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Polygonum japonicum

Common name:	Japanese Knotweed	Family:	Polygonaceae
Author:	Meissn.	Botanical references:	17, 58, 200
Synonyms:	Reynoutria japonica (Houtt.), Polygonum sieboldii (De Vriese.), Polygonum cuspidatum (Siebold.& Zucc.)
Known Hazards:	Although no specific mention has been made for this species, there have been reports that some members of this genus can cause photosensitivity in susceptible people. Many species also contain oxalic acid (the distinctive lemony flavour of sorrel) - whilst not toxic this substance can bind up other minerals making them unavailable to the body and leading to mineral deficiency. Having said that, a number of common foods such as sorrel and rhubarb contain oxalic acid and the leaves of most members of this genus are nutritious and beneficial to eat in moderate quantities. Cooking the leaves will reduce their content of oxalic acid. People with a tendency to rheumatism, arthritis, gout, kidney stones or hyperacidity should take especial caution if including this plant in their diet since it can aggravate their condition[238].
Range:	E. Asia - Japan. Commonly naturalized in S. England.
Habitat:	Wet grassy places in lowland all over Japan[58].
Edibility Rating (1-5):	2	Medicinal Rating (1-5):	3

Other Possible Synonyms:	From various places across the web, may not be correct. See below.
	Fallopia japonica[B,G,H,Mh] P. cuspidatum var. compactum[B,P] P. zuccarinii[B,P] Pleuropterus cuspidatus[B,P] Pleuropterus zuccarinii[B,P]
Other Common Names:	From various places around the Web, may not be correct. See below.
	Hu Chang [E], Itadori [E], Japanese Fleeceflower [E], Japanese Knotweed [B,P,MS], Pan Chang [E], Ts'An Chien Ts'Ao [E],
Systematics:	From a USDA Plants Database
	Order: Polygonales. Buckwheat family
Other Range Info:	From the Ethnobotany Database
	China
Noxious, Invasive and Injurious Weeds	From USDA PLANTS database, Weeds Australia , DEFRA Injurious Weeds
	Listed as noxious/invasive for: California, Oregon, Washington.

Physical Characteristics

Perennial growing to 3m by 5m at a fast rate. It is hardy to zone 5. It is in flower from July to October, and the seeds ripen from August to October. The flowers are dioecious (individual flowers are either male or female, but only one sex is to be found on any one plant so both male and female plants must be grown if seed is required). The plant not is self-fertile. It is noted for attracting wildlife. We rate it 2/5 for edibility and 3/5 for medicinal use.

The plant prefers light (sandy), medium (loamy) and heavy (clay) soils. The plant prefers acid, neutral and basic (alkaline) soils. It can grow in semi-shade (light woodland) or no shade. It requires moist soil.

Habitats and Possible Locations

Woodland, Dappled Shade, Shady Edge.

Edible Uses

Leaves Root Seed Stem.

Young shoots in spring - cooked. They can be used as an asparagus substitute[55, 62, 116, 1599, 179, 183]. They have an acid flavour and can also be used as a rhubarb substitute in pies, fruit soups, jams etc[183].
Older stems and shoot tips - cooked[62, 159]. They taste like a mild version of rhubarb[K].
Seed - raw or cooked. It is rather small and fiddly to utilize. The seed can also be ground into a powder and used as a flavouring and thickener in soups etc, or can be mixed with cereals when making bread, cakes etc.
The root is sometimes eaten[183].

Medicinal Uses

Disclaimer

Antiphlogistic Antitumor Depurative Diuretic Emollient Febrifuge Stomachic Vulnerary Women's complaints.

The root is antiphlogistic, bechic, depurative, diuretic, emmenagogue, emollient, febrifuge, stomachic and vulnerary[46, 61, 147, 178, 218]. It is also used in the treatment of women's complaints[147]. A decoction is used in the treatment of burn injuries, boils and abscesses, poisonous snakebites, acute hepatitis, appendicitis, traumatic injuries and menstrual irregularities[147]. The leaves can be crushed and applied externally as a poultice to abscesses, cuts etc, whilst the dried roots can be ground into a powder and applied externally[147].
Extracts of the plant have shown antitumour activity[218].

Other Uses

Biomass Dye.

A yellow dye is obtained from the root[46, 61].
The plant is potentially a good source of biomass.
Plants can be grown to form a ground cover that will exclude all other growth[208]. It is best to use the sub-species compactum since this is less invasive[208].

Cultivation details

A very easily grown plant, it succeeds in ordinary garden soil in sun or shade[1, 111], though it prefers a moist soil in partial shade[187].
Japanese knotweed is an extremely invasive plant capable of sending up new shoots at a considerable distance from the main clump and also of growing up through tarmac[200]. The sub-species cuspidatum is the form that is most likely to cause problems. It is considered to be a pest in many areas, where it is forming large clumps in the wild and out-competing native species. If grown in the garden it should be planted within a strong barrier to contain its roots. The sub-species compacta is only about 70cm tall and is far less invasive, but should still not be grown in small gardens[200].
A report on the Natural History Programme stated that Japanese knotweed is actually becoming a very valuable habitat for spiders, frogs, grass snakes and many other creatures. Its hollow stems allow a wide variety of insects and other small creatures to overwinter and find hiding places, thus a greater abundance of food is provided for insectivores such as frogs, who are themselves eaten by grass snakes. In areas of north Wales where Japanese knotweed has run rife, it is now the primary habitat for grass snakes[215].
Plants seem to be immune to the predations of rabbits[233].
Often cultivated as a dye and a medicinal plant[46, 61].
Very closely related to P. conspicuum[58].
Dioecious, male and female plants must be grown if seed is required[58].

Propagation

Seed - sow spring in a cold frame. Seed - sow spring in a cold frame. Germination is usually free and easy. When they are large enough to handle, prick the seedlings out into individual pots and plant them out in the summer if they have reached sufficient size. If not, overwinter them in a cold frame and plant them out the following spring after the last expected frosts.
Division in spring or autumn. Very easy, larger divisions can be planted out direct into their permanent positions. We have found that it is better to pot up the smaller divisions and grow them on in light shade in a cold frame until they are well established before planting them out in late spring or early summer.

Suppliers

For more details of plant suppliers please see our Suppliers Page which lists many more places to look.

Web References

[E] Ethnobotany Data (common names, uses, countries) from the Ethnobotany Database.
[HP] Links, Photos, Suppliers from Hortiplex Plant Database

References for Fallopia japonica (a possible synonym).

[H] Details of Scandanavian and European Common names in Henriette's names database

References for Polygonum cuspidatum (a possible synonym).

[H] Details of Scandanavian and European Common names in Henriette's names database
[E] Ethnobotany Data (common names, uses, countries) from the Ethnobotany Database.
[V] Images from the Vascular Plant Image Gallery of the Texas A& M Bioinformatics Working Group.
[B] Data (Latin & Common names, other references) from the BONAP's Synonymized Checklist of the Vascular Flora of the United States, Puerto Rico, and the Virgin Islands.
[G] Data (Common Names, Uses, Distribution) from the USDA/ARS NPGS's GRIN taxonomic database.
[PHARM] Phytochemical Data (common names, uses, countries) from Dr Duke's Phytochemical Database.
[P] Data. (uses, distribution, wetland) from the USDA'a Plants database.

References for Reynoutria japonica (a possible synonym).

[H] Details of Scandanavian and European Common names in Henriette's names database

References for the family Polygonaceae.

[CHINA] Draft Data Very detailed botanical/identification info. from the Flora of China.

See the PFAF Links Pages for other sources.

Also try Photos and info from the The Plants Database which has 14,000 images.

References

[K] Ken Fern
Notes from observations, tasting etc at Plants For A Future and on field trips.

[1] F. Chittendon. RHS Dictionary of Plants plus Supplement. 1956 Oxford University Press 1951
Comprehensive listing of species and how to grow them. Somewhat outdated, it has been replaces in 1992 by a new dictionary (see [200]).

[17] Clapham, Tootin and Warburg. Flora of the British Isles. Cambridge University Press 1962
A very comprehensive flora, the standard reference book but it has no pictures.

[46] Uphof. J. C. Th. Dictionary of Economic Plants. Weinheim 1959
An excellent and very comprehensive guide but it only gives very short descriptions of the uses without any details of how to utilize the plants. Not for the casual reader.

[55] Harris. B. C. Eat the Weeds. Pivot Health 1973
Interesting reading.

[58] Ohwi. G. Flora of Japan. (English translation) Smithsonian Institution 1965
The standard work. Brilliant, but not for the casual reader.

[61] Usher. G. A Dictionary of Plants Used by Man. Constable 1974 ISBN 0094579202
Forget the sexist title, this is one of the best books on the subject. Lists a very extensive range of useful plants from around the world with very brief details of the uses. Not for the casual reader.

[62] Elias. T. and Dykeman. P. A Field Guide to N. American Edible Wild Plants. Van Nostrand Reinhold 1982 ISBN 0442222009
Very readable.

[111] Sanders. T. W. Popular Hardy Perennials. Collingridge 1926
A fairly wide range of perennial plants that can be grown in Britain and how to grow them.

[116] Brooklyn Botanic Garden Oriental Herbs and Vegetables, Vol 39 No. 2. Brooklyn Botanic Garden 1986
A small booklet packed with information.

[147] ? A Barefoot Doctors Manual. Running Press ISBN 0-914294-92-X
A very readable herbal from China, combining some modern methods with traditional chinese methods.

[159] McPherson. A. and S. Wild Food Plants of Indiana. Indiana University Press 1977 ISBN 0-253-28925-4
A nice pocket guide to this region of America.

[178] Stuart. Rev. G. A. Chinese Materia Medica. Taipei. Southern Materials Centre
A translation of an ancient Chinese herbal. Fascinating.

[179] Reid. B. E. Famine Foods of the Chiu-Huang Pen-ts'ao. Taipei. Southern Materials Centre 1977
A translation of an ancient Chinese book on edible wild foods. Fascinating.

[183] Facciola. S. Cornucopia - A Source Book of Edible Plants. Kampong Publications 1990 ISBN 0-9628087-0-9
Excellent. Contains a very wide range of conventional and unconventional food plants (including tropical) and where they can be obtained (mainly N. American nurseries but also research institutes and a lot of other nurseries from around the world.

[187] Phillips. R. & Rix. M. Perennials Volumes 1 and 2. Pan Books 1991 ISBN 0-330-30936-9
Photographs of over 3,000 species and cultivars of ornamental plants together with brief cultivation notes, details of habitat etc.

[200] Huxley. A. The New RHS Dictionary of Gardening. 1992. MacMillan Press 1992 ISBN 0-333-47494-5
Excellent and very comprehensive, though it contains a number of silly mistakes. Readable yet also very detailed.

[208] Thomas. G. S. Plants for Ground Cover J. M. Dent & Sons 1990 ISBN 0-460-12609-1
An excellent detailled book on the subject, very comprehensive.

[215] BBC The Natural History Programme 16th September 1994. BBC 1994
A report on the Natural History radio 4 programme about the value of Japanese knot weed to British wildlife.

[218] Duke. J. A. and Ayensu. E. S. Medicinal Plants of China Reference Publications, Inc. 1985 ISBN 0-917256-20-4
Details of over 1,200 medicinal plants of China and brief details of their uses. Often includes an analysis, or at least a list of constituents. Heavy going if you are not into the subject.

[233] Thomas. G. S. Perennial Garden Plants J. M. Dent & Sons, London. 1990 ISBN 0 460 86048 8
A concise guide to a wide range of perennials. Lots of cultivation guides, very little on plant uses.

[238] Bown. D. Encyclopaedia of Herbs and their Uses. Dorling Kindersley, London. 1995 ISBN 0-7513-020-31
A very well presented and informative book on herbs from around the globe. Plenty in it for both the casual reader and the serious student. Just one main quibble is the silly way of having two separate entries for each plant.

Readers Comments

Polygonum japonicum

Trevor Renals (trevor.renals@environment-agency.gov.uk) Mon Mar 18 09:55:11 2002

Link: &ndndndnd Knotweed Forum

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