Common Name: Japanese Knotweed

General Information

Reynoutria japonica is a perennial plant that can grow up to 3.00 metres tall.
The plant is harvested from the wild for local use as a food, medicine and source of materials.
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[]. 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.

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[].

Botanical References

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Range

E. Asia - Russian Far East, eastern China, Japan, Korea

Habitat

Wet grassy places in lowland all over Japan[].

Properties

Weed Potential	Yes
Edibility Rating
Medicinal Rating
Other Uses Rating
Habit	Perennial
Height	3.00 m
Growth Rate	Fast
Self-fertile	No
Cultivation Status	Wild

Cultivation Details

Reynoutria japonica is a very cold-hardy plant, being able to tolerate temperatures down to around -25°c when fully dormant.
A very easily grown plant, it succeeds in ordinary garden soil in sun or shade[, ], though it prefers a moist soil in partial shade[].
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[]. 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[].
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[].
Plants seem to be immune to the predations of rabbits[].
Often cultivated as a dye and a medicinal plant[, ].
Very closely related to P. conspicuum[].
Dioecious, male and female plants must be grown if seed is required[].

Edible Uses

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[].

Medicinal

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[].

Agroforestry Uses:

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[].

Other Uses

A yellow dye is obtained from the root[, ].
The plant is potentially a good source of biomass.

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.