Herb: Japanese Knotweed

Latin name: Polygonum japonicum

Synonyms: Polygonum cuspidatum, Polygonum sieboldii, Reynoutria japonica

Family: Polygonaceae (Buckwheat Family)

Medicinal use of Japanese Knotweed:

The root is antiphlogistic, bechic, depurative, diuretic, emmenagogue, emollient, febrifuge, stomachic and vulnerary. It is also used in the treatment of women's complaints. A decoction is used in the treatment of burn injuries, boils and abscesses, poisonous snakebites, acute hepatitis, appendicitis, traumatic injuries and menstrual irregularities. 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. Extracts of the plant have shown antitumour activity.

Description of the plant:


3 m
(9 3/4 foot)

July to

Habitat of the herb:

Wet grassy places in lowland all over Japan.

Edible parts of Japanese Knotweed:

Young shoots in spring - cooked. They can be used as an asparagus substitute. They have an acid flavour and can also be used as a rhubarb substitute in pies, fruit soups, jams etc. Older stems and shoot tips - cooked. They taste like a mild version of rhubarb. 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.

Other uses of the herb:

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

Propagation of Japanese Knotweed:

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.

Cultivation of the herb:

Wet grassy places in lowland all over Japan.

Known hazards of Polygonum japonicum:

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.

Plant information taken from the Plants For A Future.