is hairy or not. Giant Knotweed has leaves 20-40 cm long, with the base deeply curved or heart-shaped; the margins and veins on the under-surface have scattered elongated multicellular hairs. Both Japanese Knotweed and the hybrid have smaller leaves (3-10 cm long in Japanese Knotweed; 5–30 in the hybrid), with scattered, sparse smaller hairs or no hairs visible on the under-surface (small ridges or lumps in the case of Japanese Knotweed; short hairs in the case of the hybrid) (Wilson 2007). Given the close similarities, identifications should be checked at a herbarium, although any Knotweeds are invasive and populations should be destroyed when found (Ainsworth & Weiss 2002; Bailey 2003; Gillespie & Faithfull 2004; Barney et al. 2006; Wilson 2007).

Why Is It A Weed?

What are its impacts?

Native ecosystems: Japanese Knotweed is considered a notoriously invasive plant in Britain, having spread extensively since its introduction into gardens there in the 1850s, where it has formed dense monospecific (single species) stands. Japanese Knotweed forms monospecific stands on riverbanks and other disturbed sites, and the annual dying back in winter leaves the ground susceptible to erosion. (Anon. 2008; Ainsworth & Weiss 2002; Barney et al. 2006; Doll & Doll 2002; Gillespie & Faithfull 2004; Hollingsworth & Bailey 2000). Japanese knotweed is regarded as a potentially serious environmental weed in Victoria and Tasmania, and as a "sleeper weed" in other parts of southern Australia (Weeds of Australia 2016).

Japanese knotweed can cause blockages in waterways, as the dead stems accumulate in rivers and streams channels, increasing the risk of flooding. It can also contribute to the erosion of riverbanks, restrict access to waterways, and interfere with irrigation channels in cropping areas (Weeds of Australia 2016).

Urban environments: The underground stems, and emerging shoots, can cause damage to paving, bitumin roadways and other structures (Weeds of Australia 2016).

How does it spread?

Japanese Knotweed commonly spreads by rhizomes, with even a small length of rhizome capable of regeneration. The plants are either functionally female or bisexual, so seed may, or may not, be produced. It appears that hybrids with F. sachalinensis may produce viable seed. Seed may be dispersed by wind or water (Anon. 2008; Barney et al. 2006; Beerling et al. 1994; Hollingsworth & Bailey 2000).

What is its history in Australia?

Japanese Knotweed has been cultivated in south-eastern Australia for more than a century (Wilson 2008 pers. comm.), but records do not make it clear when or how it was introduced and spread. This partly arises from confusion between this species, Giant Knotweed (Fallopia sachalinensis), and the hybrid produced by these two species, Fallopia x bohemica. Specimens in the National Herbarium of New South Wales dating back to 1957 may refer to the hybrid rather than to Japanese Knotweed (Wilson 2008 pers. comm.). Japanese Knotweed was first recorded as naturalised in Tasmania in 1979 as a localised colony on a roadside at Ilfraville (Hosking 1997).

How To Manage It?

Best practice management

Japanese Knotweed is very hard to eradicate because it has the ability to re-grow from vegetative pieces and from seeds.

Non-chemical control: Single young plants can be pulled by hand when soil is moist and roots are small. Roots and runners must be removed to prevent re-sprouting (Plant Invaders of Mid-Atlantic Natural Areas 2010). Cutting, mowing and grazing can be used to to control new growth.

Chemical control: Chemical control measures are being used for the control of Japanese Knotweed in Australia (Ainsworth & Weiss 2002; Gillespie & Faithfull 2004; Hansford 2008 pers.comm; Tasmanian Department of Primary Industries and Water 2007). Herbicides have been used effectively, applied to freshly cut stems or foliage  (Plant Invaders of Mid-Atlantic Natural Areas 2010).

See the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au 

Biological control: A biological control agent exists for Japanese Knotweed and was released in the United Kingdom in 2010 (Plant Invaders of Mid-Atlantic Natural Areas 2010).

Does it have a biological control agent?

YES

When does it grow? (lifecycle/growth calendar)

Japanese Knotweed is a vigorous grower in cooler climates. Its new stems start growing from the rootstock in early spring. It flowers and fruits in late summer to autumn, then dies back in late autumn to winter (Beerling et al. 1994; Doll & Doll 2002; Gillespie & Faithfull 2004; Barney et al. 2006; Anon. 2008; Thurlow Countryside Management 2008).

Where Is It Found?

Which states and territories is it found?

NSW, TAS, VIC

What areas within states and territories is it found?

Japanese Knotweed is cultivated in cooler climate areas of south-eastern Australia but is as yet rarely naturalised (Hansford 2008 pers. comm.; Tasmanian Department of Primary Industries and Water 2007; Gillespie & Faithfull 2004).

Assessment of distribution is difficult because of confusion between Japanese Knotweed, Giant Knotweed (F. sachalinensis) and the hybrid produced by these two species, Fallopia x bohemica. Japanese Knotweed is recorded as cultivated or possibly naturalised in various places in Victoria and Tasmania (Spencer 1997; Hosking 1997; Ainsworth & Weiss 2002; Gillespie & Faithfull 2004; Baker 2008 pers.comm.; M. Hansford 2008 pers.comm.; Lane 2008 pers.comm.; Tasmanian Department of Primary Industries and Water 2007). A few isolated infestations have also been recorded in Queensland and the Northern Territory (Navie 2004).

Where does it originate?

Japanese Knotweed is native to temperate east Asia but is widely spread in cultivation in cool temperate regions of the world. (Bailey & Conolly 2000; Barney et al. 2006; Wilson 2000).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

TAS, VIC, WA

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Fallopia japonica

Other scientific names (synonyms)?

• Polygonum cuspidatum Siebold & Zucc.
• Reynoutria japonica Houtt.
• Tiniaria japonica (Houtt.) Hedberg

Does it have other known common name(s)?

Asiatic Knotweed, Japanese Polygonum, Japanese Grass, Japanese Bamboo