Common Name: Giant Knotweed

General Information

Reynoutria sachalinensis is a very vigrous, herbaceous perennial plant growing from a rhizomatous rootstock with long stolons. It produces a large, dense clump of sparingly-branched, stout, erect, hollow stems around 200 - 400cm tall[, , ].
The plant is harvested from the wild for local use as a food, medicine and source of materials.
Reynoutria sachalinensis spreads freely at the roots, forming dense clumps of growth up to 400cm tall. It can become a highly invasive plant, capable of outcompeting and displacing native species, especially in riparian zones which it prefers. It has spread to many areas of the temperate zone, especially in N. America and Europe, usually without the aid of seed dispersal though it does provide pollen for the fertilization of Reynoutria japonica plants to produce the highly invasive Reynoutria x bohemica. Though not considered as problematic as Japanese knotweed (Reynoutria japonica), eradication can be a real challenge.

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 - Sakhalin Island, Japan, Korea

Habitat

Along ravines and by streams in montane areas of Sakhalin Island[].

Properties

Weed Potential	Yes
Edibility Rating
Medicinal Rating
Other Uses Rating
Habit	Perennial
Height	2.50 m
Growth Rate	Fast
Self-fertile	No
Cultivation Status	Wild

Cultivation Details

Reynoutria sachalinensis is a very cold-hardy plant, being able to tolerate temperatures down to around -40°c when fully dormant[, , ].
Succeeds in ordinary garden soil in sun or shade[, ]. Very tolerant of a wide range of soils from volcanic ash to muddy riverbanks[].
An extremely invasive plant, capable of sending up new shoots at a considerable distance from the main clump[]. Considered a pest in many areas, if grown in the garden it should be planted within a barrier to contain its roots.
Plants seem to be immune to the predations of rabbits[].
Some plants only bear all female flowers whilst others bear hermaphrodite flowers[]. Plants are self-incompatible and, since the two forms are rarely found growing together outside the plant's native range, fertile seed is rarely propduced. However, this species does hybridize with the related Reynoutria japonica to form self-sustaining and invasive populations of Reynoutria x bohemica[].

Edible Uses

Young shoots in spring - raw or cooked[, , , ]. They can be added to salads or cooked as an asparagus substitute[]. They have an acid flavour and we find that they are more like a rhubarb substitute[].
Older stems and shoot tips - cooked. The stems are best peeled. Tasting like a mild version of rhubarb, they have a superior quality with a hint of lemon in the flavour[].

Seed - cooked[]. The seed can be ground into a powder and used as a thickener and flavouring in soups etc, or as an extender in flour. It is rather small and fiddly to utilize. The seed is often not produced in cultivation because plants are self-sterile and most plantings are of a single clone.

Medicinal

The plant (part not specified) is viewed as an effective treatment for arthralgia, jaundice, amenorrhea, coughs, scalds and burns, traumatic injuries, carbuncles and sores[].

The antioxidant activity of anthraquinones and flavonoids from flower extracts has been described[].
The stems and roots are a good source of resveratrol, a compound that shows activity against cancerous cells[]
The flowers and stems have been shown to contain various compounds with antioxidative activity and the ability to regulate the lneurotransmitter glutamate. They have potential in the treatment of conditions such as Alzheimer's Disease, Parkinson’s disease, ischaemic stroke, and spinal cord trauma[]

Agroforestry Uses:

The plant spreads very freely at the roots and was originally planted outside its native range for its ability to bind soil and prevent erosion[]. Whilst it does have the ability to bind the soil, it should only be used for this purpose within its native range since it can so easily become an invasive weed in many other parts of the world[].
Plants are very vigorous and could be grown as a tall ground cover[].
The plant has been shown to accumulate heavy metals from the soils in which it grows[].
The plant has been promoted as a living firebreak since the leaves and foliage contain around 82% moisture. Trials in Sakhalin of its fire-resistant properties indicated that thickets of the plant are a completely effective barrier against low forest fires[]

Other Uses

The plant is a potential source of biomass. It can produce up to 68.2 - 69.5 tonnes of dry matter per hectatr, with an above ground:below ground ratio of 1:3 and an annual growth rate of 31.8-32.8 tonnes per hectare[].

Leaf extracts can be used as a plant fungicide against powdery mildew[]. Leaf extracts, applied to growing plants, have been shown to greatly reduce the occurrence of leaf mildew on plants such as tomatoes and cucumbers. It works by preventing the disease and has little effect on reducing the incidence of mildew that is already present[].

The plant contains useful quantities of tannins[].

Propagation

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.