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Polygonum japonicum - Meissn.

Common Name Japanese Knotweed, Mexican Bamboo, Japanese Knotweed
Family Polygonaceae
USDA hardiness 4-10
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].
Habitats Wet grassy places in lowland all over Japan[58].
Range E. Asia - Japan. Commonly naturalized in S. England.
Edibility Rating    (3 of 5)
Other Uses    (3 of 5)
Weed Potential No
Medicinal Rating    (3 of 5)
Care (info)
Fully Hardy Moist Soil Semi-shade Full sun
Polygonum japonicum Japanese Knotweed, Mexican Bamboo,  Japanese  Knotweed

Polygonum japonicum Japanese Knotweed, Mexican Bamboo,  Japanese  Knotweed


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Bloom Color: Pink. Main Bloom Time: Early fall, Late summer. Form: Upright or erect.

Physical Characteristics

 icon of manicon of flower
Polygonum japonicum is a PERENNIAL growing to 3 m (9ft) by 5 m (16ft) at a fast rate.
See above for USDA hardiness. It is hardy to UK zone 5. It is in flower from July to October, and the seeds ripen from August to October. The species is 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 is not self-fertile.
It is noted for attracting wildlife.
Suitable for: light (sandy), medium (loamy) and heavy (clay) soils. Suitable pH: mildly acid, neutral and basic (mildly alkaline) soils. It can grow in semi-shade (light woodland) or no shade. It prefers moist soil.

UK Hardiness Map US Hardiness Map


P. cuspidatum. Sieb.&Zucc. P. sieboldii.

Plant Habitats

Woodland Garden Dappled Shade; Shady Edge;

Edible Uses

Edible Parts: Leaves  Root  Seed  Shoots  Stem
Edible Uses:

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].

References   More on Edible Uses

Medicinal Uses

Plants For A Future can not take any responsibility for any adverse effects from the use of plants. Always seek advice from a professional before using a plant medicinally.
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].

References   More on Medicinal Uses

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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].

Special Uses

Attracts Wildlife

References   More on Other Uses

Cultivation details

Landscape Uses:Container, Specimen, Woodland garden. 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]. Special Features:Attractive foliage, Invasive, Wetlands plant.

References   Carbon Farming Information and Carbon Sequestration Information

Temperature Converter

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Plant 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.

Other Names

If available other names are mentioned here

Weed Potential

Right plant wrong place. We are currently updating this section. Please note that a plant may be invasive in one area but may not in your area so it’s worth checking.

Conservation Status

IUCN Red List of Threatened Plants Status :

Related Plants
Latin NameCommon NameHabitHeightHardinessGrowthSoilShadeMoistureEdibleMedicinalOther
Polygonum alaskanumAlaska Wild RhubarbPerennial1.8 -  LMHSNM21 
Polygonum alpinumAlpine Knotweed, Alaska wild rhubarbPerennial1.0 4-8  LMHSNM21 
Polygonum amphibiumWillow Grass, Water knotweed, Longroot smartweed, Water smartweedPerennial0.3 4-8  LMHSNWeWa121
Polygonum arenastrumSmall-Leaved Knotweed, Oval-leaf knotweedAnnual0.3 4-8  LMHSNM231
Polygonum aviculareKnotweed, Prostrate knotweedAnnual0.3 4-8  LMHSNM231
Polygonum barbatumJoint WeedPerennial0.8 -  LMHSNM11 
Polygonum bistortaBistort, Meadow bistort, SnakeweedPerennial0.5 4-7 FLMHSNMWe332
Polygonum bistortoidesAmerican BistortPerennial0.5 4-8  LMHSNM31 
Polygonum bungeanumBunge's smartweedAnnual0.8 0-0  LMHSNM10 
Polygonum coccineumWater SmartweedPerennial0.5 4-8  LMHSNWeWa10 
Polygonum conspicuum Perennial0.6 -  LMHSNM10 
Polygonum convolvulusBlack BindweedAnnual1.2 0-0  LMHSNM100
Polygonum divaricatum Perennial1.0 -  LMHSNDM10 
Polygonum douglasiiKnotweed, Douglas' knotweed, Austin knotweed, Engelmann's knotweed, Johnston's knotweed, Large knoAnnual0.3 0-0  LMHSNDM20 
Polygonum dumetorumClimbing false buckwheatAnnual1.8 0-0  LMHSNM11 
Polygonum equisetiforme Perennial1.0 7-10  LMHNDM102
Polygonum fugax Perennial0.0 -  LMHSNM30 
Polygonum hydropiperSmartweed, Marshpepper knotweedAnnual0.8 0-0  LMHNWeWa221
Polygonum lapathifoliumCurlytop KnotweedAnnual0.8 4-8  LMHSNMWe111
Polygonum limosum Perennial1.8 -  LMHSNM10 
Polygonum longisetumOriental lady's thumbAnnual0.5 0-0  LMHSNMWe10 
Polygonum maackianum Annual0.8 -  LMHSNMWe10 
Polygonum manshurienseAsian BistortPerennial0.8 -  LMHSNMwe01 
Polygonum microcephalum Perennial0.5 -  LMHSNM10 
Polygonum minusPygmy smartweedAnnual0.3 0-0  LMHSNMWe10 
Polygonum molle Perennial2.5 6-9 FLMHSNM212
Polygonum multiflorumHe Shou Wu, Tuber fleeceflowerPerennial Climber4.5 6-9  LMHSNM23 
Polygonum nepalenseNepalese smartweedAnnual0.3 0-0  LMHSNMWe111
Polygonum orientalePrince's Feather, Kiss me over the garden gateAnnual1.5 0-0  LMHSNM220

Growth: S = slow M = medium F = fast. Soil: L = light (sandy) M = medium H = heavy (clay). pH: A = acid N = neutral B = basic (alkaline). Shade: F = full shade S = semi-shade N = no shade. Moisture: D = dry M = Moist We = wet Wa = water.


Expert comment



Botanical References


Links / References

For a list of references used on this page please go here

Readers comment

Trevor Renals   Mon Mar 18 09:55:11 2002

Link: Cornwall Knotweed Forum

Peter G. Becker   Sun Dec 18 2005

Also great as a raw item in salats. The mass consumption of this plant would protect the native flora of regions japanese Knotweed invades

NewTritionInk My Appetite........Your Herbicide. The Manual of Bionic Knotweed Control

David Beaulieu   Wed Jan 11 2006

Japanese Knotweed Plants Information about Japanese knotweed in the landscape.

Michele Tsuji   Fri Jul 7 2006

Thanks for the information on the edible and medicinal uses of this plant. As so much of it grows in my yard, I would like to find a good use for it. Besides eating, any chance it's good for producing ethanol? :)

Bob Smith   Wed Oct 25 2006

Thurlow Countryside Management These guys eradicate Japanese Knotweed in one growing seaon!

Ethan Descoteau   Fri Mar 21 2008

is there any other information on edibility of the seeds & leaves? before i found this page, i considered wrapping fish or other meat in the leaves for roasting on stones; but have yet to build up the courage. i would VERY much like to try eating the seeds this year... but i'd like to see someone else first, :) any perennial seed crop is vital... i'm surprised this website hasn't looked into this plant further for alternative grain. i've found this plant very useful for compost. chop when green, right before at the beginning of flowering. the leaves and stems balance out well in terms of Nitrogen/Carbon, and the hollow stems (and their shape) allow good aeration during the composting.

   Wed Oct 15 2008

This has been reclassified as Fallopia. the roots are used a commercial source of resveratrol for the supplement industry.

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