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Japanese Knotweed
Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Caryophyllales
Family: Polygonaceae
Genus: Fallopia
Species: F. japonica
Binomial name
Fallopia japonica
Houtt. (Ronse Decr.)

Japanese Knotweed (Fallopia japonica, syn. Polygonum cuspidatum, Reynoutria japonica) is a large, herbaceous perennial plant, native to eastern Asia in Japan, China and Korea. In North America and Europe the species is very successful and has been classified as invasive in several countries.

A member of the family Polygonaceae, Japanese knotweed has hollow stems with distinct raised nodes that give it the appearance of bamboo, though it is not closely related. While stems may reach a maximum height of 3–4 m each growing season, it is typical to see much smaller plants in places where they sprout through cracks in the pavement or are repeatedly cut down. The leaves are broad oval with a truncated base, 7–14 cm long and 5–12 cm broad, with an entire margin. The flowers are small, creamy white, produced in erect racemes 6–15 cm long in late summer and early autumn.

Closely related species include giant knotweed (Fallopia sachalinensis, syn. Polygonum sachalinense) and Russian vine (Fallopia baldschuanica, syn. Polygonum aubertii, Polygonum baldschuanicum).

Other English names for Japanese knotweed include fleeceflower, Himalayan fleece vine, monkeyweed, Huzhang (Chinese: 虎杖pinyin: Hǔzhàng), Hancock's curse, elephant ears, pea shooters, donkey rhubarb (although it is not a rhubarb), sally rhubarb, Japanese bamboo, American bamboo, and Mexican bamboo (though it is not a bamboo). There are also regional names, and it is sometimes confused with sorrel.

In Japanese, the name is itadori (虎杖, イタドリ?).[1]

Japanese Knotweed is a commercial source of resveratrol supplements. Hu Zhang root extract is a traditional Chinese medicinal treatment.


Invasive species

Old stems remain in place as new growth appears

In the U.S.A. and Europe, Japanese knotweed is widely considered an invasive species or weed.[2] It is listed by the World Conservation Union as one of the world's 100 worst invasive species.[3]

The invasive root system and strong growth can damage foundations, buildings, flood defences, roads, paving, retaining walls and architectural sites. It can also reduce the capacity of channels in flood defences to carry water.[4]

It is a frequent colonizer of temperate riparian ecosystems, roadsides and waste places. It forms thick, dense colonies that completely crowd out any other herbaceous species and is now considered one of the worst invasive exotics in parts of the eastern United States. The success of the species has been partially attributed to its tolerance of a very wide range of soil types, pH and salinity. Its rhizomes can survive temperatures of −35 °C (−31 °F) and can extend 7 metres (23 ft) horizontally and 3 metres (9.8 ft) deep, making removal by excavation extremely difficult. The plant is also resilient to cutting, vigorously re-sprouting from the roots. The most effective method of control is by herbicide application close to the flowering stage in late summer or autumn. In some cases it is possible to eradicate Japanese knotweed in one growing season using only herbicides. Trials in the Queen Charlotte Islands (Haida Gwaii) of British Columbia using sea water sprayed on the foliage have demonstrated promising results, which may prove to be a viable option for eradication where concerns over herbicide application are too great.[citation needed]

It can be found in 39 of the 50 United States[5] and in six provinces in Canada. It is listed as an invasive weed in Ohio, Vermont, Virginia, New York, Alaska, Pennsylvania, Oregon and Washington state.[6] The species is also common in Europe. In the UK it was made illegal to spread Japanese knotweed by the Wildlife and Countryside Act 1981. It is also classed as "controlled waste" in Britain under part 2 of the Environmental Protection Act 1990. This requires disposal at licensed landfill sites.[7]

Two biological pest control agents that show promise in the control of the plant are the psyllid Aphalara itadori and a leaf spot fungus from genus Mycosphaerella.[8]


Japanese knotweed flowers are valued by some beekeepers as an important source of nectar for honeybees, at a time of year when little else is flowering. Japanese knotweed yields a monofloral honey, usually called bamboo honey by northeastern U.S. beekeepers, like a mild-flavored version of buckwheat honey (a related plant also in the Polygonaceae).

Erect inflorescence

The young stems are edible as a spring vegetable, with a flavor similar to mild rhubarb. In some locations, semi-cultivating Japanese knotweed for food has been used as a means of controlling knotweed populations that invade sensitive wetland areas and drive out the native vegetation.[9] Some caution should be exercised when consuming this plant because it contains oxalic acid, which may aggravate conditions such as rheumatism, arthritis, gout, kidney stones or hyperacidity.[10]

Both Japanese knotweed and giant knotweed are important concentrated sources of resveratrol, replacing grape byproducts. Many large supplement sources of resveratrol now use Japanese knotweed and use its scientific name in the supplement labels. The plant is useful because of its year-round growth and robustness in different climates.[11]

Japanese knotweed is a concentrated source of emodin, used as a nutritional supplement to regulate bowel motility. The roots of Japanese knotweed are used in traditional Chinese and Japanese herbal medicines as a natural laxative. The active principle responsible for the laxative effect is emodin, present in its natural form as a complex of its analogs. Emodin has a mild laxative effect in doses of 20 to 50 mg per day.


This antique locomotive at Beekbergen (The Netherlands) is overgrown by knotweed. A few years ago, this place was still Knotweed-free, see google maps

Japanese knotweed has a large underground network of roots (rhizomes). To eradicate the plant the roots need to be killed. Picking the right herbicide is essential, as it must travel through the plant and into the root system below. Glyphosate is the best active ingredient in herbicide for use on Japanese knotweed as it is ’systemic’; it penetrates through the whole plant and travels to the roots. Glyphosate is available under several trade names - all label the product as a "weed and grass killer". Commercial glyphosate concentrates contain approximately 20%-40% glyphosate; the balance is mostly water. Such concentrates need to be diluted in water.

The most effective spraying solution contains about 5%-10% glyphosate in water. (To make a 5% solution from a 40% concentrate mix 1 part concentrate with 7 parts water.) Ready-to-use solutions that contain less than 5% glyphosate are too weak and do not work. A small amount of liquid dish-washing detergent can be added to improve wetting of the leaves. If possible, both sides of the leaves should be sprayed until they are completely wet. It takes about 3 weeks for most of the plants to die. After 3 weeks, all remaining plants should be sprayed again. This process needs to be repeated until all the plants die.

The US federal government will come and spray Japanese knotweed for no charge in many areas under the Invasive Species Act. Local county extension agencies can be contacted for more details.

More ecologically friendly means are being tested as an alternative to chemical treatments. Soil steaming [12] involves injecting steam into contaminated soil in order to kill subterranean plant parts. Research has also been carried out on Mycosphaerella leafspot fungus, which devastates knotweed in its native Japan. Research with Mycosphaerella, has been relatively slow, due to its complex life cycle.[13].

In the UK, where over 150M pounds is spent annually on Japanese knotweed control, a decision was taken on 9 March 2010 to release a psyllid insect, Aphalara itadori.[14] It is highly specific to Japanese knotweed and shows good potential for its control.[15][16]


Detail of the stalk
  1. ^ "itadori". Denshi Jisho — Online Japanese dictionary. Retrieved 9 March 2010. 
  2. ^ Biological control of invasive plants in the Eastern United States
  3. ^ IUCN Global Invasive Species Database
  4. ^ Article on the costs of Japanese Knotweed
  5. ^ PUSDA
  6. ^ USDA weed profile for Japanese knotweed
  7. ^ Relevant Legislation for handling and disposing of japanese knotweed
  8. ^ Morelle, R. Alien invaders hit the UK. BBC News October 13, 2008.
  9. ^ Pilot project of Bionic Knotweed Control in Wiesbaden, Germany
  10. ^ Edible Plants - Japanese Knotweed
  11. ^ Pest Diagnostic Unit, University of Guelph
  12. ^ Soil-Steaming-Report, 03. Okt. 2009
  13. ^ Notes on Biological control and Japanese knotweed
  14. ^
  15. ^ The life history and host range of the Japanese knotweed psyllid, Aphalara itadori Shinji: Potentially the first classical biological weed control agent for the European Union. Richard H. Shaw, Sarah Bryner and Rob Tanner. UK Biological Control. Volume 49, Issue 2, May 2009, Pages 105-113.
  16. ^ Natural control of Japanese knotweed: information event

External links


Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary


Proper noun

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Japanese knotweed

  1. a large, herbaceous perennial plant, Fallopia japonica, native to Asia; it is a highly invasive colonizer

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